add by zhj: 有些地方不正确，有时间再改吧
Cheat sheet of Python. Some basic concepts for Python programmer need to know.
Python Naming Styles
# see: PEP8 # for public use var # for internal use _var # convention to avoid conflict keyword var_ # for private use in class __var # for protect use in class _var_ # "magic" method or attributes # ex: __init__, __file__, __main__ __var__ # for "internal" use throwaway variable # usually used in loop # ex: [_ for _ in range(10)] # or variable not used # for _, a in [(1,2),(3,4)]: print a _
Check object attributes
# example of check list attributes >>> dir(list) ['__add__', '__class__', ...]
Define a function __doc__
# Define a function document >>> def Example(): ... """ This is an example function """ ... print "Example function" ... >>> Example.__doc__ ' This is an example function ' # Or using help function >>> help(Example)
Check instance type
>>> ex = 10
>>> isinstance(ex,int)
True
Check, Get, Set attribute
>>> class example(object): ... def __init__(self): ... self.name = "ex" ... def printex(self): ... print "This is an example" ... # Check object has attributes # hasattr(obj, 'attr') >>> ex = example() >>> hasattr(ex,"name") True >>> hasattr(ex,"printex") True >>> hasattr(ex,"print") False # Get object attribute # getattr(obj, 'attr') >>> getattr(ex,'name') 'ex' # Set object attribute # setattr(obj, 'attr', value) >>> setattr(ex,'name','example') >>> ex.name 'example'
Check inheritance
>>> class example(object): ... def __init__(self): ... self.name = "ex" ... def printex(self): ... print "This is an example" ... >>> issubclass(example,object) True
Check all global variables
# globals() return a dictionary # {'variable name': variable value} >>> globals() {'args': (1, 2, 3, 4, 5), ...}
Check "callable"
>>> a = 10 >>> def fun(): ... print "I am callable" ... >>> callable(a) False >>> callable(fun) True
Common Use "Magic"
# see python document: data model # For command class __main__ __name__ __file__ __module__ __all__ __dict__ __class__ __doc__ __init__(self, [...) __str__(self) __repr__(self) __del__(self) # For Descriptor __get__(self, instance, owner) __set__(self, instance, value) __delete__(self, instance) # For Context Manager __enter__(self) __exit__(self, exc_ty, exc_val, tb) # Emulating container types __len__(self) __getitem__(self, key) __setitem__(self, key, value) __delitem__(self, key) __iter__(self) __contains__(self, value) # Controlling Attribute Access __getattr__(self, name) __setattr__(self, name, value) __delattr__(self, name) __getattribute__(self, name) # Callable object __call__(self, [args...]) # Compare related __cmp__(self, other) __eq__(self, other) __ne__(self, other) __lt__(self, other) __gt__(self, other) __le__(self, other) __ge__(self, other) # arithmetical operation related __add__(self, other) __sub__(self, other) __mul__(self, other) __div__(self, other) __mod__(self, other) __and__(self, other) __or__(self, other) __xor__(self, other)
Arithmetic operators and Comparison
class example(object): def __init__(self,val1,val2): self.val1 = val1 self.val2 = val2 def __add__(self,other): _1 = self.val1 + other.val1 _2 = self.val2 + other.val2 return _1 + _2 def __sub__(self,other): _1 = self.val1 - other.val1 _2 = self.val2 - other.val2 return _1 - _2 def __mul__(self,other): _1 = self.val1 * other.val1 _2 = self.val2 * other.val2 return _1 * _2 def __div__(self,other): _1 = self.val1 / other.val1 _2 = self.val2 / other.val2 return _1 / _2 def __mod__(self,other): _1 = self.val1 % other.val1 _2 = self.val2 % other.val2 return _1 % _2 def __eq__(self,other): if self.val1 == other.val1 and self.val2 == other.val2: return True return False def __ne__(self,other): if self.val1 != other.val1 or self.val2 != other.val2: return True return False def __lt__(self,other): if self.val1 < other.val1 or self.val2 < other.val2: return True return False def __gt__(self,other): if self.val1 > other.val1 or self.val2 > other.val2: return True return False def __le__(self,other): if self.val1 <= other.val1 or self.val2 <= other.val2: return True return False def __ge__(self,other): if self.val1 >= other.val1 or self.val2 >= other.val2: return True return False # example result: >>> ex1 = example(1.,2.) >>> ex2 = example(3.,4.) >>> ex1+ex2 10.0 >>> ex1-ex2 0.0 >>> ex1*ex2 24.0 >>> ex1/ex2 0.6666666666666666 >>> ex1%ex2 1.0 >>> ex1>ex2 False >>> ex1<ex2 True >>> ex1==ex2 False
Representations of your class behave
>>> class example(object): ... def __str__(self): ... return "example __str__" ... def __repr__(self): ... return "example __repr__" ... >>> print str(example()) example __str__ >>> example() example __repr__
Get list item "SMART"
>>> a = [1,2,3,4,5] >>> a[0] 1 >>>a[-1] 5 >>> a[0:] [1,2,3,4,5] >>> a[:-1] [1,2,3,4] # a[start:end:step] >>> a[0:-1:2] [1,3] # using slice object # slice(start,end,step) >>> s = slice(0,-1,2) >>> a[s] [1,3] # Get index and item in loop >>> a = range(3) >>> for idx,item in enumerate(a): ... print (idx,item), ... (0, 0) (1, 1) (2, 2) # with filter >>> [x for x in range(5) if x>1] [2, 3, 4] >>> _ = ['1','2',3,'Hello',4] >>> f = lambda x: isinstance(x,int) >>> filter(f,_) [3, 4]
Get dictionary item "SMART"
# get dictionary all keys >>> a={"1":1,"2":2,"3":3} >>> b={"2":2,"3":3,"4":4} >>> a.keys() ['1', '3', '2'] # get dictionary key and value as tuple >>> a.items() [('1', 1), ('3', 3), ('2', 2)] # find same key between two dictionary >>> [_ for _ in a.keys() if _ in b.keys()] ['3', '2'] # better way >>> c = set(a).intersection(set(b)) >>> list(c) ['3', '2'] # or >>> [_ for _ in a if _ in b] ['3', '2'] # update dictionary >>> a.update(b) >>> a {'1': 1, '3': 3, '2': 2, '4': 4}
Set a list/dict "SMART"
# get a list with init value >>> ex = [0]*10 >>> ex [0, 0, 0, 0, 0, 0, 0, 0, 0, 0] # using generator >>> [x for x in range(10)] [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] >>> fn = lambda x: x**2 >>> [fn(x) for x in range(5)] [0, 1, 4, 9, 16] >>> {'{0}'.format(x): x for x in range(3)} {'1': 1, '0': 0, '2': 2} # using builtin function "map" >>> map(fn,range(5)) [0, 1, 4, 9, 16]
Delegating Iteration (__iter__)
# __iter__ return an iterator object # Be careful: list is an "iterable" object not an "iterator" >>> class example(object): ... def __init__(self,list_): ... self._list = list_ ... def __iter__(self): ... return iter(self._list) ... >>> ex = example([1,2,3,4,5]) >>> for _ in ex: print _, ... 1 2 3 4 5
Using Generator as Iterator
# see: PEP289 >>> a = (_ for _ in range(10)) >>> for _ in a: print _, ... 0 1 2 3 4 5 6 7 8 9 # equivalent to >>> def _gen(): ... for _ in range(10): ... yield _ ... >>> a = _gen() >>> for _ in a: print _, ... 0 1 2 3 4 5 6 7 8 9
Emulating a list
>>> class emulist(object): ... def __init__(self,list_): ... self._list = list_ ... def __repr__(self): ... return "emulist: "+str(self._list) ... def append(self,item): ... self._list.append(item) ... def remove(self,item): ... self._list.remove(item) ... def __len__(self): ... return len(self._list) ... def __getitem__(self,sliced): ... return self._list[sliced] ... def __setitem__(self,sliced,val): ... self._list[sliced] = val ... def __delitem__(self,sliced): ... del self._list[sliced] ... def __contains__(self,item): ... return item in self._list ... def __iter__(self): ... return iter(self._list) ... >>> emul = emulist(range(5)) >>> emul emulist: [0, 1, 2, 3, 4] >>> emul[1:3] [1, 2] >>> emul[0:4:2] [0, 2] >>> len(emul) 5 >>> emul.append(5) >>> emul emulist: [0, 1, 2, 3, 4, 5] >>> emul.remove(2) >>> emul emulist: [0, 1, 3, 4, 5] >>> emul[3] = 6 >>> emul emulist: [0, 1, 3, 6, 5] >>> 0 in emul True
Emulating a dictionary
>>> class emudict(object): ... def __init__(self,dict_): ... self._dict = dict_ ... def __repr__(self): ... return "emudict: "+str(self._dict) ... def __getitem__(self,key): ... return self._dict[key] ... def __setitem__(self,key,val): ... self._dict[key] = val ... def __delitem__(self,key): ... del self._dict[key] ... def __contains__(self,key): ... return key in self._dict ... def __iter__(self): ... return iter(self._dict.keys()) ... >>> _ = {"1":1,"2":2,"3":3} >>> emud = emudict(_) >>> emud emudict: {'1': 1, '3': 3, '2': 2} >>> emud['1'] 1 >>> emud['5'] = 5 >>> emud emudict: {'1': 1, '3': 3, '2': 2, '5': 5} >>> del emud['2'] >>> emud emudict: {'1': 1, '3': 3, '5': 5} >>> for _ in emud: print emud[_], ... 1 3 5 >>> '1' in emudict True
Decorator
# see: PEP318 >>> def decor(func): ... def wrapper(*args,**kwargs): ... print "wrapper" ... func() ... print "-------" ... return wrapper ... >>> @decor ... def example(): ... print "Example" ... >>> example() wrapper Example ------- # equivalent to >>> def example(): ... print "Example" ... >>> example = decor(example) >>> example() wrapper Example -------
Decorator with arguments
>>> def example(val): ... def decor(func): ... def wrapper(*args,**kargs): ... print "Val is {0}".format(val) ... func() ... return wrapper ... return decor ... >>> @example(10) ... def undecor(): ... print "This is undecor func" ... >>> undecor() Val is 10 This is undecor func # equivalent to >>> def undecor(): ... print "This is undecor func" ... >>> d = example(10) >>> undecor = d(undecor) >>> undecor() Val is 10 This is undecor func
for: exp else: exp
# see document: More Control Flow Tools # forloop’s else clause runs when no break occurs >>> for _ in range(5): ... print _, ... else: ... print " no break occur" ... 0 1 2 3 4 no break occur >>> for _ in range(5): ... if _ % 2 ==0: ... print "break occur" ... break ... else: ... print "else not occur" ... break occur # above statement equivalent to flag = False for _ in range(5): if _ % 2 == 0: flag = True print "break occur" break if flag == False: print "else not occur"
Lambda function
# lambda [input]: expression >>> fn = lambda x: x**2 >>> fn(3) 9 >>> (lambda x:x**2)(3) 9 >>> (lambda x: [x*_ for _ in range(5)])(2) [0, 2, 4, 6, 8] >>> (lambda x: x if x>3 else 3)(5) 5 # multiline lambda example >>> (lambda x: ... True ... if x>0 ... else ... False)(3) True
Function with option arguments (*args, **kwargs)
>>> def example(a,b=None,*args,**kwargs): ... print a, b ... print args ... print kwargs ... >>> example(1,"var",2,3,word="hello") 1 var (2, 3) {'word': 'hello'} >>> _args = (1,2,3,4,5) >>> _kwargs = {"1":1,"2":2,"3":3} >>> example(1,"var",*_args,**_kwargs) 1 var (1, 2, 3, 4, 5) {'1': 1, '3': 3, '2': 2}
Callable object
>>> class calobj(object): ... def example(self): ... print "I am callable!" ... def __call__(self): ... self.example() ... >>> ex = calobj() >>> ex() I am callable!
"with" statement (Context Manager)
# replace try: ... finally: ... # see: PEP343 # common use in open and close # example: import socket class Socket(object): def __init__(self,host,port): self.host = host self.port = port def __enter__(self): sock = socket.socket( socket.AF_INET, socket.SOCK_STREAM ) sock.bind((self.host,self.port)) sock.listen(5) self.sock = sock return self.sock def __exit__(self,*exc_info): if exc_ty is not None: import traceback traceback.print_exception( *exc_info ) self.sock.close() if __name__=="__main__": host = 'localhost' port 5566 with Socket(host,port) as s: while True: conn, addr = s.accept() msg = conn.recv(1024) print msg conn.send(msg) conn.close()
Using "contextlib" implement Context Manager
from contextlib import contextmanager @contextmanager def opening(filename): f = open(filename) try: yield f finally: f.close() with opening('example.txt','r') as fd: fd.read()
Using "with" statement open file
>>> with open("example",'r') as f: ... content = f.read()
Property - Managed attributes
class example(object): def __init__(self,value): self.val = value @property def val(self): return self._val @val.setter def val(self,value): if not isintance(value,int): raise TypeError("Expect int") self._val = value @ val.deleter def val(self): del self._val # example result: >>> ex = example(123) >>> ex.val = "str" Traceback (most recent call last): File "", line 1, in File "test.py", line 12, in val raise TypeError("Expect int") TypeError: Expect int
Computed attribures - Using property
# Concept: Attribure's value is not store in memory. Computing the value only when we need. >>> class example(object): ... @property ... def square3(self): ... return 2**3 ... >>> ex = example() >>> ex.square3 8
Descriptor - Another choice to manage attributes
# Descriptor >>> class Integer(object): ... def __init__(self,name): ... self._name = name ... def __get__(self,inst,cls): ... if inst is None: ... return self ... else: ... return inst.__dict__[self._name] ... def __set__(self,inst,value): ... if not isinstance(value,int): ... raise TypeError("Expected INT") ... inst.__dict__[self._name] = value ... def __delete__(self,inst): ... del inst.__dict__[self._name] ... >>> class example(object): ... x = Integer('x') ... def __init__(self,val): ... self.x = val ... >>> ex = example(1) >>> ex = example("str") Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 4, in __init__ File "<stdin>", line 11, in __set__ TypeError: Expected an int
@staticmethod, @classmethod
# @classmethod: bound to class # @staticmethod: like python function but in class >>> class example(object): ... @classmethod ... def clsmethod(cls): ... print "I am classmethod" ... @staticmethod ... def stmethod(): ... print "I am staticmethod" ... def instmethod(self): ... print "I am instancemethod" ... >>> ex = example() >>> ex.clsmethod() I am classmethod >>> ex.stmethod() I am staticmethod >>> ex.instmethod() I am instancemethod >>> example.clsmethod() I am classmethod >>> example.stmethod() I am staticmethod >>> example.instmethod() Traceback (most recent call last): File "", line 1, in TypeError: unbound method instmethod() ...
Abstract method - Meta class
# usually using in define methods but not implement from abc import ABCMeta, abstractmethod >>> class base(object): ... __metaclass__ = ABCMeta ... @abstractmethod ... def absmethod(self): ... """ Abstract method """ ... >>> class example(base): ... def absmethod(self): ... print "abstract" ... >>> ex = example() >>> ex.absmethod() abstract # another better way to define a meta class >>> class base(object): ... def absmethod(self): ... raise NotImplementedError ... >>> class example(base): ... def absmethod(self): ... print "abstract" ... >>> ex = example() >>> ex.absmethod() abstract