Add `typing-extensions` to the list of dependencies?

[malmans2]

typing_extensions is imported in xarray/core/npcompat.py and xarray/core/utils.py.

However, typing-extensions is not a dependency in setup.cfg, and I get the following error when I install with pip install git+https://github.com/pydata/xarray using python 3.9:

import xarray

ModuleNotFoundError: No module named 'typing_extensions'

[max-sixty]

Great spot! We should definitely add that.

I wonder how CI passes, then?

[malmans2]

Looks like there isn't an action that only installs xarray using pip, but dependencies are installed first using conda. Must be in the conda recipe of one of the packages specified in the CI environments?

[max-sixty]

Yes, good point.

Would you be up for another PR @malmans2 ?

[shoyer]

Can we consider making typing-extensions a fully optional dependency instead?

In particular, typing-extensions 3.10 is a very new version, only from May 2021. This seems overly restrictive, particularly for a dependency that isn't really needed at all at runtime and that we only use for a few type annotations.

[malmans2]

@shoyer I added typing-extensions in the docs too, so you'd have to remove it from there as well: https://github.com/shoyer/xarray/blob/typing-extensions-optional/doc/getting-started-guide/installing.rst

[shoyer]

I made an attempt at making the use of TypeGuard optional, but it doesn't pass mypy yet: #5624

[Illviljan]

Using typing_extensions seems to be a nice way to get backwards compatibility with older python versions.
I can see some typing bugs slipping through rather easily onsidering how picky mypy is and the CI doesn't do mypy checks on all python versions.