PEX warns about missing `setuptools` for `matplotlib` but it seems like it's not required

PEX warns about missing setuptools for matplotlib:

$ pip install pex
$ pex --version
2.1.92
$ pex matplotlib==3.5.2
/private/var/folders/t0/zb49xf010cq4mqzwwxw8f31h0000gp/T/tmpbwxjl1xt/.bootstrap/pex/environment.py:657: PEXWarning: The `pkg_resources` package was loaded from a pex vendored version when declaring namespace packages defined by:

1. matplotlib==3.5.2 namespace packages:
  mpl_toolkits

These distributions should fix their `install_requires` to include `setuptools`
  "should fix their `install_requires` to include `setuptools`".format(dists=dists)
Python 3.7.9 (default, Mar 12 2021, 20:11:13)
[Clang 12.0.0 (clang-1200.0.32.29)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
(InteractiveConsole)
>>>

However, a docker container without setuptools is able to install matplotlib and import from the namespace package mpl_toolkits:

$ cat Dockerfile
FROM ubuntu:18.04

# install python
RUN apt-get update && \
    apt-get install --no-install-recommends -y \
    python3.7 python3-pip python3.7-dev

RUN python3.7 -m pip install -U pip
RUN python3.7 -m pip install matplotlib
$ docker build -t check-container .
...
$ docker run --rm check-container pip list
Package           Version
----------------- -------
cycler            0.11.0
fonttools         4.33.3
kiwisolver        1.4.3
matplotlib        3.5.2
numpy             1.21.6
packaging         21.3
Pillow            9.1.1
pip               22.1.2
pyparsing         3.0.9
python-dateutil   2.8.2
six               1.16.0
typing_extensions 4.2.0
$ docker run --rm check-container python3.7 -c 'from mpl_toolkits import mplot3d; print(dir(mplot3d))'
['Axes3D', '__builtins__', '__cached__', '__doc__', '__file__', '__loader__', '__name__', '__package__', '__path__', '__spec__', 'art3d', 'axes3d', 'axis3d', 'proj3d']

I should acknoledge that without upgrading pip in the Dockerfile, it uses pip version 9, which is unable to install matplotlib because numpy and pillow fail to build and complain about missing setuptools and gcc. However with pip version 20.0.3+ there aren’t any issues.

[jsirois]

I think you should silence Pex in this case. As I pointed out, matplotlib wraps the namespace declaration in a try/except. Any import wrapped like that is, by definition, optional - the code author knows how to deal with the missing import / package it comes from. As such, they need not declare the dependency as an install_requires since they just opportunistically use it if present.

[georgipopovhs]

Thanks!