Use Literal types in argparse

[sobolevn]

No description provided.

[sobolevn]

🤦

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

cwltool (https://github.com/common-workflow-language/cwltool)
+ cwltool/argparser.py: note: In function "arg_parser":
+ cwltool/argparser.py:460:51: error: Incompatible types in assignment (expression has type "Literal['Dependency resolver ']", variable has type "Literal['==SUPPRESS==']")  [assignment]
+ cwltool/argparser.py:464:13: error: Incompatible types in assignment (expression has type "Literal['Defaut root directory used by dependency resolvers configuration.']", variable has type "Literal['==SUPPRESS==']")  [assignment]
+ cwltool/argparser.py:466:34: error: Incompatible types in assignment (expression has type "Literal['Use biocontainers for tools without an ']", variable has type "Literal['==SUPPRESS==']")  [assignment]
+ cwltool/argparser.py:469:13: error: Incompatible types in assignment (expression has type "Literal["Short cut to use Conda to resolve 'SoftwareRequirement' packages."]", variable has type "Literal['==SUPPRESS==']")  [assignment]

[Akuli]

argparse is trickier than you might expect. It uses is to compare strings, so there is a difference between passing argparse.SUPPRESS and passing "==SUPPRESS==":

akuli@akuli-desktop:~$ python3 -c 'import argparse; p = argparse.ArgumentParser(); p.add_argument("--xyz", help=argparse.SUPPRESS); p.parse_args()' --help
usage: -c [-h]

optional arguments:
  -h, --help  show this help message and exit

akuli@akuli-desktop:~$ python3 -c 'import argparse; p = argparse.ArgumentParser(); p.add_argument("--xyz", help="==SUPPRESS=="); p.parse_args()' --help
usage: -c [-h] [--xyz XYZ]

optional arguments:
  -h, --help  show this help message and exit
  --xyz XYZ   ==SUPPRESS==

With this in mind, I'm not sure if Literal is the correct type for these constants. They really should be treated as if they were typing.NewType or something.

[sobolevn]

@Akuli thanks for the interesting use-case, I was not aware of that 🙂

But, I still think that Literal is the correct type here.
Because, I am not add_argument or help, I am just fixing constants to be in-line with https://typing.readthedocs.io/en/latest/source/stubs.html#constants

[Akuli]

But the constants really aren't meant to be treated as strings, but as special sentinel objects. Literal["foo"] means that you might as well pass the string "foo", and there should be no difference.

[JelleZijlstra]

Closing per @Akuli's comment; there is also a merge conflict.