mypy errors with suggested tomli/tomllib compatibility layer

[JonathonReinhart]

Hello, I'm attempting to write forward-looking code which uses the documented tomli/tomllib compatibility layer:

try:
    import tomllib
except ModuleNotFoundError:
    import tomli as tomllib

result = tomllib.loads("")
reveal_type(result)

When I check this code with mypy, I am met with the following errors:
(Python 3.10.9, mypy 0.991)

$ mypy example.py
example.py:2: error: Cannot find implementation or library stub for module named "tomllib"  [import]
example.py:2: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
example.py:4: error: Name "tomllib" already defined (possibly by an import)  [no-redef]
example.py:7: note: Revealed type is "Any"
Found 2 errors in 1 file (checked 1 source file

• Line 2: The tomllib import error is expected since I am running Python 3.10 (and it was added to the standard lib in 3.11). This is resolved by ignoring the import error.

• Line 4: The Name "tomllib" already defined error is resolved by upgrading to mypy 1.2.0 or ingoring the redef error.

try:
    import tomllib  # type: ignore[import]
except ModuleNotFoundError:
    import tomli as tomllib  # type: ignore[no-redef]

result = tomllib.loads("")
reveal_type(result)

$ mypy example.py
example.py:7: note: Revealed type is "Any"
Success: no issues found in 1 source file

• Line 7: This is the unfortunate part. When using the prescribed pattern, tomllib is Any, so all functions and their return values are Any as well.

When the compatibility layer is not used, the typing is correctly provided (presumably via typeshed, although I can't find it?)

import tomli
reveal_type(tomli)

result = tomli.loads("")
reveal_type(result)

$ mypy example_nolayer.py
example_nolayer.py:2: note: Revealed type is "types.ModuleType"
example_nolayer.py:5: note: Revealed type is "builtins.dict[builtins.str, Any]"

---

Is this a limitation of mypy that we have to live with, or can the pattern be somehow updated to resolve these issues?

[hauntsaninja]

Try:

if sys.version_info >= (3, 11):
    import tomllib
else:
    import tomli as tomllib

[encukou]

This sounds like a bug in mypy: python/mypy#13914