Importing IPython confuses mypy about package '__main__.py'

[dmcooke]

I have a package with a __main__.py in it. I discovered that if

• you run mypy in the package directory on the __main__.py
• and at some point your code (or another package's code) imports IPython
• then I get the following error:

__main__.py: error: Source file found twice under different module names: "foopy.__main__" and "__main__"
__main__.py: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#mapping-file-paths-to-modules for more info
__main__.py: note: Common resolutions include: a) adding `__init__.py` somewhere, b) using `--explicit-package-bases` or adjusting MYPYPATH
Found 1 error in 1 file (errors prevented further checking)

I should note that adding --explicit-package-bases, as suggested, does prevent this error. However, the specific set of circumstances required to trigger this seems problematic, especially as there is a top-level __init__.py.

Exact minimal steps to reproduce:

$ mkdir foo; cd foo
$ mkdir foopy

# Create the source: an empty __init__.py, and a __main__.py
# that only imports IPython.

$ touch foopy/__init__.py
$ echo 'import IPython' > foopy/__main__.py

# Running mypy in the project directory works

$ mypy foopy/__main__.py
Success: no issues found in 1 source file

# But not when in the foopy package directory

$ cd foopy
$ mypy __main__.py
__main__.py: error: Source file found twice under different module names: "foopy.__main__" and "__main__"
<...rest as above...>

The error occurs with both my system-installed mypy 1.15.0 and ipython 8.35.0 (Debian unstable), and with the current versions mypy 1.17.0 and ipython 9.4.0 installed in a new venv with pip. The Python version is 3.13.5.

Some observations:

• You have to run mypy in the package directory (this is what the flycheck package
  for emacs does, which is how I discovered/got bothered by it).
• So far, only IPython appears to trigger this.
  • importing IPython's dependencies instead doesn't trigger it
  • Other packages with a __main__.py that I've tried can be imported without issue

[emmatyping]

I believe IPython modifies the entry for __main__ in sys.modules, so I wonder if that is related?

[brianschubert]

I'm not an expert on how mypy's import resolution works, but I think this is what's going on: when you pass __main__.py on the command line, mypy uses the presense of the __init__.py file to infer that the file __main__.py corresponds to the module foopy.__main__. Later on, while following the import of IPython, mypy encounters an internal import of import __main__. To resolve this import, mypy scans its search path, which includes the directories containing the source files that were passed on the command line. Here, the search path includes the directory foopy, so __main__.py is discovered again, which mypy now maps to the top-level module __main__. The result is that the same file (foopy/__main__.py) corresponds to two different module names (foopy.__main__ and __main__), hence the error.

This doesn't seem to be unique to __main__.py or IPython; the same behavior can be reproduced if you swap __main__ with any other name. For example, here's a simpler setup that reproduces the problem:

$ mkdir foo
$ cd foo
$ touch __init__.py
$ echo 'import a' > a.py

$ mypy a.py
a.py: error: Source file found twice under different module names: "foo.a" and "a"