为什么`__iter__`在定义为实例变量时不起作用?

2022-01-24 00:00:00 python iteration iterable

问题描述

如果我如下定义 __iter__ 方法，它将不起作用:

If I define the __iter__ method as follows, it won't work:

class A:

    def __init__(self):
        self.__iter__ = lambda: iter('text')


for i in A().__iter__():
    print(i)

iter(A())

结果:

t
e
x
t
Traceback (most recent call last):
  File "...mytest.py", line 10, in <module>
    iter(A())
TypeError: 'A' object is not iterable

如您所见，调用 A().__iter__() 有效，但 A() 不可迭代.

As you can see, calling A().__iter__() works, but A() is not iterable.

但是，如果我为该类定义 __iter__，那么它将起作用:

However if I define __iter__ for the class, then it will work:

class A:

    def __init__(self):

        self.__class__.__iter__ = staticmethod(lambda: iter('text'))
        # or:
        # self.__class__.__iter__ = lambda s: iter('text')


for i in A():
    print(i)

iter(A())

# will print:
# t
# e
# x
# t

有谁知道为什么 python 是这样设计的?即为什么 __iter__ 作为实例变量不起作用?你不觉得它不直观吗?

Does anyone know why python has been designed like this? i.e. why __iter__ as instance variable does not work? Don't you find it unintuitive?

解决方案

这是按设计完成的.您可以在此处找到详细说明:https://docs.python.org/3/reference/datamodel.html#special-method-lookup

It is done by design. You can find the thorough description here: https://docs.python.org/3/reference/datamodel.html#special-method-lookup

简短的回答:必须在类对象本身上设置特殊方法，以便解释器始终如一地调用.

Short answer: the special method must be set on the class object itself in order to be consistently invoked by the interpreter.

长答案:这背后的想法是加快众所周知的建设.在你的例子中:

Long answer: the idea behind this is to speed up well-known constructions. In your example: 

class A:
    def __init__(self):
        self.__iter__ = lambda: iter('text')

在现实生活中，您多久会编写一次这样的代码?所以，Python 做了什么 - 它跳过了实例的字典查找，即 iter(A()) 根本没有看到"那个 self.__iter__，它实际上是self.__dict__['__iter__'] 在这种情况下.

How often are you going to write a code like this in real life? So, what Python does - it skips a dictionary lookup of the instance, i.e. iter(A()) simply does not "see" that self.__iter__, which is actually self.__dict__['__iter__'] in this case.

它还跳过了所有 __getattribute__ 实例和元类查找，获得了显着的加速.

It also skips all the __getattribute__ instance and metaclass lookup gaining a significant speedup.

相关文章