Weird results from PyIter_Check

Sat Apr 2 19:07:14 GMT 2022

PyIter_Check appears to be the C-level equivalent of
isinstance(..., collections.abc.Iterator), testing whether the python
next() or the C PyIter_Next will succeed. However, PyIter_Check
disagrees with collections.abc.Iterator and next about whether a given
object is an iterator. 
$ cat test_iterator.pyx
from cpython.iterator cimport PyIter_Check
def is_iterator(obj: object) -> bool:
 """Check whether obj is an iterator.
 Should agree with isinstance(obj, collections.abc.Iterator).
 Parameters
 ----------
 obj : object
 Returns
 -------
 bool
 """
 return PyIter_Check(obj)
$ cythonize --build --inplace test_iterator.pyx
running build_ext
$ python
Python 3.9.10 (main, Jan 20 2022, 21:37:52)
[GCC 11.2.0] on cygwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import test_iterator, os, fractions, collections.abc
>>> test_iterator.is_iterator(os.environ)
True
>>> isinstance(os.environ, collections.abc.Iterator)
False
>>> next(os.environ)
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: '_Environ' object is not an iterator
>>> test_iterator.is_iterator(fractions.Fraction(0, 1))
True
>>> isinstance(fractions.Fraction(0, 1), collections.abc.Iterator)
False
>>> next(fractions.Fraction(0, 1))
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: 'Fraction' object is not an iterator
On Linux, the test function using PyIter_Check agrees with
collections.abc.Iterator and next. The test case that led me to this
behaviour works the same in Windows as on Linux. Is this expected behavior?
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