Using type stubs through mypy installed via PDM?

[twoev]

Hi,

We are using PDM version 1.8.1 with Python 3.7 (we have also experimented with Python 3.9, where we have the same issue). We are also using Mypy, installed and steered through PDM, to type check our Python code.

The problem we are having starts with Mypy being unable to locate the type stub packages for external libraries, for example jsonschema needs an additional types-jsonschema package to be installed. We have added these type stub packages with pdm, and confirm they are present in the __pypackages__ structure. In order to find type stubs installed by pdm, my understanding is that the mypy_path config option needs to be added to Mypy. On adding this to the [tool.mypy] section of the pyproject.toml file Mypy does seem to include the pdm installations in its search path, but unfortunately mypy now fails with the error

mypy: "__pypackages__/3.9/lib/typing_extensions.py" shadows library module "typing_extensions"
note: A user-defined top-level module with name "typing_extensions" is not supported

The typing_extensions module is in fact required by Mypy and gets installed by PDM as part of the Mypy installation. Mypy then finds its own dependency and (incorrectly) assumes there is a name clash because it thinks the typing_extensions module it found is a user module.

The general problem seems to be that Mypy expects to be installed globally and doesn't play nicely with the local PDM installation inside __pypackages__. There is a proposed patch to Mypy for this, but it seems like the devs are not going to accept it

python/mypy#10833

I'm hoping the PDM experts have experienced this issue and maybe have a way around it for the time being?

Thanks for your help

James

[pawamoy]

Related mypy feature request: python/mypy#10633

[twoev]

Thanks. Does this mean there is nothing that can be done until mypy is fixed? Like you I have tried making symlinks and pointing my_path at the symlinks, but it doesn't really work.

[pawamoy]

In fact it does work, I was not using the right variable name: MYPYPATH. I've posted some Python code to automate it before mypy runs.

[twoev]

Instead of using the MYPYPATH environment variable, we set mypy_path in the tool.mypy section. Mypy is responding to that config change because we can see its behaviour changes. Unfortunately, with the symlinks mypy seems unable to reliably follow all the import paths and complains about a bunch of missing definitions in standard packages.

[pawamoy]

Ah yes, I have to ignore the errors for packages that are not mine indeed. Well, this is definitely a hack 😅
We'll have to wait on mypy to bring proper support 😕