ModuleNotFoundError: `matplotlib._contour` with matplotlib v3.6.0

[khaeru]

With matplotlib v3.6.0 (just released) and plotnine 0.9.0, I see the following (in this GitHub Actions job):

Traceback:
/opt/hostedtoolcache/Python/3.10.6/x64/lib/python3.10/importlib/__init__.py:126: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
genno/tests/compat/test_plotnine.py:5: in <module>
    import plotnine as p9
/opt/hostedtoolcache/Python/3.10.6/x64/lib/python3.10/site-packages/plotnine/__init__.py:1: in <module>
    from .qplot import qplot            # noqa: F401
/opt/hostedtoolcache/Python/3.10.6/x64/lib/python3.10/site-packages/plotnine/qplot.py:9: in <module>
    from .ggplot import ggplot
/opt/hostedtoolcache/Python/3.10.6/x64/lib/python3.10/site-packages/plotnine/ggplot.py:18: in <module>
    from .facets import facet_null
/opt/hostedtoolcache/Python/3.10.6/x64/lib/python3.10/site-packages/plotnine/facets/__init__.py:1: in <module>
    from .facet_grid import facet_grid
/opt/hostedtoolcache/Python/3.10.6/x64/lib/python3.10/site-packages/plotnine/facets/facet_grid.py:6: in <module>
    from .facet import facet, layout_null, combine_vars, add_missing_facets
/opt/hostedtoolcache/Python/3.10.6/x64/lib/python3.10/site-packages/plotnine/facets/facet.py:14: in <module>
    from ..scales.scales import Scales
/opt/hostedtoolcache/Python/3.10.6/x64/lib/python3.10/site-packages/plotnine/scales/__init__.py:83: in <module>
    from .limits import xlim, ylim, lims, expand_limits
/opt/hostedtoolcache/Python/3.10.6/x64/lib/python3.10/site-packages/plotnine/scales/limits.py:8: in <module>
    from ..geoms import geom_blank
/opt/hostedtoolcache/Python/3.10.6/x64/lib/python3.10/site-packages/plotnine/geoms/__init__.py:6: in <module>
    from .geom_abline import geom_abline
/opt/hostedtoolcache/Python/3.10.6/x64/lib/python3.10/site-packages/plotnine/geoms/geom_abline.py:9: in <module>
    from .geom import geom
/opt/hostedtoolcache/Python/3.10.6/x64/lib/python3.10/site-packages/plotnine/geoms/geom.py:3: in <module>
    from ..stats.stat import stat
/opt/hostedtoolcache/Python/3.10.6/x64/lib/python3.10/site-packages/plotnine/stats/__init__.py:7: in <module>
    from .stat_density_2d import stat_density_2d
/opt/hostedtoolcache/Python/3.10.6/x64/lib/python3.10/site-packages/plotnine/stats/stat_density_2d.py:3: in <module>
    import matplotlib._contour as _contour
E   ModuleNotFoundError: No module named 'matplotlib._contour'

More info:

• Per the release notes, matplotlib's C code for this has been split off to a separate package contourpy, which is now a requirement of matplotlib.
• The code uses a factory method to create the contour generator object: https://github.com/matplotlib/matplotlib/blob/5c4595267ccd3daf78f5fd05693b7ecbcd575c1e/lib/matplotlib/contour.py#L1437-L1441

[khaeru]

I almost started a PR, but I'm not sure exactly what the practice of plotnine is in a case like this:

• If it's not desired to maintain compatibility with older matplotlib, plotnine can simply import contourpy directly.
• If it is, then I guess some more complicated try/except checking would be necessary.

[khaeru]

Many thanks for the quick fix!

[tacaswell]

@has2k1 What could we have done to have caught this earlier?