Calling built-in open() with **args

[warsaw]

I have some code that does this:

        if encoding is None:
            args = dict(mode='rb')
        else:
            args = dict(mode='r', encoding=encoding, errors=errors)
        try:
            return open(full_path, **args)   # type: ignore

If I don't include the # type: ignore comment, mypy complains:

importlib_resources/_py3.py:71: error: Argument 2 to "open" has incompatible type "**Dict[str, str]"; expected "int"
importlib_resources/_py3.py:71: error: Argument 2 to "open" has incompatible type "**Dict[str, str]"; expected "bool"

I don't know what if anything can be done about this, short of adding the ignore. I can imagine it's not so easy to support **keywords.

[ilevkivskyi]

Another possible workaround is to give an explicit type for args. For example adding this line somewhere at beginning seems to fix the problem:

args: Dict[str, Any]

maybe it is possible to give a more precise type using TypedDict. I think the problem is that mypy does not know what are the keys in args so it tries to substitute a str for every parameter of open.

[ethanhs]

Here is a minimal repro.

def f(a: str, b: str = '', c: int = 5) -> int:
    ...
args = dict(mode='rb')
f('', **args)

If you change the last argument to be eg, c: str = '', the error is no longer reported.

Unfortunately, I'm not sure there is a better alternative than requiring an annotation of args or casting, unless we improve our knowledge of the key/value pair types.

[warsaw]

Thanks. Unfortunately we also need to support Python 3.4 and 3.5 which don't have PEP 526.

I'm not sure changing that to

        if encoding is None:
            args = dict(mode='rb')                  # type: Dict[str, Any]
        else:
            args = dict(mode='r', encoding=encoding, errors=errors) # type: Dict[str, Any]

is better because it means we still have to import Dict and Any without actually using them, so then flake8 complains.

[ethanhs]

You can do

from typing import Dict, Any  # noqa
...
        args = dict()  # Dict[str, Any]
        if encoding is None:
            args = dict(mode='rb')
        else:
            args = dict(mode='r', encoding=encoding, errors=errors)

[ethanhs]

I believe mypy did the correct thing here, so I am going to close this. If you have further concerns @warsaw, feel free to comment and I'll reopen.