python中魔法方法repr_Python的两个魔法方法：__repr__和__str__
目录
例子
关系
使用
官方文档
参考
__repr__ 和 __str__ 是 Python 的两个魔法方法(Magic/Special method)，更多魔法方法可以参考 A Guide to Python's Magic Methods 以及 Special method names。
提前说明下面的 str() 和 repr() 都是 Python 中的内置函数，分别调用 __str__() 和 __repr__()。至于内置函数的源码实现可以参考 Finding the source code for built-in Python functions?。
例子
>>> class A:
pass
>>> a1 = A()
>>> print(repr(a1))
>>> print(str(a1))
这里的 0x0000000003166EB8 是什么呢？我们可以用 hex(id(a1)) 方法验证一下，发现结果和它是相同的。说明这就是 a1 对象的 id。更具体一点：
CPython implementation detail: For CPython, id(x) is the memory address where x is stored.
可以参考：In Python, what does '' mean?
>>> class A:
def __str__(self):
#__str__使用：被打印的时候需要以字符串的形式输出的时候，就会找到这个方法，并将返回值打印出来
return "我是一个字符串"
# 要想显示对象的属性，可以
# (1)return 后加上你想要格式化输出的属性,比如： return "%d %s" % (int("123"), str(123))
# (2)利用字符串的format方法，比如："{},{}".format(1,2)
>>> a1 = A()
>>> print(repr(a1))
>>> print(str(a1))
我是一个字符串
与下例对比
>>> class A:
def __repr__(self): #返回一个可以用来表示对象的可打印字符串
return "我是一个字符串"
>>> a1 = A()
>>> print(repr(a1))
我是一个字符串
>>> print(str(a1))
我是一个字符串
此例表明 __str__ 方法默认调用了 __repr__ 方法。
关系
__str__ 方法其实调用了 __repr__ 方法。
使用
在 SO 上的解释是：
__repr__ 目的是为了表示清楚，是为开发者准备的。
__str__ 目的是可读性好，是为使用者准备的。
我的理解是 __repr__ 应该尽可能的表示出一个对象来源的类以及继承关系，方便程序员们了解这个对象。而 __str__ 就简单的表示对象，而不要让不懂编程的以为输出的是 bug。
>>> import datetime
>>> today = datetime.datetime.now()
>>> str(today)
'2012-03-14 09:21:58.130922'
>>> repr(today)
'datetime.datetime(2012, 3, 14, 9, 21, 58, 130922)'
上表是来自 bitoffdev 的回答。
我们可以看到 str(today) 输出的很正常，哪个都看得懂。但是 repr(today) 的输出对不懂编程的就来说有点奇怪了，只懂一丢丢的还可能会以为自己搞出了啥幺蛾子呢。但是这对于有点经验的人来说这个就表示 today 对象是由 datetime 类实例化出来的。
官方文档
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> 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.
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__().
repr(object)
Return a string containing a printable representation of an object. For many types, this function makes an attempt to return a string that would yield an object with the same value when passed to eval(), otherwise the representation is a string enclosed in angle brackets that contains the name of the type of the object together with additional information often including the name and address of the object. A class can control what this function returns for its instances by defining a __repr__() method.
str(object='')
Return a string containing a nicely printable representation of an object. For strings, this returns the string itself. The difference with repr(object) is that str(object) does not always attempt to return a string that is acceptable to eval(); its goal is to return a printable string. If no argument is given, returns the empty string, ''.
参考
http://blog..net/luckytanggu/article/details/53649156
https://www..com/superxuezhazha/p/5746922.html
http://blog..net/DucklikeJAVA/article/details/73478307
https://docs.python.org/3/library/functions.html
https://docs.python.org/3/reference/datamodel.html