scatter plotting tests fail for recent matplotlib version

[jkikstra]

With matplotlib version 3.1.1 installed, some pyam plotting tests fail.
More specifically, test_scatter, test_scatter_variables_with_meta_color, test_scatter_with_lines, and test_scatter_meta fail, because of IndexError: list index our of range, pyam\plotting.py:605
when:

pyam\core.py:1293: in scatter
    ax = plotting.scatter(df.dropna(), x, y, **kwargs)
pyam\plotting.py:605: in scatter
    handles = [handles[i] for i in idxs]

With matplotlib version 3.0.2 all of the aforementioned tests pass.

[khaeru]

The "quickfix" for this in 7d6b7ef / #280 broke message_ix builds.

The last build for iiasa/message_ix#249, Travis build #1167, succeeded; but build #1180 failed just now on OS X with the following error caused by pyam:

___________________ ERROR collecting tests/test_reporting.py ___________________
ImportError while importing test module '/Users/travis/build/iiasa/message_ix/tests/test_reporting.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
tests/test_reporting.py:15: in <module>
    import pyam
../../../miniconda/envs/testing/lib/python3.7/site-packages/pyam/__init__.py:1: in <module>
    from pyam.core import *
../../../miniconda/envs/testing/lib/python3.7/site-packages/pyam/core.py:16: in <module>
    from pyam import plotting
../../../miniconda/envs/testing/lib/python3.7/site-packages/pyam/plotting.py:4: in <module>
    import matplotlib.pyplot as plt
../../../miniconda/envs/testing/lib/python3.7/site-packages/matplotlib/pyplot.py:2374: in <module>
    switch_backend(rcParams["backend"])
../../../miniconda/envs/testing/lib/python3.7/site-packages/matplotlib/pyplot.py:207: in switch_backend
    backend_mod = importlib.import_module(backend_name)
../../../miniconda/envs/testing/lib/python3.7/importlib/__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
../../../miniconda/envs/testing/lib/python3.7/site-packages/matplotlib/backends/backend_macosx.py:14: in <module>
    from matplotlib.backends import _macosx

E   ImportError: Python is not installed as a framework. The Mac OS X backend will not be able to function correctly if Python is not installed as a framework. See the Python documentation for more information on installing Python as a framework on Mac OS X. Please either reinstall Python as a framework, or try one of the other backends. If you are using (Ana)Conda please install python.app and replace the use of 'python' with 'pythonw'. See 'Working with Matplotlib on OSX' in the Matplotlib FAQ for more information.

Looking at the build log, this failure happens with matplotlib 3.0.2; the previous, successful build was using matplotlib 3.1.1. #280 was merged in between these two builds.

I will add a workaround to message_ix by forcing matplotlib to again update to the latest after pyam downgrades it.

IMHO, the actual fix should be to adjust pyam to work properly with the latest matplotlib; not to force any other software using pyam to run an outdated matplotlib.

[khaeru]

A few more thoughts about the failure mode here:

1. The message:

   Python is not installed as a framework. The Mac OS X backend will not be able to function correctly if Python is not installed as a framework. See the Python documentation for more information on installing Python as a framework on Mac OS X. Please either reinstall Python as a framework, or try one of the other backends. If you are using (Ana)Conda please install python.app and replace the use of 'python' with 'pythonw'. See 'Working with Matplotlib on OSX' in the Matplotlib FAQ for more information.

   …mentions the conda package python.app.

2. python.app is not mentioned as a conda dependency of pyam.

3. However, inspecting the pyam build logs (e.g. of release candidate 0.3.0 #282) shows that it does get installed on Travis/macOS.

4. Thus, it must be a conda dependency of some conda dependency of pyam.

5. Because message_ix is currently working around Bug fix release #259 (or hotfix for pandas 0.25.0 update, closes #245 #246 I guess), pyam is not installed via conda, but from source.

6. Thus the setup.py dependencies of pyam are installed from PyPI—not the conda dependencies…

7. …and so python.app does not get installed (in contrast to point 3), and eventually the build fails.

When the next release comes out, the workaround in point 5 will get reverted, so point 6/7 cease to happen, python.app will likely get pulled in for message_ix builds, and they may pass even with matplotlib 3.0.2. However, the suggested fix in my previous comment would still be preferred.

[awais307]

The pyam requirement for an outdated (3.0.2) version of matplotlib is causing difficulties on my system, where another code requires the latest matplotlib. So I am forced to have two conda environments; one for any code using pyam and other one requiring latest matplotlib version.

[znicholls]

A possible quickfix for the build failures discussed by @khaeru could be to tell matplotlib to use a different backend (e.g. https://github.com/znicholls/silicone/blob/2e64d8a8a20b00ec545b280e7e3253feaa51761e/.github/workflows/test-pypi-install.yml#L23)

[znicholls]

IMHO, the actual fix should be to adjust pyam to work properly with the latest matplotlib; not to force any other software using pyam to run an outdated matplotlib.

Although I fully agree with this (but acknowledge that anything to do with plotting and plotting tests is super hard).