typing_extensions needed for Python 3.7

[dejanceltra]

• I have searched the issue tracker and believe that this is not a duplicate.

pdm doesn't have typing_extensions specified as a dependency, but it's needed for Python 3.7:

pdm/pdm/_types.py

Lines 4 to 9 in 1f83b7e

	if sys.version_info >= (3, 8):
	from importlib.metadata import Distribution
	from typing import Literal, Protocol, TypedDict
	else:
	from importlib_metadata import Distribution
	from typing_extensions import Literal, Protocol, TypedDict

Steps to reproduce

$ /usr/local/python3.7/bin/pip install pdm==1.8.5
$ /usr/local/python3.7/bin/pdm

Actual behavior

$ /usr/local/python3.7/bin/pdm
Traceback (most recent call last):
  File "/usr/local/python3.7/bin/pdm", line 5, in <module>
    from pdm.core import main
  File "/usr/local/python3.7/lib/python3.7/site-packages/pdm/__init__.py", line 5, in <module>
    from pdm.cli.commands.base import BaseCommand
  File "/usr/local/python3.7/lib/python3.7/site-packages/pdm/cli/commands/base.py", line 5, in <module>
    from pdm.cli.options import Option, global_option, project_option, verbose_option
  File "/usr/local/python3.7/lib/python3.7/site-packages/pdm/cli/options.py", line 9, in <module>
    from pdm._types import Protocol
  File "/usr/local/python3.7/lib/python3.7/site-packages/pdm/_types.py", line 9, in <module>
    from typing_extensions import Literal, Protocol, TypedDict
ModuleNotFoundError: No module named 'typing_extensions'

[blueyed]

Might be worth only importing it with/for typing.TYPE_CHECKING…

[frostming]

Can you double check the dependency tree? since typing-extensions is required by importlib-metadata, which is required on Python < 3.8

[pawamoy]

@frostming I think you shouldn't rely on a package being a dependency of another dependency. If you import it directly, list it as a dependency. This could prevent bad surprises :) And it gives you control over the accepted versions.