Python类的内建方法-即所谓的魔术方法

在Python中被双下划线包围的方法，__init__、__str__、__doc__、__new__等，被称为"魔术方法"（Magic methods）#更严谨的说法是内建方法。魔术方法在类或对象的某些事件出发后会自动执行，如果希望根据自己的程序定制自己特殊功能的类，那么就需要对这些方法进行重写。Python 将所有以 __包起来（即前后皆有__）的类方法保留为魔术方法。

注：如果只前面有双下划线__，则为私有方法或私有变量

 

一，

A Guide to Python's Magic Methods

Rafe Kettler

Copyright © 2012 Rafe Kettler

Version 1.17

A PDF version of this guide can be obtained from my site or Github. The magic methods guide has a git repository at http://www.github.com/RafeKettler/magicmethods. Any issues can be reported there, along with comments, (or even contributions!).

Table of Contents

1. Introduction
2. Construction and Initialization
3. Making Operators Work on Custom Classes
   • Comparison magic methods
   • Numeric magic methods
4. Representing your Classes
5. Controlling Attribute Access
6. Making Custom Sequences
7. Reflection
8. Abstract Base Classes
9. Callable Objects
10. Context Managers
11. Building Descriptor Objects
12. Copying
13. Pickling your Objects
14. Conclusion
15. Appendix 1: How to Call Magic Methods
16. Appendix 2: Changes in Python 3

Introduction

This guide is the culmination of a few months' worth of blog posts. The subject is magic methods.

What are magic methods? They're everything in object-oriented Python. They're special methods that you can define to add "magic" to your classes. They're always surrounded by double underscores (e.g. __init__ or __lt__). They're also not as well documented as they need to be. All of the magic methods for Python appear in the same section in the Python docs, but they're scattered about and only loosely organized. There's hardly an example to be found in that section (and that may very well be by design, since they're all detailed in the language reference, along with boring syntax descriptions, etc.).

So, to fix what I perceived as a flaw in Python's documentation, I set out to provide some more plain-English, example-driven documentation for Python's magic methods. I started out with weekly blog posts, and now that I've finished with those, I've put together this guide.

I hope you enjoy it. Use it as a tutorial, a refresher, or a reference; it's just intended to be a user-friendly guide to Python's magic methods.

Construction and Initialization

Everyone knows the most basic magic method, __init__. It's the way that we can define the initialization behavior of an object. However, when I call x = SomeClass(), __init__ is not the first thing to get called. Actually, it's a method called __new__, which actually creates the instance, then passes any arguments at creation on to the initializer. At the other end of the object's lifespan, there's __del__. Let's take a closer look at these 3 magic methods:

__new__(cls, [...)

__new__ is the first method to get called in an object's instantiation. It takes the class, then any other arguments that it will pass along to __init__. __new__ is used fairly rarely, but it does have its purposes, particularly when subclassing an immutable type like a tuple or a string. I don't want to go in to too much detail on __new__ because it's not too useful, but it is covered in great detail in the Python docs.

__init__(self, [...)

The initializer for the class. It gets passed whatever the primary constructor was called with (so, for example, if we called x = SomeClass(10, 'foo'), __init__ would get passed 10 and 'foo' as arguments. __init__ is almost universally used in Python class definitions.

__del__(self)

If __new__ and __init__ formed the constructor of the object, __del__ is the destructor. It doesn't implement behavior for the statement del x (so that code would not translate to x.__del__()). Rather, it defines behavior for when an object is garbage collected. It can be quite useful for objects that might require extra cleanup upon deletion, like sockets or file objects. Be careful, however, as there is no guarantee that __del__ will be executed if the object is still alive when the interpreter exits, so __del__ can't serve as a replacement for good coding practices (like always closing a connection when you're done with it. In fact, __del__ should almost never be used because of the precarious circumstances under which it is called; use it with caution!

Putting it all together, here's an example of __init__ and __del__ in action:

from os.path import join

class FileObject:
    '''Wrapper for file objects to make sure the file gets closed on deletion.'''

    def __init__(self, filepath='~', filename='sample.txt'):
        # open a file filename in filepath in read and write mode
        self.file = open(join(filepath, filename), 'r+')

    def __del__(self):
        self.file.close()
        del self.file

Making Operators Work on Custom Classes

One of the biggest advantages of using Python's magic methods is that they provide a simple way to make objects behave like built-in types. That means you can avoid ugly, counter-intuitive, and nonstandard ways of performing basic operators. In some languages, it's common to do something like this:

if instance.equals(other_instance):
    # do something

You could certainly do this in Python, too, but this adds confusion and is unnecessarily verbose. Different libraries might use different names for the same operations, making the client do way more work than necessary. With the power of magic methods, however, we can define one method (__eq__, in this case), and say what we mean instead:

if instance == other_instance:
    #do something

That's part of the power of magic methods. The vast majority of them allow us to define meaning for operators so that we can use them on our own classes just like they were built in types.

Comparison magic methods

Python has a whole slew of magic methods designed to implement intuitive comparisons between objects using operators, not awkward method calls. They also provide a way to override the default Python behavior for comparisons of objects (by reference). Here's the list of those methods and what they do:

__cmp__(self, other)

__cmp__ is the most basic of the comparison magic methods. It actually implements behavior for all of the comparison operators (<, ==, !=, etc.), but it might not do it the way you want (for example, if whether one instance was equal to another were determined by one criterion and and whether an instance is greater than another were determined by something else). __cmp__ should return a negative integer if self < other, zero if self == other, and positive if self > other. It's usually best to define each comparison you need rather than define them all at once, but __cmp__ can be a good way to save repetition and improve clarity when you need all comparisons implemented with similar criteria.

__eq__(self, other)

Defines behavior for the equality operator, ==.

__ne__(self, other)

Defines behavior for the inequality operator, !=.

__lt__(self, other)

Defines behavior for the less-than operator, <.

__gt__(self, other)

Defines behavior for the greater-than operator, >.

__le__(self, other)

Defines behavior for the less-than-or-equal-to operator, <=.

__ge__(self, other)

Defines behavior for the greater-than-or-equal-to operator, >=.

For an example, consider a class to model a word. We might want to compare words lexicographically (by the alphabet), which is the default comparison behavior for strings, but we also might want to do it based on some other criterion, like length or number of syllables. In this example, we'll compare by length. Here's an implementation:

class Word(str):
    '''Class for words, defining comparison based on word length.'''

    def __new__(cls, word):
        # Note that we have to use __new__. This is because str is an immutable
        # type, so we have to initialize it early (at creation)
        if ' ' in word:
            print "Value contains spaces. Truncating to first space."
            word = word[:word.index(' ')] # Word is now all chars before first space
        return str.__new__(cls, word)

    def __gt__(self, other):
        return len(self) > len(other)
    def __lt__(self, other):
        return len(self) < len(other)
    def __ge__(self, other):
        return len(self) >= len(other)
    def __le__(self, other):
        return len(self) <= len(other)

Now, we can create two Words (by using Word('foo') and Word('bar')) and compare them based on length. Note, however, that we didn't define __eq__ and __ne__. This is because this would lead to some weird behavior (notably that Word('foo') == Word('bar') would evaluate to true). It wouldn't make sense to test for equality based on length, so we fall back on str's implementation of equality.

Now would be a good time to note that you don't have to define every comparison magic method to get rich comparisons. The standard library has kindly provided us with a class decorator in the module functools that will define all rich comparison methods if you only define __eq__ and one other (e.g. __gt__, __lt__, etc.) This feature is only available in Python 2.7, but when you get a chance it saves a great deal of time and effort. You can use it by placing @total_ordering above your class definition.

Numeric magic methods

Just like you can create ways for instances of your class to be compared with comparison operators, you can define behavior for numeric operators. Buckle your seat belts, folks...there's a lot of these. For organization's sake, I've split the numeric magic methods into 5 categories: unary operators, normal arithmetic operators, reflected arithmetic operators (more on this later), augmented assignment, and type conversions.

Unary operators and functions

Unary operators and functions only have one operand, e.g. negation, absolute value, etc.

__pos__(self)

Implements behavior for unary positive (e.g. +some_object)

__neg__(self)

Implements behavior for negation (e.g. -some_object)

__abs__(self)

Implements behavior for the built in abs() function.

__invert__(self)

Implements behavior for inversion using the ~ operator. For an explanation on what this does, see the Wikipedia article on bitwise operations.

__round__(self, n)

Implements behavior for the built in round() function. n is the number of decimal places to round to.

__floor__(self)

Implements behavior for math.floor(), i.e., rounding down to the nearest integer.

__ceil__(self)

Implements behavior for math.ceil(), i.e., rounding up to the nearest integer.

__trunc__(self)

Implements behavior for math.trunc(), i.e., truncating to an integral.

Normal arithmetic operators

Now, we cover the typical binary operators (and a function or two): +, -, * and the like. These are, for the most part, pretty self-explanatory.

__add__(self, other)

Implements addition.

__sub__(self, other)

Implements subtraction.

__mul__(self, other)

Implements multiplication.

__floordiv__(self, other)

Implements integer division using the // operator.

__div__(self, other)

Implements division using the / operator.

__truediv__(self, other)

Implements true division. Note that this only works when from __future__ import division is in effect.

__mod__(self, other)

Implements modulo using the % operator.

__divmod__(self, other)

Implements behavior for long division using the divmod() built in function.

__pow__

Implements behavior for exponents using the ** operator.

__lshift__(self, other)

Implements left bitwise shift using the << operator.

__rshift__(self, other)

Implements right bitwise shift using the >> operator.

__and__(self, other)

Implements bitwise and using the & operator.

__or__(self, other)

Implements bitwise or using the | operator.

__xor__(self, other)

Implements bitwise xor using the ^ operator.

Reflected arithmetic operators

You know how I said I would get to reflected arithmetic in a bit? Some of you might think it's some big, scary, foreign concept. It's actually quite simple. Here's an example:

some_object + other

That was "normal" addition. The reflected equivalent is the same thing, except with the operands switched around:

other + some_object

So, all of these magic methods do the same thing as their normal equivalents, except the perform the operation with other as the first operand and self as the second, rather than the other way around. In most cases, the result of a reflected operation is the same as its normal equivalent, so you may just end up defining __radd__ as calling __add__ and so on. Note that the object on the left hand side of the operator (other in the example) must not define (or return NotImplemented) for its definition of the non-reflected version of an operation. For instance, in the example, some_object.__radd__ will only be called if other does not define __add__.

__radd__(self, other)

Implements reflected addition.

__rsub__(self, other)

Implements reflected subtraction.

__rmul__(self, other)

Implements reflected multiplication.

__rfloordiv__(self, other)

Implements reflected integer division using the // operator.

__rdiv__(self, other)

Implements reflected division using the / operator.

__rtruediv__(self, other)

Implements reflected true division. Note that this only works when from __future__ import division is in effect.

__rmod__(self, other)

Implements reflected modulo using the % operator.

__rdivmod__(self, other)

Implements behavior for long division using the divmod() built in function, when divmod(other, self) is called.

__rpow__

Implements behavior for reflected exponents using the ** operator.

__rlshift__(self, other)

Implements reflected left bitwise shift using the << operator.

__rrshift__(self, other)

Implements reflected right bitwise shift using the >> operator.

__rand__(self, other)

Implements reflected bitwise and using the & operator.

__ror__(self, other)

Implements reflected bitwise or using the | operator.

__rxor__(self, other)

Implements reflected bitwise xor using the ^ operator.

Augmented assignment

Python also has a wide variety of magic methods to allow custom behavior to be defined for augmented assignment. You're probably already familiar with augmented assignment, it combines "normal" operators with assignment. If you still don't know what I'm talking about, here's an example:

x = 5
x += 1 # in other words x = x + 1

Each of these methods should return the value that the variable on the left hand side should be assigned to (for instance, for a += b, __iadd__ might return a + b, which would be assigned to a). Here's the list:

__iadd__(self, other)

Implements addition with assignment.

__isub__(self, other)

Implements subtraction with assignment.

__imul__(self, other)

Implements multiplication with assignment.

__ifloordiv__(self, other)

Implements integer division with assignment using the //= operator.

__idiv__(self, other)

Implements division with assignment using the /= operator.

__itruediv__(self, other)

Implements true division with assignment. Note that this only works when from __future__ import division is in effect.

__imod__(self, other)

Implements modulo with assignment using the %= operator.

__ipow__

Implements behavior for exponents with assignment using the **= operator.

__ilshift__(self, other)

Implements left bitwise shift with assignment using the <<= operator.

__irshift__(self, other)

Implements right bitwise shift with assignment using the >>= operator.

__iand__(self, other)

Implements bitwise and with assignment using the &= operator.

__ior__(self, other)

Implements bitwise or with assignment using the |= operator.

__ixor__(self, other)

Implements bitwise xor with assignment using the ^= operator.

Type conversion magic methods

Python also has an array of magic methods designed to implement behavior for built in type conversion functions like float(). Here they are:

__int__(self)

Implements type conversion to int.

__long__(self)

Implements type conversion to long.

__float__(self)

Implements type conversion to float.

__complex__(self)

Implements type conversion to complex.

__oct__(self)

Implements type conversion to octal.

__hex__(self)

Implements type conversion to hexadecimal.

__index__(self)

Implements type conversion to an int when the object is used in a slice expression. If you define a custom numeric type that might be used in slicing, you should define __index__.

__trunc__(self)

Called when math.trunc(self) is called. __trunc__ should return the value of `self truncated to an integral type (usually a long).

__coerce__(self, other)

Method to implement mixed mode arithmetic. __coerce__ should return None if type conversion is impossible. Otherwise, it should return a pair (2-tuple) of self and other, manipulated to have the same type.

Representing your Classes

It's often useful to have a string representation of a class. In Python, there are a few methods that you can implement in your class definition to customize how built in functions that return representations of your class behave.

__str__(self)

Defines behavior for when str() is called on an instance of your class.

__repr__(self)

Defines behavior for when repr() is called on an instance of your class. The major difference between str() and repr() is intended audience. repr() is intended to produce output that is mostly machine-readable (in many cases, it could be valid Python code even), whereas str() is intended to be human-readable.

__unicode__(self)

Defines behavior for when unicode() is called on an instance of your class. unicode() is like str(), but it returns a unicode string. Be wary: if a client calls str() on an instance of your class and you've only defined __unicode__(), it won't work. You should always try to define __str__() as well in case someone doesn't have the luxury of using unicode.

__format__(self, formatstr)

Defines behavior for when an instance of your class is used in new-style string formatting. For instance, "Hello, {0:abc}!".format(a) would lead to the call a.__format__("abc"). This can be useful for defining your own numerical or string types that you might like to give special formatting options.

__hash__(self)

Defines behavior for when hash() is called on an instance of your class. It has to return an integer, and its result is used for quick key comparison in dictionaries. Note that this usually entails implementing __eq__ as well. Live by the following rule: a == b implies hash(a) == hash(b).

__nonzero__(self)

Defines behavior for when bool() is called on an instance of your class. Should return True or False, depending on whether you would want to consider the instance to be True or False.

__dir__(self)

Defines behavior for when dir() is called on an instance of your class. This method should return a list of attributes for the user. Typically, implementing __dir__ is unnecessary, but it can be vitally important for interactive use of your classes if you redefine __getattr__ or __getattribute__ (which you will see in the next section) or are otherwise dynamically generating attributes.

__sizeof__(self)

Defines behavior for when sys.getsizeof() is called on an instance of your class. This should return the size of your object, in bytes. This is generally more useful for Python classes implemented in C extensions, but it helps to be aware of it.

We're pretty much done with the boring (and example-free) part of the magic methods guide. Now that we've covered some of the more basic magic methods, it's time to move to more advanced material.

Controlling Attribute Access

Many people coming to Python from other languages complain that it lacks true encapsulation for classes; that is, there's no way to define private attributes with public getter and setters. This couldn't be farther than the truth: it just happens that Python accomplishes a great deal of encapsulation through "magic", instead of explicit modifiers for methods or fields. Take a look:

__getattr__(self, name)

You can define behavior for when a user attempts to access an attribute that doesn't exist (either at all or yet). This can be useful for catching and redirecting common misspellings, giving warnings about using deprecated attributes (you can still choose to compute and return that attribute, if you wish), or deftly handing an AttributeError. It only gets called when a nonexistent attribute is accessed, however, so it isn't a true encapsulation solution.

__setattr__(self, name, value)

Unlike __getattr__, __setattr__ is an encapsulation solution. It allows you to define behavior for assignment to an attribute regardless of whether or not that attribute exists, meaning you can define custom rules for any changes in the values of attributes. However, you have to be careful with how you use __setattr__, as the example at the end of the list will show.

__delattr__(self, name)

This is the exact same as __setattr__, but for deleting attributes instead of setting them. The same precautions need to be taken as with __setattr__ as well in order to prevent infinite recursion (calling del self.name in the implementation of __delattr__ would cause infinite recursion).

__getattribute__(self, name)

After all this, __getattribute__ fits in pretty well with its companions __setattr__ and __delattr__. However, I don't recommend you use it. __getattribute__ can only be used with new-style classes (all classes are new-style in the newest versions of Python, and in older versions you can make a class new-style by subclassing object. It allows you to define rules for whenever an attribute's value is accessed. It suffers from some similar infinite recursion problems as its partners-in-crime (this time you call the base class's __getattribute__ method to prevent this). It also mainly obviates the need for __getattr__, which, when __getattribute__ is implemented, only gets called if it is called explicitly or an AttributeError is raised. This method can be used (after all, it's your choice), but I don't recommend it because it has a small use case (it's far more rare that we need special behavior to retrieve a value than to assign to it) and because it can be really difficult to implement bug-free.

You can easily cause a problem in your definitions of any of the methods controlling attribute access. Consider this example:

def __setattr__(self, name, value):
    self.name = value
    # since every time an attribute is assigned, __setattr__() is called, this
    # is recursion.
    # so this really means self.__setattr__('name', value). Since the method
    # keeps calling itself, the recursion goes on forever causing a crash

def __setattr__(self, name, value):
    self.__dict__[name] = value # assigning to the dict of names in the class
    # define custom behavior here

Again, Python's magic methods are incredibly powerful, and with great power comes great responsibility. It's important to know the proper way to use magic methods so you don't break any code.

So, what have we learned about custom attribute access in Python? It's not to be used lightly. In fact, it tends to be excessively powerful and counter-intuitive. But the reason why it exists is to scratch a certain itch: Python doesn't seek to make bad things impossible, but just to make them difficult. Freedom is paramount, so you can really do whatever you want. Here's an example of some of the special attribute access methods in action (note that we use super because not all classes have an attribute __dict__):

class AccessCounter(object):
    '''A class that contains a value and implements an access counter.
    The counter increments each time the value is changed.'''

    def __init__(self, val):
        super(AccessCounter, self).__setattr__('counter', 0)
        super(AccessCounter, self).__setattr__('value', val)

    def __setattr__(self, name, value):
        if name == 'value':
            super(AccessCounter, self).__setattr__('counter', self.counter + 1)
        # Make this unconditional.
        # If you want to prevent other attributes to be set, raise AttributeError(name)
        super(AccessCounter, self).__setattr__(name, value)

    def __delattr__(self, name):
        if name == 'value':
            super(AccessCounter, self).__setattr__('counter', self.counter + 1)
        super(AccessCounter, self).__delattr__(name)

Making Custom Sequences

There's a number of ways to get your Python classes to act like built in sequences (dict, tuple, list, str, etc.). These are by far my favorite magic methods in Python because of the absurd degree of control they give you and the way that they magically make a whole array of global functions work beautifully on instances of your class. But before we get down to the good stuff, a quick word on requirements.

Requirements

Now that we're talking about creating your own sequences in Python, it's time to talk about protocols. Protocols are somewhat similar to interfaces in other languages in that they give you a set of methods you must define. However, in Python protocols are totally informal and require no explicit declarations to implement. Rather, they're more like guidelines.

Why are we talking about protocols now? Because implementing custom container types in Python involves using some of these protocols. First, there's the protocol for defining immutable containers: to make an immutable container, you need only define __len__ and __getitem__ (more on these later). The mutable container protocol requires everything that immutable containers require plus __setitem__ and __delitem__. Lastly, if you want your object to be iterable, you'll have to define __iter__, which returns an iterator. That iterator must conform to an iterator protocol, which requires iterators to have methods called __iter__(returning itself) and next.

The magic behind containers

Without any more wait, here are the magic methods that containers use:

__len__(self)

Returns the length of the container. Part of the protocol for both immutable and mutable containers.

__getitem__(self, key)

Defines behavior for when an item is accessed, using the notation self[key]. This is also part of both the mutable and immutable container protocols. It should also raise appropriate exceptions: TypeError if the type of the key is wrong and KeyError if there is no corresponding value for the key.

__setitem__(self, key, value)

Defines behavior for when an item is assigned to, using the notation self[nkey] = value. This is part of the mutable container protocol. Again, you should raise KeyError and TypeError where appropriate.

__delitem__(self, key)

Defines behavior for when an item is deleted (e.g. del self[key]). This is only part of the mutable container protocol. You must raise the appropriate exceptions when an invalid key is used.

__iter__(self)

Should return an iterator for the container. Iterators are returned in a number of contexts, most notably by the iter() built in function and when a container is looped over using the form for x in container:. Iterators are their own objects, and they also must define an __iter__ method that returns self.

__reversed__(self)

Called to implement behavior for the reversed() built in function. Should return a reversed version of the sequence. Implement this only if the sequence class is ordered, like list or tuple.

__contains__(self, item)

__contains__ defines behavior for membership tests using in and not in. Why isn't this part of a sequence protocol, you ask? Because when __contains__ isn't defined, Python just iterates over the sequence and returns True if it comes across the item it's looking for.

__missing__(self, key)

__missing__ is used in subclasses of dict. It defines behavior for whenever a key is accessed that does not exist in a dictionary (so, for instance, if I had a dictionary d and said d["george"] when "george" is not a key in the dict, d.__missing__("george") would be called).

An example

For our example, let's look at a list that implements some functional constructs that you might be used to from other languages (Haskell, for example).

class FunctionalList:
    '''A class wrapping a list with some extra functional magic, like head,
    tail, init, last, drop, and take.'''

    def __init__(self, values=None):
        if values is None:
            self.values = []
        else:
            self.values = values

    def __len__(self):
        return len(self.values)

    def __getitem__(self, key):
        # if key is of invalid type or value, the list values will raise the error
        return self.values[key]

    def __setitem__(self, key, value):
        self.values[key] = value

    def __delitem__(self, key):
        del self.values[key]

    def __iter__(self):
        return iter(self.values)

    def __reversed__(self):
        return reversed(self.values)

    def append(self, value):
        self.values.append(value)
    def head(self):
        # get the first element
        return self.values[0]
    def tail(self):
        # get all elements after the first
        return self.values[1:]
    def init(self):
        # get elements up to the last
        return self.values[:-1]
    def last(self):
        # get last element
        return self.values[-1]
    def drop(self, n):
        # get all elements except first n
        return self.values[n:]
    def take(self, n):
        # get first n elements
        return self.values[:n]

There you have it, a (marginally) useful example of how to implement your own sequence. Of course, there are more useful applications of custom sequences, but quite a few of them are already implemented in the standard library (batteries included, right?), like Counter, OrderedDict, and NamedTuple.

Reflection

You can also control how reflection using the built in functions isinstance() and issubclass()behaves by defining magic methods. The magic methods are:

__instancecheck__(self, instance)

Checks if an instance is an instance of the class you defined (e.g. isinstance(instance, class).

__subclasscheck__(self, subclass)

Checks if a class subclasses the class you defined (e.g. issubclass(subclass, class)).

The use case for these magic methods might seem small, and that may very well be true. I won't spend too much more time on reflection magic methods because they aren't very important, but they reflect something important about object-oriented programming in Python and Python in general: there is almost always an easy way to do something, even if it's rarely necessary. These magic methods might not seem useful, but if you ever need them you'll be glad that they're there (and that you read this guide!).

Callable Objects

As you may already know, in Python, functions are first-class objects. This means that they can be passed to functions and methods just as if they were objects of any other kind. This is an incredibly powerful feature.

A special magic method in Python allows instances of your classes to behave as if they were functions, so that you can "call" them, pass them to functions that take functions as arguments, and so on. This is another powerful convenience feature that makes programming in Python that much sweeter.

__call__(self, [args...])

Allows an instance of a class to be called as a function. Essentially, this means that x() is the same as x.__call__(). Note that __call__ takes a variable number of arguments; this means that you define __call__ as you would any other function, taking however many arguments you'd like it to.

__call__ can be particularly useful in classes with instances that need to often change state. "Calling" the instance can be an intuitive and elegant way to change the object's state. An example might be a class representing an entity's position on a plane:

class Entity:
    '''Class to represent an entity. Callable to update the entity's position.'''

    def __init__(self, size, x, y):
        self.x, self.y = x, y
        self.size = size

    def __call__(self, x, y):
        '''Change the position of the entity.'''
        self.x, self.y = x, y

    # snip...

Context Managers

In Python 2.5, a new keyword was introduced in Python along with a new method for code reuse: the with statement. The concept of context managers was hardly new in Python (it was implemented before as a part of the library), but not until PEP 343 was accepted did it achieve status as a first-class language construct. You may have seen with statements before:

with open('foo.txt') as bar:
    # perform some action with bar

Context managers allow setup and cleanup actions to be taken for objects when their creation is wrapped with a with statement. The behavior of the context manager is determined by two magic methods:

__enter__(self)

Defines what the context manager should do at the beginning of the block created by the with statement. Note that the return value of __enter__ is bound to the target of the with statement, or the name after the as.

__exit__(self, exception_type, exception_value, traceback)

Defines what the context manager should do after its block has been executed (or terminates). It can be used to handle exceptions, perform cleanup, or do something always done immediately after the action in the block. If the block executes successfully, exception_type, exception_value, and traceback will be None. Otherwise, you can choose to handle the exception or let the user handle it; if you want to handle it, make sure __exit__ returns True after all is said and done. If you don't want the exception to be handled by the context manager, just let it happen.

__enter__ and __exit__ can be useful for specific classes that have well-defined and common behavior for setup and cleanup. You can also use these methods to create generic context managers that wrap other objects. Here's an example:

class Closer:
    '''A context manager to automatically close an object with a close method
    in a with statement.'''

    def __init__(self, obj):
        self.obj = obj

    def __enter__(self):
        return self.obj # bound to target

    def __exit__(self, exception_type, exception_val, trace):
        try:
           self.obj.close()
        except AttributeError: # obj isn't closable
           print 'Not closable.'
           return True # exception handled successfully

Here's an example of Closer in action, using an FTP connection to demonstrate it (a closable socket):

>>> from magicmethods import Closer
>>> from ftplib import FTP
>>> with Closer(FTP('ftp.somesite.com')) as conn:
...     conn.dir()
...
# output omitted for brevity
>>> conn.dir()
# long AttributeError message, can't use a connection that's closed
>>> with Closer(int(5)) as i:
...     i += 1
...
Not closable.
>>> i
6

See how our wrapper gracefully handled both proper and improper uses? That's the power of context managers and magic methods. Note that the Python standard library includes a module contextlib that contains a context manager, contextlib.closing(), that does approximately the same thing (without any handling of the case where an object does not have a close() method).

Abstract Base Classes

See http://docs.python.org/2/library/abc.html.

Building Descriptor Objects

Descriptors are classes which, when accessed through either getting, setting, or deleting, can also alter other objects. Descriptors aren't meant to stand alone; rather, they're meant to be held by an owner class. Descriptors can be useful when building object-oriented databases or classes that have attributes whose values are dependent on each other. Descriptors are particularly useful when representing attributes in several different units of measurement or representing computed attributes (like distance from the origin in a class to represent a point on a grid).

To be a descriptor, a class must have at least one of __get__, __set__, and __delete__ implemented. Let's take a look at those magic methods:

__get__(self, instance, owner)

Define behavior for when the descriptor's value is retrieved. instance is the instance of the owner object. owner is the owner class itself.

__set__(self, instance, value)

Define behavior for when the descriptor's value is changed. instance is the instance of the owner class and value is the value to set the descriptor to.

__delete__(self, instance)

Define behavior for when the descriptor's value is deleted. instance is the instance of the owner object.

Now, an example of a useful application of descriptors: unit conversions.

class Meter(object):
    '''Descriptor for a meter.'''

    def __init__(self, value=0.0):
        self.value = float(value)
    def __get__(self, instance, owner):
        return self.value
    def __set__(self, instance, value):
        self.value = float(value)

class Foot(object):
    '''Descriptor for a foot.'''

    def __get__(self, instance, owner):
        return instance.meter * 3.2808
    def __set__(self, instance, value):
        instance.meter = float(value) / 3.2808

class Distance(object):
    '''Class to represent distance holding two descriptors for feet and
    meters.'''
    meter = Meter()
    foot = Foot()

Copying

Sometimes, particularly when dealing with mutable objects, you want to be able to copy an object and make changes without affecting what you copied from. This is where Python's copy comes into play. However (fortunately), Python modules are not sentient, so we don't have to worry about a Linux-based robot uprising, but we do have to tell Python how to efficiently copy things.

__copy__(self)

Defines behavior for copy.copy() for instances of your class. copy.copy() returns a shallow copy of your object -- this means that, while the instance itself is a new instance, all of its data is referenced -- i.e., the object itself is copied, but its data is still referenced (and hence changes to data in a shallow copy may cause changes in the original).

__deepcopy__(self, memodict={})

Defines behavior for copy.deepcopy() for instances of your class. copy.deepcopy() returns a deep copy of your object -- the object and its data are both copied. memodict is a cache of previously copied objects -- this optimizes copying and prevents infinite recursion when copying recursive data structures. When you want to deep copy an individual attribute, call copy.deepcopy() on that attribute with memodict as the first argument.

What are some use cases for these magic methods? As always, in any case where you need more fine-grained control than what the default behavior gives you. For instance, if you are attempting to copy an object that stores a cache as a dictionary (which might be large), it might not make sense to copy the cache as well -- if the cache can be shared in memory between instances, then it should be.

Pickling Your Objects

If you spend time with other Pythonistas, chances are you've at least heard of pickling. Pickling is a serialization process for Python data structures, and can be incredibly useful when you need to store an object and retrieve it later (usually for caching). It's also a major source of worries and confusion.

Pickling is so important that it doesn't just have its own module (pickle), but its own protocol and the magic methods to go with it. But first, a brief word on how to pickle existing types(feel free to skip it if you already know).

Pickling: A Quick Soak in the Brine

Let's dive into pickling. Say you have a dictionary that you want to store and retrieve later. You couldwrite it's contents to a file, carefully making sure that you write correct syntax, then retrieve it using either exec() or processing the file input. But this is precarious at best: if you store important data in plain text, it could be corrupted or changed in any number of ways to make your program crash or worse run malicious code on your computer. Instead, we're going to pickle it:

import pickle

data = {'foo': [1, 2, 3],
        'bar': ('Hello', 'world!'),
        'baz': True}
jar = open('data.pkl', 'wb')
pickle.dump(data, jar) # write the pickled data to the file jar
jar.close()

Now, a few hours later, we want it back. All we have to do is unpickle it:

import pickle

pkl_file = open('data.pkl', 'rb') # connect to the pickled data
data = pickle.load(pkl_file) # load it into a variable
print data
pkl_file.close()

What happens? Exactly what you expect. It's just like we had data all along.

Now, for a word of caution: pickling is not perfect. Pickle files are easily corrupted on accident and on purpose. Pickling may be more secure than using flat text files, but it still can be used to run malicious code. It's also incompatible across different versions of Python, so don't expect to distribute pickled objects and expect people to be able to open them. However, it can also be a powerful tool for caching and other common serialization tasks.

Pickling your own Objects

Pickling isn't just for built-in types. It's for any class that follows the pickle protocol. The pickle protocol has four optional methods for Python objects to customize how they act (it's a bit different for C extensions, but that's not in our scope):

__getinitargs__(self)

If you'd like for __init__ to be called when your class is unpickled, you can define __getinitargs__, which should return a tuple of the arguments that you'd like to be passed to __init__. Note that this method will only work for old-style classes.

__getnewargs__(self)

For new-style classes, you can influence what arguments get passed to __new__ upon unpickling. This method should also return a tuple of arguments that will then be passed to __new__.

__getstate__(self)

Instead of the object's __dict__ attribute being stored, you can return a custom state to be stored when the object is pickled. That state will be used by __setstate__ when the object is unpickled.

__setstate__(self, state)

When the object is unpickled, if __setstate__ is defined the object's state will be passed to it instead of directly applied to the object's __dict__. This goes hand in hand with __getstate__: when both are defined, you can represent the object's pickled state however you want with whatever you want.

__reduce__(self)

When defining extension types (i.e., types implemented using Python's C API), you have to tell Python how to pickle them if you want them to pickle them. __reduce__() is called when an object defining it is pickled. It can either return a string representing a global name that Python will look up and pickle, or a tuple. The tuple contains between 2 and 5 elements: a callable object that is called to recreate the object, a tuple of arguments for that callable object, state to be passed to __setstate__ (optional), an iterator yielding list items to be pickled (optional), and an iterator yielding dictionary items to be pickled (optional).

__reduce_ex__(self)

__reduce_ex__ exists for compatibility. If it is defined, __reduce_ex__ will be called over __reduce__ on pickling. __reduce__ can be defined as well for older versions of the pickling API that did not support __reduce_ex__.

An Example

Our example is a Slate, which remembers what its values have been and when those values were written to it. However, this particular slate goes blank each time it is pickled: the current value will not be saved.

import time

class Slate:
    '''Class to store a string and a changelog, and forget its value when
    pickled.'''

    def __init__(self, value):
        self.value = value
        self.last_change = time.asctime()
        self.history = {}

    def change(self, new_value):
        # Change the value. Commit last value to history
        self.history[self.last_change] = self.value
        self.value = new_value
        self.last_change = time.asctime()

    def print_changes(self):
        print 'Changelog for Slate object:'
        for k, v in self.history.items():
            print '%s	 %s' % (k, v)

    def __getstate__(self):
        # Deliberately do not return self.value or self.last_change.
        # We want to have a "blank slate" when we unpickle.
        return self.history

    def __setstate__(self, state):
        # Make self.history = state and last_change and value undefined
        self.history = state
        self.value, self.last_change = None, None

Conclusion

The goal of this guide is to bring something to anyone that reads it, regardless of their experience with Python or object-oriented programming. If you're just getting started with Python, you've gained valuable knowledge of the basics of writing feature-rich, elegant, and easy-to-use classes. If you're an intermediate Python programmer, you've probably picked up some slick new concepts and strategies and some good ways to reduce the amount of code written by you and clients. If you're an expert Pythonista, you've been refreshed on some of the stuff you might have forgotten about and maybe picked up a few new tricks along the way. Whatever your experience level, I hope that this trip through Python's special methods has been truly magical. (I couldn't resist the final pun!)

Appendix 1: How to Call Magic Methods

Some of the magic methods in Python directly map to built-in functions; in this case, how to invoke them is fairly obvious. However, in other cases, the invocation is far less obvious. This appendix is devoted to exposing non-obvious syntax that leads to magic methods getting called.

Magic Method	When it gets invoked (example)	Explanation
__new__(cls [,...])	instance = MyClass(arg1, arg2)	__new__ is called on instance creation
__init__(self [,...])	instance = MyClass(arg1, arg2)	__init__ is called on instance creation
__cmp__(self, other)	self == other, self > other, etc.	Called for any comparison
__pos__(self)	+self	Unary plus sign
__neg__(self)	-self	Unary minus sign
__invert__(self)	~self	Bitwise inversion
__index__(self)	x[self]	Conversion when object is used as index
__nonzero__(self)	bool(self)	Boolean value of the object
__getattr__(self, name)	self.name # name doesn't exist	Accessing nonexistent attribute
__setattr__(self, name, val)	self.name = val	Assigning to an attribute
__delattr__(self, name)	del self.name	Deleting an attribute
__getattribute__(self, name)	self.name	Accessing any attribute
__getitem__(self, key)	self[key]	Accessing an item using an index
__setitem__(self, key, val)	self[key] = val	Assigning to an item using an index
__delitem__(self, key)	del self[key]	Deleting an item using an index
__iter__(self)	for x in self	Iteration
__contains__(self, value)	value in self, value not in self	Membership tests using in
__call__(self [,...])	self(args)	"Calling" an instance
__enter__(self)	with self as x:	with statement context managers
__exit__(self, exc, val, trace)	with self as x:	with statement context managers
__getstate__(self)	pickle.dump(pkl_file, self)	Pickling
__setstate__(self)	data = pickle.load(pkl_file)	Pickling

Hopefully, this table should have cleared up any questions you might have had about what syntax invokes which magic method.

Appendix 2: Changes in Python 3

Here, we document a few major places where Python 3 differs from 2.x in terms of its object model:

• Since the distinction between string and unicode has been done away with in Python 3, __unicode__ is gone and __bytes__ (which behaves similarly to __str__ and __unicode__ in 2.7) exists for a new built-in for constructing byte arrays.
• Since division defaults to true division in Python 3, __div__ is gone in Python 3
• __coerce__ is gone due to redundancy with other magic methods and confusing behavior
• __cmp__ is gone due to redundancy with other magic methods
• __nonzero__ has been renamed to __bool__

二，

Introduction

The so-called magic methods have nothing to do with wizardry. You have already seen them in previous chapters of our tutorial. They are special methods with fixed names. They are the methods with this clumsy syntax, i.e. the double underscores at the beginning and the end. They are also hard to talk about. How do you pronounce or say a method name like __init__? "Underscore underscore init underscore underscore" sounds horrible and is nearly a tongue twister. "Double underscore init double underscore" is a lot better, but the ideal way is "dunder init dunder"¹ That's why magic methods methods are sometimes called dunder methods!

So what's magic about the __init__ method? The answer is, you don't have to invoke it directly. The invocation is realized behind the scenes. When you create an instance x of a class A with the statement "x = A()", Python will do the necessary calls to __new__ and __init__.

Nearly at the end of this chapter of our tutorial we will introduce the __call__ method. It is overlooked by many beginners and even advanced programmers of Python. It is a functionality which many programming languages do not have, so programmers are generally not looking for. The __call__ method enables Python programmers to write classes where the instances behave like functions. Both functions and the instances of such classes are called callables.

We have encountered the concept of operator overloading many times in the course of this tutorial. We had used the plus sign to add numerical values, to concatenate strings or to combine lists:

>>> 4 + 5
9
>>> 3.8 + 9
12.8
>>> "Peter" + " " + "Pan"
'Peter Pan'
>>> [3,6,8] + [7,11,13]
[3, 6, 8, 7, 11, 13]
>>> 

It's even possible to overload the "+" operator as well as all the other operators for the purposes of your own class. To do this, you need to understand the underlying mechanism. There is a special (or a "magic") method for every operator sign. The magic method for the "+" sign is the __add__ method. For "-" it is "__sub__" and so on. We have a complete listing of all the magic methods a little bit further down.

The mechanism works like this: If we have an expression "x + y" and x is an instance of class K, then Python will check the class definition of K. If K has a method __add__ it will be called with x.__add__(y), otherwise we will get an error message.

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'K' and 'K'

Overview of Magic Methods

Binary Operators

Operator	Method
+	object.__add__(self, other)
-	object.__sub__(self, other)
*	object.__mul__(self, other)
//	object.__floordiv__(self, other)
/	object.__truediv__(self, other)
%	object.__mod__(self, other)
**	object.__pow__(self, other[, modulo])
<<	object.__lshift__(self, other)
>>	object.__rshift__(self, other)
&	object.__and__(self, other)
^	object.__xor__(self, other)
|	object.__or__(self, other)

Extended Assignments

Operator	Method
+=	object.__iadd__(self, other)
-=	object.__isub__(self, other)
*=	object.__imul__(self, other)
/=	object.__idiv__(self, other)
//=	object.__ifloordiv__(self, other)
%=	object.__imod__(self, other)
**=	object.__ipow__(self, other[, modulo])
<<=	object.__ilshift__(self, other)
>>=	object.__irshift__(self, other)
&=	object.__iand__(self, other)
^=	object.__ixor__(self, other)
|=	object.__ior__(self, other)

Unary Operators

Operator	Method
-	object.__neg__(self)
+	object.__pos__(self)
abs()	object.__abs__(self)
~	object.__invert__(self)
complex()	object.__complex__(self)
int()	object.__int__(self)
long()	object.__long__(self)
float()	object.__float__(self)
oct()	object.__oct__(self)
hex()	object.__hex__(self

Comparison Operators

Operator	Method
<	object.__lt__(self, other)
<=	object.__le__(self, other)
==	object.__eq__(self, other)
!=	object.__ne__(self, other)
>=	object.__ge__(self, other)
>	object.__gt__(self, other)

Example class: Length

We will demonstrate in the following Length class, how you can overload the "+" operator for your own class. To do this, we have to overload the __add__ method. Our class contains the __str__ and __repr__ methods as well. The instances of the class Length contain length or distance information. The attributes of an instance are self.value and self.unit.

This class allows us to calculate expressions with mixed units like this one:

2.56 m + 3 yd + 7.8 in + 7.03 cm

The class can be used like this:

>>> from unit_conversions import Length
>>> L = Length
>>> print(L(2.56,"m") + L(3,"yd") + L(7.8,"in") + L(7.03,"cm"))
5.57162
>>> 

The listing of the class:

class Length:

    __metric = {"mm" : 0.001, "cm" : 0.01, "m" : 1, "km" : 1000,
                "in" : 0.0254, "ft" : 0.3048, "yd" : 0.9144,
                "mi" : 1609.344 }
    
    def __init__(self, value, unit = "m" ):
        self.value = value
        self.unit = unit
    
    def Converse2Metres(self):
        return self.value * Length.__metric[self.unit]
    
    def __add__(self, other):
        l = self.Converse2Metres() + other.Converse2Metres()
        return Length(l / Length.__metric[self.unit], self.unit )
    
    def __str__(self):
        return str(self.Converse2Metres())
    
    def __repr__(self):
        return "Length(" + str(self.value) + ", '" + self.unit + "')"

if __name__ == "__main__":
    x = Length(4)
    print(x)
    y = eval(repr(x))

    z = Length(4.5, "yd") + Length(1)
    print(repr(z))
    print(z)

If we start this program, we get the following output:

4
Length(5.593613298337708, 'yd')
5.1148

We use the method__iadd__ to implement the extended assignment:

    def __iadd__(self, other):
        l = self.Converse2Metres() + other.Converse2Metres()
        self.value = l / Length.__metric[self.unit]
        return self

Now we are capable to write the following assignments:

    x += Length(1)
    x += Length(4, "yd")

We have added 1 metre in the example above by writing "x += Length(1))". Most certainly, you will agree with us that it would be more convenient to simply write "x += 1" instead. We also want to treat expressions like "Length(5,"yd") + 4.8" similarly. So, if somebody uses a type int or float, our class takes it automatically for "metre" and converts it into a Length object. It's easy to adapt our __add__ and "__iadd__" method for this task. All we have to do is to check the type of the parameter "other":

    def __add__(self, other):
        if type(other) == int or type(other) == float:
            l = self.Converse2Metres() + other
        else:
            l = self.Converse2Metres() + other.Converse2Metres()
        return Length(l / Length.__metric[self.unit], self.unit )

    def __iadd__(self, other):
        if type(other) == int or type(other) == float:
            l = self.Converse2Metres() + other
        else:
            l = self.Converse2Metres() + other.Converse2Metres()
        self.value = l / Length.__metric[self.unit]
        return self

It's a safe bet that if somebody works for a while with adding integers and floats from the right sight that he or she wants to the same from the left side! So let's try it out:

>>> from unit_conversions import Length
>>> x = Length(3, "yd") + 5
>>> x = 5 + Length(3, "yd")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'int' and 'Length'
>>> 

Of course, the left side has to be of type "Length", because otherwise Python tries to apply the __add__ method from int, which can't cope with Length objects as second arguments!

Python provides a solution for this problem as well. It's the __radd__ method. It works like this: Python tries to evaluate the expression "5 + Length(3, 'yd')". First it calls int.__add__(5,Length(3, 'yd')), which will raise an exception. After this it will try to invoke Length.__radd__(Length(3, "yd"), 5). It's easy to recognize that the implementation of __radd__ is analogue to __add__:

    def __radd__(self, other):
        if type(other) == int or type(other) == float:
            l = self.Converse2Metres() + otherLength.__radd__(Length(3, "yd"), 5)
        else:
            l = self.Converse2Metres() + other.Converse2Metres()
        return Length(l / Length.__metric[self.unit], self.unit )

It's advisable to make use of the __add__ method in the __radd__ method:

    def __radd__(self, other):
        return Length.__add__(self,other)  

The following diagram illustrates the relationship between __add__ and __radd__:

The __call__ method

The __call__ method can be used to turn the instances of the class into callables. Functions are callable objects. A callable object is an object which can be used and behaves like a function but might not be a function. By using the __call__ method it is possible to define classes in a way that the instances will be callable objects. The __call__ method is called, if the instance is called "like a function", i.e. using brackets. The following example defines a class with which we can create abitrary polynomial functions:

class Polynomial:
    
    def __init__(self, *coefficients):
        self.coefficients = coefficients[::-1]
        
    def __call__(self, x):
        res = 0
        for index, coeff in enumerate(self.coefficients):
            res += coeff * x** index
        return res

# a constant function
p1 = Polynomial(42)

# a straight Line
p2 = Polynomial(0.75, 2)

# a third degree Polynomial
p3 = Polynomial(1, -0.5, 0.75, 2)

for i in range(1, 10):
    print(i, p1(i), p2(i), p3(i))

These are the results of the previous function:

1 42 2.75 3.25
2 42 3.5 9.5
3 42 4.25 26.75
4 42 5.0 61.0
5 42 5.75 118.25
6 42 6.5 204.5
7 42 7.25 325.75
8 42 8.0 488.0
9 42 8.75 697.25

You will find further interesting examples of the __call__ function in our tutorial in the chapters Decorators and Memoization with Decorators. You may also consult our chapter on Polynomials

Standard Classes as Base Classes

It's possible to use standard classes - like int, float, dict or lists - as base classes as well.

We extend the list class by adding a push method:

class Plist(list):

    def __init__(self, l):
        list.__init__(self, l)

    def push(self, item):
        self.append(item)


if __name__ == "__main__":
    x = Plist([3,4])
    x.push(47)
    print(x)

This means that all the previously introduced binary and extended assignment operators exist in the "reversed" version as well:

__radd__
__rsub__
__rmul__
...
and so on

Exercises

1. Write a class with the name Ccy, similar to the previously defined Length class.
   
   Ccy should contain values in various currencies, e.g. "EUR", "GBP" or "USD". An instance should contain the amount and the currency unit.
   
   The class, you are going to design as an excercise, might be best described with the following example session:
   
   

   >>> from currencies import Ccy
   >>> v1 = Ccy(23.43, "EUR")
   >>> v2 = Ccy(19.97, "USD")
   >>> print(v1 + v2)
   32.89 EUR
   >>> print(v2 + v1)
   31.07 USD
   >>> print(v1 + 3) # an int or a float is considered to be a EUR value
   27.43 EUR
   >>> print(3 + v1)
   27.43 EUR
   >>> 
   

Solutions to our Exercises

1. First exercise:

   """
   
   The class "Ccy" can be used to define money values in various currencies. A Ccy instance has the string attributes 'unit' (e.g. 'CHF', 'CAD' od 'EUR' and the 'value' as a float. 
   A currency object consists of a value and the corresponding unit.
   
   
   
   """
   
       
   
   class Ccy:
   
       currencies =  {'CHF': 1.0821202355817312, 
                      'CAD': 1.488609845538393, 
                      'GBP': 0.8916546282920325, 
                      'JPY': 114.38826536281809, 
                      'EUR': 1.0, 
                      'USD': 1.11123458162018}
       
       def __init__(self, value, unit="EUR"):
           self.value = value
           self.unit = unit
   
       def __str__(self):
           return "{0:5.2f}".format(self.value) + " " + self.unit
   
       def changeTo(self, new_unit):
           """
           An Ccy object is transformed from the unit "self.unit" to "new_unit"
           """
           self.value = (self.value / Ccy.currencies[self.unit] * Ccy.currencies[new_unit])
           self.unit = new_unit
           
       def __add__(self, other):
           """
           Defines the '+' operator.
           If other is a CCy object the currency values 
           are added and the result will be the unit of 
           self. If other is an int or a float, other will
           be treated as a Euro value. 
           """
           if type(other) == int or type(other) == float:
               x = (other * Ccy.currencies[self.unit])
           else:
               x = (other.value / Ccy.currencies[other.unit] * Ccy.currencies[self.unit]) 
           return Ccy(x + self.value, self.unit)
   
   
       def __iadd__(self, other):
           """
           Similar to __add__
           """
           if type(other) == int or type(other) == float:
               x = (other * Ccy.currencies[self.unit])
           else:
               x = (other.value / Ccy.currencies[other.unit] * Ccy.currencies[self.unit])
           self.value += x
           return self
   
       def __radd__(self, other):
           res = self + other
           if self.unit != "EUR":
               res.changeTo("EUR")
           return res
   
       # __sub__, __isub__ and __rsub__ can be defined analogue
   
   x = Ccy(10,"USD")
   y = Ccy(11)
   z = Ccy(12.34, "JPY")
   z = 7.8 + x + y + 255 + z
   print(z)
   
   lst = [Ccy(10,"USD"), Ccy(11), Ccy(12.34, "JPY"), Ccy(12.34, "CAD")]
   
   z = sum(lst)
   
   print(z)
   
   

   The program returns:

   282.91 EUR
   28.40 EUR
   

   Another interesting aspect of this currency converter class in Python can be shown, if we add multiplication. You will easily understand that it makes no sense to allow expressions like "12.4 € * 3.4 $" (or in praefix notation: "€ 12.4 * $ 3.4"), but it makes perfect sense to evaluate "3 * 4.54 €". You can find the new currency converter class with the newly added methods for __mul__, __imul__ and __rmul__ in the following listing:

   """
   
   The class "Ccy" can be used to define money values in various currencies. A Ccy instance has the string attributes 'unit' (e.g. 'CHF', 'CAD' od 'EUR' and the 'value' as a float. 
   A currency object consists of a value and the corresponding unit.
   
   
   
   """
   
       
   
   class Ccy:
   
       currencies =  {'CHF': 1.0821202355817312, 
                      'CAD': 1.488609845538393, 
                      'GBP': 0.8916546282920325, 
                      'JPY': 114.38826536281809, 
                      'EUR': 1.0, 
                      'USD': 1.11123458162018}
       
       def __init__(self, value, unit="EUR"):
           self.value = value
           self.unit = unit
   
       def __str__(self):
           return "{0:5.2f}".format(self.value) + " " + self.unit
   
       def __repr__(self):
           return 'Ccy(' + str(self.value) + ', "' + self.unit + '")'
   
       def changeTo(self, new_unit):
           """
           An Ccy object is transformed from the unit "self.unit" to "new_unit"
           """
           self.value = (self.value / Ccy.currencies[self.unit] * Ccy.currencies[new_unit])
           self.unit = new_unit
           
       def __add__(self, other):
           """
           Defines the '+' operator.
           If other is a CCy object the currency values 
           are added and the result will be the unit of 
           self. If other is an int or a float, other will
           be treated as a Euro value. 
           """
           if type(other) == int or type(other) == float:
               x = (other * Ccy.currencies[self.unit])
           else:
               x = (other.value / Ccy.currencies[other.unit] * Ccy.currencies[self.unit]) 
           return Ccy(x + self.value, self.unit)
   
   
       def __iadd__(self, other):
           """
           Similar to __add__
           """
           if type(other) == int or type(other) == float:
               x = (other * Ccy.currencies[self.unit])
           else:
               x = (other.value / Ccy.currencies[other.unit] * Ccy.currencies[self.unit])
           self.value += x
           return self
   
       def __radd__(self, other):
           res = self + other
           if self.unit != "EUR":
               res.changeTo("EUR")
           return res
       
       # __sub__, __isub__ and __rsub__ can be defined analogue
       
   
       def __mul__(self, other):
           """
           Multiplication is only defined as a scalar multiplication, 
           i.e. a money value can be multiplied by an int or a float.
           It is not possible to multiply to money values
           """
           if type(other)==int or type(other)==float:
               return Ccy(self.value * other, self.unit)
           else:
               raise TypeError("unsupported operand type(s) for *: 'Ccy' and " + type(other).__name__)  
           
       def __rmul__(self, other):
           return self.__mul__(other)
       
       def __imul__(self, other):
           if type(other)==int or type(other)==float:
               self.value *= other
               return self
           else:
               raise TypeError("unsupported operand type(s) for *: 'Ccy' and " + type(other).__name__)      
   
   

   Assuming that you have saved the class under the name currency_converter, you can use it in the following way in the command shell:

   >>> from currency_converter import Ccy
   >>> x = Ccy(10.00, "EUR")
   >>> y = Ccy(10.00, "GBP")
   >>> x + y
   Ccy(21.215104685942173, "EUR")
   >>> print(x + y)
   21.22 EUR
   >>> print(2*x + y*0.9)
   30.09 EUR
   >>> 
   

   We can further improve our currency converter class by using a function get_currencies, which downloads the latest exchange rates from finance.yahoo.com. This function returns an exchange rates dictionary in our previously used format. The function is in a module called exchange_rates.py This is the code of the function exchange_rates.py:

   from urllib.request import urlopen
   from bs4 import BeautifulSoup
   
   def get_currency_rates(base="USD"):
       """ The file at location url is read in and the exchange rates are extracted """
       url = "https://finance.yahoo.com/webservice/v1/symbols/allcurrencies/quote"
       data = urlopen(url).read()
       data = data.decode('utf-8')
       soup = BeautifulSoup(data, 'html.parser')
       data = soup.get_text()
   
       flag = False
       currencies = {}
       for line in data.splitlines():
           if flag:
               value = float(line)
               flag = False
               currencies[currency] = value
           if line.startswith("USD/"):
               flag = True
               currency = line[4:7]
       
       currencies["USD"] =  1 # we must add it, because it's not included in file
       if base != "USD":
           base_currency_rate = currencies[base] 
           for currency in currencies:
               currencies[currency] /= base_currency_rate
   
       return currencies
   

   We can import this function from our module. (You have to save it somewhare in your Python path or the directory where your program runs):

   from exchange_rates import get_currency_rates
   
   class Ccy:
       
       currencies =  get_currencies()
   
       #   continue with the code from the previous version
   

   We save this version as currency_converter_web.

   >>> from currency_converter_web import Ccy
   >>> x = Ccy(1000, "JPY")
   >>> y = Ccy(10, "CHF")
   >>> z = Ccy(15, "CAD")
   >>> print(2*x + 4.11*y + z)
   7722.98 JPY
   >>> 
   

三，

Python - Magic Methods

Magic methods in Python are the special methods which add "magic" to your class. Magic methods are not meant to be invoked directly by you, but the invocation happens internally from the class on a certain action. For example, when you add two numbers using the + operator, internally, the __add__() method will be called.

Built-in classes in Python define many magic methods. Use the dir() function to see the number of magic methods inherited by a class. For example, the following lists all the attributes and methods defined in the int class.

>>> dir(int)
['__abs__', '__add__', '__and__', '__bool__', '__ceil__', '__class__', '__delattr__', '__dir__', '__divmod__', '__doc__', '__eq__', '__float__', '__floor__', '__floordiv__', '__format__', '__ge__', '__getattribute__', '__getnewargs__', '__gt__', '__hash__', '__index__', '__init__', '__init_subclass__', '__int__', '__invert__', '__le__', '__lshift__', '__lt__', '__mod__', '__mul__', '__ne__', '__neg__', '__new__', '__or__', '__pos__', '__pow__', '__radd__', '__rand__', '__rdivmod__', '__reduce__', '__reduce_ex__', '__repr__', '__rfloordiv__', '__rlshift__', '__rmod__', '__rmul__', '__ror__', '__round__', '__rpow__', '__rrshift__', '__rshift__', '__rsub__', '__rtruediv__', '__rxor__', '__setattr__', '__sizeof__', '__str__', '__sub__', '__subclasshook__', '__truediv__', '__trunc__', '__xor__', 'bit_length', 'conjugate', 'denominator', 'from_bytes', 'imag', 'numerator', 'real', 'to_bytes']

As you can see above, the int class includes various magic methods surrounded by double underscores. For example, the __add__ method is a magic method which gets called when we add two numbers using the + operator. Consider the following example.

>>> num=10
>>> num + 5
15
>>> num.__add__(5)
15

As you can see, when you do num+10, the + operator calls the __add__(10) method. You can also call num.__add__(5) directly which will give the same result. However, as mentioned before, magic methods are not meant to be called directly, but internally, through some other methods or actions.

Magic methods are most frequently used to define overloaded behaviours of predefined operators in Python. For instance, arithmetic operators by default operate upon numeric operands. This means that numeric objects must be used along with operators like +, -, *, /, etc. The + operator is also defined as a concatenation operator in string, list and tuple classes. We can say that the + operator is overloaded.

In order to make the overloaded behaviour available in your own custom class, the corresponding magic method should be overridden. For example, in order to use the + operator with objects of a user-defined class, it should include the __add__() method.

Let's see how to implement and use some of the important magic methods.

__new__() method

Languages such as Java and C# use the new operator to create a new instance of a class. In Python the __new__() magic method is implicitly called before the __init__() method. The __new__() method returns a new object, which is then initialized by __init__().

Example: __new__()

class employee:
                def __new__(cls):
        print ("__new__ magic method is called")
        inst = object.__new__(cls)
                return inst
                def __init__(self):
        print ("__init__ magic method is called")
        self.name='Satya'

The above example will produce the following output when you create an instance of the Employee class.

>>> e1=employee()
__new__ magic method is called
__init__ magic method is called

Thus, the __new__() method is called before the __init__() method.

__str__() method

Another useful magic method is __str__(). It is overridden to return a printable string representation of any user defined class. We have seen str() built-in function which returns a string from the object parameter. For example, str(12) returns '12'. When invoked, it calls the __str__() method in the int class.

>>> num=12
>>> str(num)
'12'
>>> #This is equivalent to
>>> int.__str__(num)
'12'

Let us now override the __str__() method in the employee class to return a string representation of its object.

Example:

class employee:
                def __init__(self):
        self.name='Swati'
        self.salary=10000
                def __str__(self):
                return 'name='+self.name+' salary=$'+str(self.salary)

See how the str() function internally calls the __str__() method defined in the employee class. This is why it is called a magic method!

>>> e1=employee()
>>> print(e1)
name=Swati salary=$10000

__add__() method

In following example, a class named distance is defined with two instance attributes - ft and inch. The addition of these two distance objects is desired to be performed using the overloading + operator.

To achieve this, the magic method __add__() is overridden, which performs the addition of the ft and inch attributes of the two objects. The __str__() method returns the object's string representation.

Example: __add__()

class distance:
                def __init__(self, x=None,y=None):
        self.ft=x
        self.inch=y
                def __add__(self,x):
        temp=distance()
        temp.ft=self.ft+x.ft
        temp.inch=self.inch+x.inch
                if temp.inch>=12:
            temp.ft+=1
            temp.inch-=12
                return temp
                def __str__(self):
                return 'ft:'+str(self.ft)+' in: '+str(self.inch)

Run the above Python script to verify the overloaded operation of the + operator.

>>> d1=distance(3,10)
>>> d2=distance(4,4)
>>> print("d1= {} d2={}".format(d1, d2))
d1= ft:3 in: 10 d2=ft:4 in: 4
>>>d3=d1+d2
>>>print(d3)
ft:8 in: 2

__ge__() method

The following method is added in the distance class to overload the >= operator.

Example: __ge__()

class distance:
                def __init__(self, x=None,y=None):
        self.ft=x
        self.inch=y
                def __ge__(self, x):
        val1=self.ft*12+self.inch
        val2=x.ft*12+x.inch
                if val1>=val2:
                return True
                else:
                return False

This method gets invoked when the >= operator is used and returns True or False. Accordingly, the appropriate message can be displayed

>>>d1=distance(2,1)
>>>d2=distance(4,10)
>>>d1>=d2
False

Important Magic Methods

The following tables list important magic methods in Python 3.

Initialization and Construction	Description
__new__(cls, other)	To get called in an object's instantiation.
__init__(self, other)	To get called by the __new__ method.
__del__(self)	Destructor method.

Unary operators and functions	Description
__pos__(self)	To get called for unary positive e.g. +someobject.
__neg__(self)	To get called for unary negative e.g. -someobject.
__abs__(self)	To get called by built-in abs() function.
__invert__(self)	To get called for inversion using the ~ operator.
__round__(self,n)	To get called by built-in round() function.
__floor__(self)	To get called by built-in math.floor() function.
__ceil__(self)	To get called by built-in math.ceil() function.
__trunc__(self)	To get called by built-in math.trunc() function.

Augmented Assignment	Description
__iadd__(self, other)	To get called on addition with assignment e.g. a +=b.
__isub__(self, other)	To get called on subtraction with assignment e.g. a -=b.
__imul__(self, other)	To get called on multiplication with assignment e.g. a *=b.
__ifloordiv__(self, other)	To get called on integer division with assignment e.g. a //=b.
__idiv__(self, other)	To get called on division with assignment e.g. a /=b.
__itruediv__(self, other)	To get called on true division with assignment
__imod__(self, other)	To get called on modulo with assignment e.g. a%=b.
__ipow__(self, other)	To get called on exponentswith assignment e.g. a **=b.
__ilshift__(self, other)	To get called on left bitwise shift with assignment e.g. a<<=b.
__irshift__(self, other)	To get called on right bitwise shift with assignment e.g. a >>=b.
__iand__(self, other)	To get called on bitwise AND with assignment e.g. a&=b.
__ior__(self, other)	To get called on bitwise OR with assignment e.g. a|=b.
__ixor__(self, other)	To get called on bitwise XOR with assignment e.g. a ^=b.

Type Conversion Magic Methods	Description
__int__(self)	To get called by built-int int() method to convert a type to an int.
__float__(self)	To get called by built-int float() method to convert a type to float.
__complex__(self)	To get called by built-int complex() method to convert a type to complex.
__oct__(self)	To get called by built-int oct() method to convert a type to octal.
__hex__(self)	To get called by built-int hex() method to convert a type to hexadecimal.
__index__(self)	To get called on type conversion to an int when the object is used in a slice expression.
__trunc__(self)	To get called from math.trunc() method.

String Magic Methods	Description
__str__(self)	To get called by built-int str() method to return a string representation of a type.
__repr__(self)	To get called by built-int repr() method to return a machine readable representation of a type.
__unicode__(self)	To get called by built-int unicode() method to return an unicode string of a type.
__format__(self, formatstr)	To get called by built-int string.format() method to return a new style of string.
__hash__(self)	To get called by built-int hash() method to return an integer.
__nonzero__(self)	To get called by built-int bool() method to return True or False.
__dir__(self)	To get called by built-int dir() method to return a list of attributes of a class.
__sizeof__(self)	To get called by built-int sys.getsizeof() method to return the size of an object.

Attribute Magic Methods	Description
__getattr__(self, name)	Is called when the accessing attribute of a class that does not exist.
__setattr__(self, name, value)	Is called when assigning a value to the attribute of a class.
__delattr__(self, name)	Is called when deleting an attribute of a class.

Operator Magic Methods	Description
__add__(self, other)	To get called on add operation using + operator
__sub__(self, other)	To get called on subtraction operation using - operator.
__mul__(self, other)	To get called on multiplication operation using * operator.
__floordiv__(self, other)	To get called on floor division operation using // operator.
__div__(self, other)	To get called on division operation using / operator.
__mod__(self, other)	To get called on modulo operation using % operator.
__pow__(self, other[, modulo])	To get called on calculating the power using ** operator.
__lt__(self, other)	To get called on comparison using < operator.
__le__(self, other)	To get called on comparison using <= operator.
__eq__(self, other)	To get called on comparison using == operator.
__ne__(self, other)	To get called on comparison using != operator.
__ge__(self, other)	To get called on comparison using >= operator.

Thus, you can use the appropriate magic methods to add various functionalities in your custom class.

四，

魔法方法	含义
	基本的魔法方法
__new__(cls[, ...])	1. __new__ 是在一个对象实例化的时候所调用的第一个方法 2. 它的第一个参数是这个类，其他的参数是用来直接传递给 __init__ 方法 3. __new__ 决定是否要使用该 __init__ 方法，因为 __new__ 可以调用其他类的构造方法或者直接返回别的实例对象来作为本类的实例，如果 __new__ 没有返回实例对象，则 __init__ 不会被调用 4. __new__ 主要是用于继承一个不可变的类型比如一个 tuple 或者 string
__init__(self[, ...])	构造器，当一个实例被创建的时候调用的初始化方法
__del__(self)	析构器，当一个实例被销毁的时候调用的方法
__call__(self[, args...])	允许一个类的实例像函数一样被调用：x(a, b) 调用 x.__call__(a, b)
__len__(self)	定义当被 len() 调用时的行为
__repr__(self)	定义当被 repr() 调用或者直接执行对象时的行为
__str__(self)	定义当被 str() 调用或者打印对象时的行为
__bytes__(self)	定义当被 bytes() 调用时的行为
__hash__(self)	定义当被 hash() 调用时的行为
__bool__(self)	定义当被 bool() 调用时的行为，应该返回 True 或 False
__format__(self, format_spec)	定义当被 format() 调用时的行为
	有关属性
__getattr__(self, name)	定义当用户试图获取一个不存在的属性时的行为
__getattribute__(self, name)	定义当该类的属性被访问时的行为
__setattr__(self, name, value)	定义当一个属性被设置时的行为
__delattr__(self, name)	定义当一个属性被删除时的行为
__dir__(self)	定义当 dir() 被调用时的行为
__get__(self, instance, owner)	定义当描述符的值被取得时的行为
__set__(self, instance, value)	定义当描述符的值被改变时的行为
__delete__(self, instance)	定义当描述符的值被删除时的行为
	比较操作符
__lt__(self, other)	定义小于号的行为：x < y 调用 x.__lt__(y)
__le__(self, other)	定义小于等于号的行为：x <= y 调用 x.__le__(y)
__eq__(self, other)	定义等于号的行为：x == y 调用 x.__eq__(y)
__ne__(self, other)	定义不等号的行为：x != y 调用 x.__ne__(y)
__gt__(self, other)	定义大于号的行为：x > y 调用 x.__gt__(y)
__ge__(self, other)	定义大于等于号的行为：x >= y 调用 x.__ge__(y)
	算数运算符
__add__(self, other)	定义加法的行为：+
__sub__(self, other)	定义减法的行为：-
__mul__(self, other)	定义乘法的行为：*
__truediv__(self, other)	定义真除法的行为：/
__floordiv__(self, other)	定义整数除法的行为：//
__mod__(self, other)	定义取模算法的行为：%
__divmod__(self, other)	定义当被 divmod() 调用时的行为
__pow__(self, other[, modulo])	定义当被 power() 调用或 ** 运算时的行为
__lshift__(self, other)	定义按位左移位的行为：<<
__rshift__(self, other)	定义按位右移位的行为：>>
__and__(self, other)	定义按位与操作的行为：&
__xor__(self, other)	定义按位异或操作的行为：^
__or__(self, other)	定义按位或操作的行为：|
	反运算
__radd__(self, other)	（与上方相同，当左操作数不支持相应的操作时被调用）
__rsub__(self, other)	（与上方相同，当左操作数不支持相应的操作时被调用）
__rmul__(self, other)	（与上方相同，当左操作数不支持相应的操作时被调用）
__rtruediv__(self, other)	（与上方相同，当左操作数不支持相应的操作时被调用）
__rfloordiv__(self, other)	（与上方相同，当左操作数不支持相应的操作时被调用）
__rmod__(self, other)	（与上方相同，当左操作数不支持相应的操作时被调用）
__rdivmod__(self, other)	（与上方相同，当左操作数不支持相应的操作时被调用）
__rpow__(self, other)	（与上方相同，当左操作数不支持相应的操作时被调用）
__rlshift__(self, other)	（与上方相同，当左操作数不支持相应的操作时被调用）
__rrshift__(self, other)	（与上方相同，当左操作数不支持相应的操作时被调用）
__rand__(self, other)	（与上方相同，当左操作数不支持相应的操作时被调用）
__rxor__(self, other)	（与上方相同，当左操作数不支持相应的操作时被调用）
__ror__(self, other)	（与上方相同，当左操作数不支持相应的操作时被调用）
	增量赋值运算
__iadd__(self, other)	定义赋值加法的行为：+=
__isub__(self, other)	定义赋值减法的行为：-=
__imul__(self, other)	定义赋值乘法的行为：*=
__itruediv__(self, other)	定义赋值真除法的行为：/=
__ifloordiv__(self, other)	定义赋值整数除法的行为：//=
__imod__(self, other)	定义赋值取模算法的行为：%=
__ipow__(self, other[, modulo])	定义赋值幂运算的行为：**=
__ilshift__(self, other)	定义赋值按位左移位的行为：<<=
__irshift__(self, other)	定义赋值按位右移位的行为：>>=
__iand__(self, other)	定义赋值按位与操作的行为：&=
__ixor__(self, other)	定义赋值按位异或操作的行为：^=
__ior__(self, other)	定义赋值按位或操作的行为：|=
	一元操作符
__pos__(self)	定义正号的行为：+x
__neg__(self)	定义负号的行为：-x
__abs__(self)	定义当被 abs() 调用时的行为
__invert__(self)	定义按位求反的行为：~x
	类型转换
__complex__(self)	定义当被 complex() 调用时的行为（需要返回恰当的值）
__int__(self)	定义当被 int() 调用时的行为（需要返回恰当的值）
__float__(self)	定义当被 float() 调用时的行为（需要返回恰当的值）
__round__(self[, n])	定义当被 round() 调用时的行为（需要返回恰当的值）
__index__(self)	1. 当对象是被应用在切片表达式中时，实现整形强制转换 2. 如果你定义了一个可能在切片时用到的定制的数值型,你应该定义 __index__ 3. 如果 __index__ 被定义，则 __int__ 也需要被定义，且返回相同的值
	上下文管理（with 语句）
__enter__(self)	1. 定义当使用 with 语句时的初始化行为 2. __enter__ 的返回值被 with 语句的目标或者 as 后的名字绑定
__exit__(self, exc_type, exc_value, traceback)	1. 定义当一个代码块被执行或者终止后上下文管理器应该做什么 2. 一般被用来处理异常，清除工作或者做一些代码块执行完毕之后的日常工作
	容器类型
__len__(self)	定义当被 len() 调用时的行为（返回容器中元素的个数）
__getitem__(self, key)	定义获取容器中指定元素的行为，相当于 self[key]
__setitem__(self, key, value)	定义设置容器中指定元素的行为，相当于 self[key] = value
__delitem__(self, key)	定义删除容器中指定元素的行为，相当于 del self[key]
__iter__(self)	定义当迭代容器中的元素的行为
__reversed__(self)	定义当被 reversed() 调用时的行为
__contains__(self, item)	定义当使用成员测试运算符（in 或 not in）时的行为