Categorical fillna with custom objects raises TypeError #21097
Description
This behavior appears new in 0.23.0.
Setup copied from #21002:
from pandas.core.base import StringMixin class County(StringMixin): name = u'San Sebastián' state = u'PR' def __unicode__(self): return self.name + u', ' + self.state cat = pd.Categorical([County() for n in range(61)]) >>> cat.fillna(cat[0]) Traceback (most recent call last): File "<stdin>", line 1, in <module> File "/usr/local/lib/python2.7/site-packages/pandas/util/_decorators.py", line 177, in wrapper return func(*args, **kwargs) File "/usr/local/lib/python2.7/site-packages/pandas/core/arrays/categorical.py", line 1769, in fillna '"{0}"'.format(type(value).__name__)) TypeError: "value" parameter must be a scalar, dict or Series, but you passed a "County" I don't see any reason why passing one of the categories to fillna should be disallowed, so it looks like the issue is with being too strict about what qualifies as a scalar.