Coverage increased (+0.05%) to 85.443% when pulling ec19105 on jorisvandenbossche:deprecate-custom-attributes into 8eaac9d on Toblerity:master.

@sgillies @mwtoews you're fine with this additional deprecation?

The only thing we can't warn against is users that extend geometry types:

from shapely.geometry import Point

class MyPoint(Point):
    extra = 123

mp = MyPoint(2, 4)
print(mp.extra)  # 123
mp.extra = 456
print(mp.extra)  # 456

I suppose that in theory we could also add a warning when subclassing the geometry classes (with a metaclass, see eg https://stackoverflow.com/questions/16056574/how-does-python-prevent-a-class-from-being-subclassed), but not directly sure how that impacts other things.

I am going to interpret the thumbs up of @sgillies at #1181 as an approval of the general idea, and merge this.