Why does iter(v,w) return a class when the callable is a generator?

Question

Consider the following minimum working example:

import itertools

for i in iter(itertools.count, 10):
    print(i)

I have expected that the output counts to 10. However, the output was count(0) over and over again. Printing the type instead gives <class 'itertools.count'>.

The documentation of iter(object, sentinel) says the following:

The iterator created in this case will call object with no arguments for each call to its __next__() method; if the value returned is equal to sentinel, StopIteration will be raised, otherwise the value will be returned.

Which reads to me like the behaviour I have expected. What have I overlooked? Optional bonus question: How is it possible with iter to make object a generator and get the expected behaviour?

Answer 1

Fleshing out the docs for 2-arg iter, it works like:

def two_arg_iter(object, sentinel):
    while True:
        temp = object()
        if temp == sentinel:
            break
        yield temp

Which does exactly what you see it doing in the case you gave. Note:

>>> itertools.count()
count(0)

and count(0) is never equal to 10 so you keep getting count(0) forever.

It's not clear to me what you're asking, though. Either of these, I believe, would do what you actually want:

for i in itertools.count(10):

or, redundantly,

for i in iter(itertools.count(10)):

It may help to realize that 2-arg iter is a very different beast than 1-arg iter.