Importing IPython confuses mypy about package '__main__.py'

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

Riproduci il problema eseguendo mypy main.py dall’interno del pacchetto foopy, quindi confrontalo con l’esecuzione dello stesso percorso dalla directory del progetto e con --explicit-package-bases. Traccia il modo in cui mypy associa i nomi dei moduli a foopy/main.py quando viene importato IPython. Il lavoro è completato quando la riproduzione minima non segnala più nomi di moduli duplicati senza richiedere quel flag, con una copertura di regressione per i comandi mostrati.