Importing ArrayLike or DTypeLike raises a NameError in numpy 1.21.1

[hhtong]

With python 3.7.9, importing ArrayLike or DTypeLike raises a NameError in numpy==1.21.1.

I had expected that the TypeError raised with numpy==1.21.0 would have been resolved with this PR : #19311, but now a NameError comes up instead.

Reproducing code example:

from numpy.typing import ArrayLike, DTypeLike

Error message:

>>> from numpy.typing import ArrayLike, DTypeLike
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/htong/env379/lib/python3.7/site-packages/numpy/typing/__init__.py", line 316, in <module>
    from ._dtype_like import (
  File "/Users/htong/env379/lib/python3.7/site-packages/numpy/typing/_dtype_like.py", line 95, in <module>
    class _SupportsDType(Generic[_DType_co]):
NameError: name '_DType_co' is not defined

NumPy/Python version information:

>>> import sys, numpy; print(numpy.__version__, sys.version)
1.21.1 3.7.9 (default, Jul 16 2021, 10:15:27) 
[Clang 11.0.0 (clang-1100.0.33.16)]

[sourabh69733]

Try to update your python to 3.8+ or 3.9.1.

It is not throwing an error in 3.9.1.

[sourabh69733]

Check here the documentation reference for the previous version of python

[BvB93]

The condition for triggering the exception is as following:

• Python 3.7 is used.
• typing-extensions is not installed.

I'm currently working a fix for the issue. Annoyingly enough the relevant code paths have proven difficult to test,
as typing-extensions is an (indirect) dependency of pytest. #19525 should help with this though.

[BvB93]

Closed by #19533.