series.dt.tz_localize() on Categorical operates on categories, not values

[adamhooper]

Code Sample, a copy-pastable example if possible

datetimes = pd.Series(['2019-01-01', '2019-01-01', '2019-01-02'], dtype='datetime64[ns]')
categorical = datetimes.astype('category')
categorical.dt.tz_localize(None)

Produces:

0   2019-01-01
1   2019-01-02
dtype: datetime64[ns]

Problem description

.dt.tz_localize() is operating on categorical.cat.categories. It should be operating on categorical.astype('datetime64[ns]').values. This is just plain wrong.

Expected Output

According to Categorical docs, "The returned Series (or DataFrame) is of the same type as if you used the .str. / .dt. on a Series of that type (and not of type category!).". So I think the expected value to be:

>>> datetimes.dt.tz_localize(None)
0   2019-01-01
1   2019-01-01
2   2019-01-02
dtype: datetime64[ns]

Output of pd.show_versions()

INSTALLED VERSIONS ------------------ commit : None python : 3.7.2.final.0 python-bits : 64 OS : Linux OS-release : 5.2.8-200.fc30.x86_64 machine : x86_64 processor : byteorder : little LC_ALL : None LANG : C.UTF-8 LOCALE : en_US.UTF-8

pandas : 0.25.0
numpy : 1.17.0
pytz : 2019.2
dateutil : 2.8.0
pip : 19.0.2
setuptools : 40.8.0
Cython : None
pytest : None
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : 4.3.0
html5lib : 1.0.1
pymysql : None
psycopg2 : 2.8.3 (dt dec pq3 ext lo64)
jinja2 : None
IPython : None
pandas_datareader: None
bs4 : 4.7.1
bottleneck : None
fastparquet : 0.3.2
gcsfs : None
lxml.etree : 4.3.0
matplotlib : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : None
pytables : None
s3fs : None
scipy : None
sqlalchemy : None
tables : None
xarray : None
xlrd : 1.2.0
xlwt : None
xlsxwriter : None

[mroeschke]

The line that needs fixing is here:

pandas/pandas/core/indexes/accessors.py

Line 327 in 9f93d57

data = Series(orig.values.categories, name=orig.name, copy=False)

PR's welcome!

[jbrockmendel]

@mroeschke are you sure thats the problem? I think that might be a problem, but it looks like _delegate_method might be doing something weird too

[mroeschke]

When I tried debugging quickly that's the first spot where the behavior was incorrect. You're probably right in that the behavior is also incorrect up/down stream somewhere