Bug: not properly parsing native namespace package imports, reporting "incompatible type"

[jamesbraza]

I am reporting a bug where mypy doesn't seem to be able to understand native namespace packages.

The current behavior is mypy seems to confuse import paths:

• namespace.package.spam.ham.Key
• package.spam.ham.Key

#8944 is a related issue, but in this case, mypy reports an arg-type error, saying incompatible type (not Source file found twice). Please see my repro below for more details.

The expected behavior I would like is for mypy not to raise an error.

---

Repro

The package follows the native namespace package folder structure from this Python packaging guide.

Folder structure as output by tree:

.
├── namespace
│   └── package
│       ├── __init__.py
│       ├── foo
│       │   ├── __init__.py
│       │   └── bar.py
│       └── spam
│           ├── __init__.py
│           └── ham.py
├── setup.cfg
└── setup.py

setup.cfg

mypy is invoked from . in the above folder structure, via simply entering mypy (thanks to files)

[mypy]

# Specifies the Python version used to parse and check the target program.
python_version = 3.8

# Shows error codes in error messages.
show_error_codes = True

# Comma-separated list of paths which should be checked by mypy if none are
# given on the command line.
files = namespace

# Enables PEP 420 style namespace packages.
namespace_packages = True

door.py

from typing import TYPE_CHECKING

if TYPE_CHECKING:
    from typing import Optional

    from namespace.package.spam.keys import Key


class DoorWithKeyhole:
    def __init__(self):
        self.key: "Optional[Key]" = None

    def specify_key(self, key: "Key") -> None:
        self.key = key


if __name__ == "__main__":
    print("This module actually runs.")

keys.py

from abc import ABC
from typing import Sequence

from namespace.package.foo.door import DoorWithKeyhole


class Key(ABC):
    def __init__(self, compatible_doors: Sequence[DoorWithKeyhole]):
        for door in compatible_doors:
            door.specify_key(self)


if __name__ == "__main__":
    print("This module also actually runs.")

mypy output:

namespace/package/spam/ham.py:10: error: Argument 1 to "add_key" of "DoorWithKeyhole" has incompatible type "package.spam.ham.Key"; expected "namespace.package.spam.ham.Key"  [arg-type]

My temporary workaround is a type: ignore[arg-type] comment.

---

Versions

Python==3.8.5
mypy==0.782

(also current master of 0.790+dev.2b704df25ca299e6936927be8d41d2ab945cee2a)

[gvanrossum]

Seems to be because you pass a directory for the ‘files’ argument. Mypy looks inside the directory for files and it doesn’t treat it as a namespace package at that point — the PEP 420 support only works for imports.

[jamesbraza]

Ah, that sounds like it could be the problem.

Invoking mypy via: mypy namespace seems to raise the same error message. How should mypy be invoked in this case?

[gvanrossum]

Use -p.

[jamesbraza]

Using mypy -p namespace -m setup outputs: Success: no issues found in 7 source files.

Case closed, thank you @gvanrossum !! :)