Semantic equivalent of async for

From the docs relative to async for syntax in Python 3.5, I gathered that it was introduced to iterate over an awaitable iterator.

There is something I don’t get in the semantic equivalent that follow the description though:

iter = (ITER)
iter = type(iter).__aiter__(iter)
running = True
while running:
    try:
        TARGET = await type(iter).__anext__(iter)
    except StopAsyncIteration:
        running = False
    else:
       BLOCK
else:
    BLOCK2 

What is the line iter = type(iter).__aiter__(iter) doing? Why is it necessary?

Answer

Magic methods in python, e.g. __add__, are always looked up on the class. For example, len(foo) will actually call resolve to type(foo).__len__(foo). This means it will never use a foo.__len__ if __len__ is defined on foo.

Since __aiter__ is a magic method, it is thus always looked up as type(foo).__aiter__(foo).

---

Example for lookup:

class Thingy(object):
  def __len__(self):
    return 10

lengthy_thingy = Thingy()
lengthy_thingy.__len__ = lambda self: 20

print(len(lengthy_thingy))  # gives `10`
print(lengthy_thingy.__len__(lengthy_thingy))  # gives `20` instead
print(type(lengthy_thingy).__len__(lengthy_thingy))  # gives `10` as for using `len`