What type to use for itertools.count in Python?

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I am trying to specify the type of an itertool.count object in Python, like so:

from itertools import count

c: count = count()

However, running mypy yields the following error:

test.py:3: error: Function "itertools.count" is not valid as a type
test.py:3: note: Perhaps you need "Callable[...]" or a callback protocol?
Found 1 error in 1 file (checked 1 source file)

It seems to be caused by the fact that itertools.count behaves like a function. However, it returns an itertools.count object, as shown by

In [1]: import itertools
In [2]: type(itertools.count()) is itertools.count
Out[2]: True

Then, how should I specify the type of the result of count() ?

mypy python-3.8 type-hinting python
1 Answers

There is following annotation in itertools.pyi:

_N = TypeVar('_N', int, float)

def count(start: _N = ...,
          step: _N = ...) -> Iterator[_N]: ...  # more general types?

Thus, in your code, you could do like this:

from typing import Iterator
from itertools import count

c: Iterator[int] = count()
c_i: Iterator[int] = count(start=1, step=1)
c_f: Iterator[float] = count(start=1.0, step=0.1)  # since python 3.1 float is allowed
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