mypy: different error message for the same missing package

Question

basically, with flask_api and flask_sqlalchemy installed (via pip3), if I run mypy on this

import flask_sqalchemy
import flask_api

the error message is this

testMypy.py:1: error: No library stub file for module 'flask_sqlalchemy'
testMypy.py:1: note: (Stub files are from https://github.com/python/typeshed)
testMypy.py:2: error: Cannot find module named 'flask_api'
testMypy.py:2: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports

now, I do know that as of Jan/19/2019, flask_sqlalchemy and flask_api doesn't have stub files in the typeshed (yet), but I think if both modules are installed in the system, the error messages should both be "No library stub file for module ..", what are the difference between these two?

Answer 1

The two error messages mean the same thing. The only difference is that you get the first error message if the module you're importing is considered to be a "popular" third party library -- specifically, if it's one of the modules in this list. In this case, flask_sqlalchemy is a member of that list, but flask_api isn't.

The reason why mypy special-cases these modules is largely for usability: it's somewhat confusing to get an error message like "Cannot find module named 'blah'" when you've seemingly installed it via pip. So, it special-cases 3rd party libraries that are likely to be commonly used so it can at least improve the user experience there.

A follow-up question you might have is "why doesn't mypy just look at what's pip-installed and use the first error message whenever you try importing anything that's pip-installed?". After all, mypy needs to do this anyways to try and find PEP 561 compatible packages -- packages that declare they come bundled with type hints.

Well, mypy probably could be doing that -- but this section of the codebase was added before PEP 561 was a thing (before mypy really needed to be scanning pip-installed packages), and I suspect nobody's really thought about this corner of the codebase for a long time.