Why `__iter__` does not work when defined as an instance variable?

Question

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())

Result:

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

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

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

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?

Answer 1

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

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

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')

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.

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