ExtensionArray.astype docstring

[andrewgsavage]

pandas/pandas/core/arrays/base.py

Line 324 in 6d3565a

def astype(self, dtype, copy=True):

Should the docstring say that this should return an ExtensionArray when an ExtensionType is passed into it?
I note that PeriodArray behaves like this.

import pandas as pd
a=pd.core.arrays.period_array([2000, 2001, 2002], freq='D')
type(a.dtype)

pandas.core.dtypes.dtypes.PeriodDtype

type(a.astype(a.dtype))

pandas.core.arrays.period.PeriodArray

[TomAugspurger]

Yes, it should return an ExtensionArray when passed an ExtensionDtype.

This ends up making .astype quite complicated in practice. Pandas should provide some helpers here (though we had an issue for that, but couldn't quickly find one).

In the meantime, the EA docs / code should recommend a few things

1. Use pandas_dtype(dtype) to convert strings like Period[D] to the ExtensionDtype or numba dtype
2. For extension types, recommending dtype.construct_array_type()._from_sequence may make sense, but I'm not sure. That's supposed to take sequences of the scalar, so in theory it'll only work when you have to different ExtensionDtypes for the same scalar type. Not sure how common that'll be.