Literal-types

[janosh]

Follow up to #2247.

@CompRhys informed me that due to #2247, Pymatgen is no longer installable in Google Colab which is locked to Python 3.7 while typing.Literal was only added in Python 3.8.

There's a relatively recent but quiet issue tracking the ability to select different Python versions in Colab: googlecolab/colabtools#2165

In the meantime though, I assume Pymatgen wants to keep supporting Google Colab. I think the best solution for that would be to protect Literal imports with

from typing import TYPE_CHECKING

if TYPE_CHECKING:
    from typing import Literal

and then use the string form "Literal" in type hints. Happy to add those changes to this PR if that's acceptable.

[mkhorton]

You can install typing_extensions, which is actually already a requirement of pymatgen if you're running in Python 3.7.

The correct way of resolving this is following this example:

pymatgen/pymatgen/io/qchem/inputs.py

Lines 19 to 22 in f978687

	if sys.version_info >= (3, 8):
	from typing import Literal
	else:
	from typing_extensions import Literal

Apologies, I should have mentioned this in your earlier PR but forgot.

[janosh]

@mkhorton Thanks for that advice. Changed all the Literal imports from #2247.

[CompRhys]

pulled this branch everything seems to work

[mkhorton]

Thanks both!