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() ?
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