Using Mypy with `.pyi` stub files

Question

Lets say I have a simple function inside the file home/func.py, and I make one call to it.

def f(x):
   return x

print(f("example"))

Now lets say I want to type check this function using a stub, home/stubs/func.pyi which looks like:

def f(x: int): ...

Now I want to check my code with mypy to see if I've made any typing mistakes. Running mypy on home/func.py I would expect an warning/error like: Got string where int was expected - however I can't get this functionality to work.

What I've read online and in the docs suggests I should create a home/mypy.ini config file with the following contents:

[mypy]
mypy_path = $MYPY_CONFIG_FILE_DIR/stubs

and then I should run mypy while pointing to this config, so something like: python -m mypy --config mypy.ini func.py. This has not worked.

I know I could use typing annotations in the function definition but in this case the use of stubs is non-optional. Any help is really really appreciated.

Answer 1

To this day (Python 3.10 and MyPy 0.982), as you, I haven't been able to separate stubs from code and make it work in my projects.

For now I either put the annotations in the code or use the alternatives (such as pytype, pyright, Pyre).

I hope someone else to correct me, or maybe understand how is it done in others projects (such as Panda, or Django).

A few hints to understand

Quoting Pragy Agarwal

This makes stub files a way of "lying" to mypy.

Is there a way to keep the code and type info in separate files (for readability) while still being able to check code against the type info?

Which, in turn, reference the yet unclosed issue "Test stub files against real code".

Out of context quote from the BDFL:

If you want type annotations, put then in main.py and get rid of main.pyi

Related question: How to point to custom stubs directory in mypy via MYPY environment variable or mypy_path variable