python: __str__ v.s. __repr__

__str__ v.s. __repr__

• 具体阅读 stackoverflow上的问题: Difference between str and repr in Python
  对应 Top 1 回答的翻译: Python 中 str 和 repr 的区别

做几点摘要：

• My rule of thumb: __repr__ is for developers, __str__ is for customers.

• Note that there is one default which is true: if __repr__ is defined, and __str__ is not, the object will behave as though __str__=__repr__

>>> class Sic(object): pass
... 
>>> print str(Sic())
<__main__.Sic object at 0x8b7d0>
>>> print repr(Sic())
<__main__.Sic object at 0x8b7d0>
>>> 

>>> class Sic(object): 
...   def __repr__(object): return 'foo'
... 
>>> print str(Sic())
foo
>>> print repr(Sic())
foo
>>> class Sic(object):
...   def __str__(object): return 'foo'
... 
>>> print str(Sic())
foo
>>> print repr(Sic())
<__main__.Sic object at 0x2617f0>

• In short, the goal of __repr__ is to be unambiguous and __str__ is to be readable.

>>> import datetime
>>> 
>>> now = datetime.datetime.now()
>>> str(now)
'2017-12-06 14:42:27.403028'
>>> repr(now)
'datetime.datetime(2017, 12, 6, 14, 42, 27, 403028)'

• The default object __repr__ is (C Python source) something like:

def __repr__(self):
    return '<{0}.{1} object at {2}>'.format(
      self.__module__, type(self).__name__, hex(id(self)))