I hit an issue today with a

ModuleNotFoundError

for

flask_compress

while using Dash.

ImportError: To use the compress option, you need to install dash[compress]

I added an override for the 3rd party optional dependency

python_requirements(
  name="reqs",
  overrides={
    "dash": {
      "dependencies": [":reqs#Flask-Compress"],      
    }
  }
)

but it still wasn't working. Turns out I had both

flask-compress=1.13

and

Flask-Compress

in requirements.txt. After removing the latter, it works. It works with just one or the other, or both with versions specified, but not in this first scenario. I'm not sure why. I'm mainly posting for anyone else that searches for this specifically in future, but curious if Pants can do anything to better help here or if it's a bug.

TIL pip is case-insensitive. I think the error might have to do with ambiguous dependency ownership. When I try adding both

flask-compress==1.13

and

Flask-Compress

to a requirements.txt, and

import flask_compress

, I get the ambiguity warning below, and then Pants fails because it can't determine an owner.

[WARN] The target testprojects/src/python/sources/sources.py imports `flask_compress`, but Pants cannot safely infer a dependency because more than one target owns this module, so it is ambiguous which to use: ['testprojects/src/python:reqs#Flask-Compress', 'testprojects/src/python:reqs#flask-compress'].

If instead the import exists in a "weak" form (for example, inside of a try-except ImportError like would happen with a package's extra) then Pants will not mandate that a unique owner can be found (because you might not be using that extra). Unfortunately, the warning can be cached and not re-emitted, which is be frustrating. There's the source-analysis extra, which is a lot but really details the inner thoughts of Pants https://www.pantsbuild.org/docs/reference-python-dump-source-analysis . Although it looks like the weak-ignore took precedence over the ambiguous resolution

I did notice that cached warnings disappear which is a bit annoying, but I was also invalidating the cache when requirements change, and not seeing anything. Hmm. When I run with

pants --no-local-cache test

I don't get the import error. When I run with cache I do. I don't see flask-compress in the source analysis dump in any case.

oh, right, of course. Pants won't analyse the source of the dependencies, it uses the requirements for them. not sure why --no-local-cache would not give the error, though...