Can 'confdir' arg to sphinx.application.Sphinx.__init__() be None?

[mthuurne]

Describe the bug

We ran into a problem when type-checking a Sphinx extension we're working on. mypy reports Cannot determine type of 'config' for app.config, where app is the sphinx.application.Sphinx object passed to the extension.

To Reproduce

Run mypy on the following program:

from typing import Any, Dict
from sphinx.application import Sphinx

def setup(app: Sphinx) ->  Dict[str, Any]:
    reveal_type(app.config)
    return {}

Example output:

$ mypy repro.py 
repro.py:5: error: Cannot determine type of 'config'
repro.py:5: note: Revealed type is 'Any'
Found 1 error in 1 file (checked 1 source file)

Expected behavior

The type of app.config is sphinx.config.Config and I'd expect mypy to be able to figure this out automatically.

The problem seems to be caused by this fragment in Sphinx.__init__():

        if self.confdir is None:
            self.config = Config({}, confoverrides or {})
        else:
            self.config = Config.read(self.confdir, confoverrides or {}, self.tags)

The self.confdir attribute is assigned from the confdir argument, which is annotated as having type str. Therefore mypy concludes that self.confdir is None can never be true and the first assignment to self.config is unreachable code. But as the first assignment usually determines the type of an attribute, mypy is now confused about the type of self.config.

You could argue that this is a mypy bug and I will be reporting this issue with mypy as well, if it's not already in their tracker. However, there is still an inconsistency in how Sphinx.__init__() handles confdir and how confdir is annotated, so there is an opportunity to improve Sphinx by either removing the code that handles confdir is None or changing the argument annotation to confdir: Optional[str].

Your project

Original PR in which the problem was encountered.

Environment info

• OS: Linux (openSUSE Tumbleweed)
• Python version: 3.8
• Sphinx version: 3.3.0
• Sphinx extensions: n/a
• Extra tools: mypy 0.790

[mthuurne]

Related issue in mypy.

[tk0miya]

The confdir argument of the Sphinx class is surely optional. This is one of the usecase.

sphinx/sphinx/cmd/build.py

Line 209 in 249778e

args.confdir = None

So I'll modify the type annotation of it soon. Thanks.