Cartopy 0.18.0 USCOUNTIES '20m' invalid

[ahuang11]

metpy/plots/cartopy_utils.py

USCOUNTIES = MetPyMapFeature('us_counties', '20m', facecolor='None', edgecolor='black')
ValueError: 20m is not a valid Natural Earth scale. Valid scales are "110m", "50m", and "10m".

[jthielen]

This looks to be the same issue that caused CI to fail in #1346.

@dopplershift I've tagged it as 1.0 for now, but would this warrant a 0.12.2 as well?

[dopplershift]

Is this a full-out import failure? I haven't quite tried it out yet.

[jthielen]

Is this a full-out import failure? I haven't quite tried it out yet.

Based on when I tested it out quick, it is an import failure in metpy.plots, which further causes any of the projection helpers on the xarray accessors to fail.

I think this means that MetPy's MetPyMapFeature can no longer inherit from cartopy.feature.NaturalEarthFeature, since it really isn't a Natural Earth feature (in that it has differing scales of '500k', '5m', and '20m' instead of '10m', '50m', and '110m'), and instead needs to inherit directly from cartopy.feature.Feature and copy the other methods? Or can we monkey patch the new _validate_scale() method (although that seems risky given that it is private API)?

xref SciTools/cartopy#1506

[jthielen]

One complication I ran across when trying to implement a fix: Cartopy is LGPL licensed, so we can't directly copy/reuse any of its code in MetPy itself.

[jthielen]

Not sure if this is too hacky, but I was able to fix this (without copying any cartopy code) by modifying MetPyMapFeature.__init__() to

def __init__(self, name, scale, **kwargs):
    """Create USCountiesFeature instance."""
    super(cfeature.NaturalEarthFeature, self).__init__(ccrs.PlateCarree(), **kwargs)

    self.category = ''
    self.name = name
    self.scaler = (
        cfeature.Scaler(scale) if not isinstance(scale, cfeature.Scaler) else scale
    )

[dopplershift]

sigh I hate everything. 😭 🤣

I'd be willing to put that "fix" in for 0.12.2. I think for master we should just implement something ourselves, relying on CartoPy's shapefile-reading functionality.

[AdamTheisen]

@dopplershift Not sure if it matters, but just wanted to +1 for the quick fix for 0.12.2. Just went to do a release on our software (act-atmos) but ran into the same error on conda-forge feedstock. I'd be happy to help out in any way.