mypy throws an error when I have an editable installation (pip install -e .) of my library. It works fine with the non-editable installation (pip install .).

I was able to reproduce it with a toy example, so here are the files:

main.py

def something() -> None:
    print("I am something")

second.py

from my_ns.mylib.main import something


def something_else() -> None:
    something()
    print("I am something else")

pyproject.toml

[build-system]
requires = ["setuptools", "setuptools-scm"]
build-backend = "setuptools.build_meta"

[project]
name = "mylib"
requires-python = ">=3.10"
dependencies = [
    "pyyaml",
]
version = "0.1.0"


[project.optional-dependencies]
test = ["mypy",]

[tool.setuptools.packages.find]
where = ["src"]

[tool.setuptools.package-data]
"*" = ["*py.typed"]

mypy.ini

[mypy]

namespace_packages = True
explicit_package_bases = True

exclude = (?x)(
    ^tests/  # ignore everything in tests directory
    | ^test/  # ignore everything in test directory
    | ^setup.py$ # ignore root's setup.py
  )


# The following flags are equivalent to "--strict"

# Basic checks
warn_unused_configs = True
warn_redundant_casts = True
warn_unused_ignores = True
strict_equality = True
extra_checks = True
check_untyped_defs = True
disallow_untyped_calls = True
disallow_incomplete_defs = True
disallow_untyped_defs = True
no_implicit_reexport = True

# These shouldn't be too much additional work, but may require more manual "ignores" for untyped imports
disallow_subclassing_any = True
disallow_untyped_decorators = True
disallow_any_generics = True
warn_return_any = True

my_ns is a namespace package, so it does by intention not include a __init__.py (and must remain a namespace).

This is the result when running mypy 1.10.0:

$ mypy --config-file mypy.ini .
src/my_ns/mylib/main.py: error: Source file found twice under different module names: "src.my_ns.mylib.main" and "my_ns.mylib.main"
Found 1 error in 1 file (errors prevented further checking)

How can I make mypy work with an editable install and support namespace packages?