15 __repr__ 与 __strr__

预览一下两个方法

>>> class A(object):
...     def __repr__(self):
...         return "this is __repr__"
...     def __str__(self):
...         return "this is __str__"
...
>>> a = A()
>>> a
this is __repr__
>>> print(a)
this is __str__
>>> 

官方文档

• Python 3.7.3

__repr__()

 object.__repr__(self)

    Called by the repr() built-in function to compute the “official” string representation of an object. If at all possible, this should look like a valid Python expression that could be used to recreate an object with the same value (given an appropriate environment). If this is not possible, a string of the form <...some useful description...> should be returned. The return value must be a string object. If a class defines __repr__() but not __str__(), then __repr__() is also used when an “informal” string representation of instances of that class is required.

    This is typically used for debugging, so it is important that the representation is information-rich and unambiguous.

• __repr__() 通常用于调试
• 它能将对象转化为供解释器读取的形式（字符串）

__str__()

object.__str__(self)

    Called by str(object) and the built-in functions format() and print() to compute the “informal” or nicely printable string representation of an object. The return value must be a string object.

    This method differs from object.__repr__() in that there is no expectation that __str__() return a valid Python expression: a more convenient or concise representation can be used.

    The default implementation defined by the built-in type object calls object.__repr__().

• __str__() 是调用 __repr__() 实现的

举例

>>> import datetime
>>> today = datetime.datetime.now()
>>> today
datetime.datetime(2019, 4, 11, 20, 45, 8, 463653)
>>> repr(today)
'datetime.datetime(2019, 4, 11, 20, 45, 8, 463653)'
>>> str(today)
'2019-04-11 20:45:08.463653'
>>> 

• __repr__() 显示更全面，__str__() 表达更简洁
• print() 打印对象时，默认调用 __repr__()
  • 若没写，返回 <... object at 0x...>
  • 若有写 __str__()，则调用 __str__()