When no .mypy_cache folder exists and a user runs
mypy --install-types --non-interactive ./, mypy may fail on fundamental
Python issues but suggest that the failure was because of "no mypy cache
directory," confusing the user.
Environment
Ubuntu 20, Python 3.6, mypy 0.910. (The old Python version reflects the setup of a production codebase where this issue was discovered.)
Confirm The Failure
Using this repo as-is, first run main.py to confirm it works.
$ python3 main.py
test
Then, run mypy.
$ mypy --version
mypy 0.910
$ mypy --install-types --non-interactive ./
error: --install-types failed (no mypy cache directory)
We can provoke success by creating an empty cache directory and running mypy again.
$ mkdir .mypy_cache
$ mypy --install-types --non-interactive ./
b/lib.py: error: Duplicate module named "lib" (also at "./a/lib.py")
b/lib.py: note: Are you missing an __init__.py? Alternatively, consider using --exclude to avoid checking one of them.
Found 1 error in 1 file (errors prevented further checking)
Address the errors, remove .mypy_cache, and run mypy again.
$ touch a/__init__.py
$ touch b/__init__.py
$ rm -r .mypy_cache/
$ mypy --install-types --non-interactive ./
main.py:2: error: Incompatible types in assignment (expression has type "str", variable has type "int")
Found 1 error in 1 file (checked 5 source files)