In a Python file, my first line is:

from flask import Flask

Which triggers an error in the gutter that shows the following message when I drag the cursor over that line:

Cannot find implementation or library stub for module named "flask"

I have installed Flask with Pipenv in a virtual environment. Then I have activated the virtual environment. Running Flask from there works so I assume I have installed it correctly.

I also run Vim from that activated virtual environment:

Running :py3 import sys, site; print('Version:', sys.version); print('Executable:', sys.executable); print('Site Packages:', site.getsitepackages()) returns:

Version: 3.9.6 (default, Jun 30 2021, 10:22:16)                                                                                                                                                                                                              
[GCC 11.1.0]                                                                                                                                                                                                                                                 
Executable: /home/bastien/.local/share/virtualenvs/flask-hxySx92r/bin/python3                                                                                                                                                                                
Site Packages: ['/home/bastien/.local/share/virtualenvs/flask-hxySx92r/lib/python3.9/site-packages'] 

So I understand Vim is correctly running from the virtual environment.

Installing Flask globally and running Vim again does not trigger that error. So I assume the package from the virtual environment cannot be found.

I use ALE, but after investigating, I'm not sure anymore what is responsible for printing those messages.

Also, youcompleteme stops working when running Vim from the virtual environment. I have set the full python path though.

Any help is appreciated.

  • Is ALE running mypy? Is that installed in the virtualenv and detected in path earlier than a global mypy?
    – D. Ben Knoble
    Jul 28, 2021 at 13:41
  • ALE is running Black. I have tried to install Black in the virtual environment, outside it and to totally uninstall it. The result is the same, the error message is always there.
    – Bastien
    Jul 28, 2021 at 15:42
  • Huh. I didn't think Black checked for stubs… thought it only formatted… must be out of my experience then. (You're sure? :ALEInfo, for one, though it doesn't consistently show me my LSPs running)
    – D. Ben Knoble
    Jul 28, 2021 at 16:40
  • @D.BenKnoble :ALEInfo returns that mypy is in the list of available linters. So I tried to install it also inside the virtualenv and indeed it works now. Thanks! Would you like to make it a full answer?
    – Bastien
    Jul 28, 2021 at 16:41
  • Although this solves my current issue, I don't really see the value of installing a tool like that inside the virtualenv. I use it on all my projects. To me it's a bit like installing vim inside each project, I don't get what are the benefits.
    – Bastien
    Jul 28, 2021 at 16:47

1 Answer 1


As we sussed out in the comments, this is from ALE+mypy, which needs to be installed in the virtual-env of the project.

Why? I'm not 100% sure, but I suspect mypy is simply ignorant of virtual-environments. It probably uses its installation location to find modules (possibly via the file-system; more likely via dynamic import/eval wizardry).

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