cattrs 1.7.0 fails to import _LiteralGenericAlias on Python 3.9.0

[ferraith]

• cattrs version: 1.7.0
• Python version: 3.9.0
• Operating System: Ubuntu Linux

Description

cattrs 1.7.0 introduced support for Literals. Therefore it imports on Python 3.9 some classes part of the typing module. For instance the _LiteralGenericAlias is imported:
https://github.com/Tinche/cattrs/blob/71a7ed2b4c72558af77487ae991f19cc5401c378/src/cattr/_compat.py#L195

These classes weren't introduced with the first release of Python 3.9.0 but with Python 3.9.1. See comparison between Python 3.9.0 and 3.9.1: python/cpython@v3.9.0...v3.9.1

The conditional import in the _compat.py module doesn't check the patch version of Python 3.9.x:
https://github.com/Tinche/cattrs/blob/71a7ed2b4c72558af77487ae991f19cc5401c378/src/cattr/_compat.py#L88
and:
https://github.com/Tinche/cattrs/blob/71a7ed2b4c72558af77487ae991f19cc5401c378/src/cattr/_compat.py#L181

What I Did

Import cattrs in user code on Python 3.9.0.

  File \"mycode.py\", line 7, in <module>
    import cattr
  File \"venv/lib/python3.9/site-packages/cattr/__init__.py\", line 1, in <module>
    from .converters import Converter, GenConverter, UnstructureStrategy
  File \"venv/lib/python3.9/site-packages/cattr/converters.py\", line 10, in <module>
    from ._compat import (
  File \"venv/lib/python3.9/site-packages/cattr/_compat.py\", line 191, in <module>
    from typing import (
ImportError: cannot import name '_LiteralGenericAlias' from 'typing' (/usr/lib/python3.9/typing.py)

[Tinche]

Bleh, you have to be kidding me.

Ok, I need to conditionally import this :/

[Tinche]

@ferraith Can you import _LiteralGenericAlias from typing_extensions if you pip install it? We could fall back to that.

[ferraith]

@Tinche
I installed typing_extensions==3.10.0.0
Successfully installed typing-extensions-3.10.0.0

But if you have a look in the typing_extensions module there is no _LiteralGenericAlias. Only a Literal class is available:
https://github.com/python/typing/blob/3.10.0.0/typing_extensions/src_py3/typing_extensions.py

Because of that also importing the _LiteralGenericAlias class is failing:

    from typing_extensions import _LiteralGenericAlias
ImportError: cannot import name '_LiteralGenericAlias' from 'typing_extensions' (/workspace/venv/lib/python3.9/site-packages/typing_extensions.py)

[Tinche]

I took a look at the internal implementation of the module, but it looks like there are several implementations of the Literal alias. In the interest of simplicity I'll just change it so literals don't work if they aren't present in typing.

[ferraith]

I took a look at the internal implementation of the module, but it looks like there are several implementations of the Literal alias. In the interest of simplicity I'll just change it so literals don't work if they aren't present in typing.

Very nice! Do you plan a bugfix release like 1.7.1 for that fix or will the change be part of the next upcoming release?

[Tinche]

I can put out a hotfix tonight!

[ferraith]

@Tinche A hotfix would be excellent. Thanks a lot for your work!

[Tinche]

I pushed a fix to master, can you install from git and verify it works for you? I can release then.

[ferraith]

@Tinche I tested your fix in my Python 3.9.0 environment and the import is now working like expected. Thanks for that fix!

[Tinche]

1.7.1 released!