python - 为什么 'in' 关键字声称它需要一个可迭代对象？

Question

>>> non_iterable = 1
>>> 5 in non_iterable
Traceback (most recent call last):
  File "<input>", line 1, in <module>
TypeError: 'int' object is not iterable

>>> class also_non_iterable:
...     def __contains__(self,thing):
...         return True

>>> 5 in also_non_iterable()
True
>>> isinstance(also_non_iterable(), Iterable)
False

当 in 关键字真正需要的是一个实现了 __contains__ 的对象时，是否有理由声称需要一个可迭代对象？

Answer 1

它声称需要一个可迭代对象，因为如果对象的类没有实现 __contains__ ，那么 in 会尝试遍历对象并检查值是否相等到它产生的值(value)。

一个例子来说明-

>>> class C:
...     def __iter__(self):
...         return iter([1,2,3,4])
>>>
>>> c = C()
>>> 2 in c
True
>>> 5 in c
False

这在 the documentation - 中有解释

For user-defined classes which define the __contains__() method, x in y is true if and only if y.__contains__(x) is true.

For user-defined classes which do not define __contains__() but do define __iter__() , x in y is true if some value z with x == z is produced while iterating over y . If an exception is raised during the iteration, it is as if in raised that exception.