BUG: Multiplication of two serieses changes the the timezone from the given serieses

[eyjay-ok]

(since this is my first bug report I am happy for any feedback :) )

import pandas as pd

cet_idx = pd.date_range(start=pd.to_datetime('today').normalize(), periods=10, freq='15min', tz='CET')
utc_idx = cet_idx.tz_convert('utc')

cet_series = pd.Series(data=2, index=cet_idx)
utc_series = pd.Series(data=2, index=utc_idx)

print(cet_series.index.tz)
>> <DstTzInfo 'CET' CET+1:00:00 STD>

out_series = utc_series * cet_series   # The output is irrelevant 

print(cet_series.index.tz)
>> <UTC>

Problem description

The timezone of the series 'cet_series' changes by the multiplication. Since it is only an input series the timezone should stay the same.

Expected Output

... # the same initialization as above

print(cet_series.index.tz)
>> <DstTzInfo 'CET' CET+1:00:00 STD>

utc_series * cet_series   # The output is irrelevant 

print(cet_series.index.tz)
>> <DstTzInfo 'CET' CET+1:00:00 STD>

Output of pd.show_versions()

INSTALLED VERSIONS

commit : None
python : 3.8.2.final.0
python-bits : 64
OS : Linux
OS-release : 5.3.0-46-generic
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 1.0.1
numpy : 1.18.1
pytz : 2019.3
dateutil : 2.8.1
pip : 19.2.3
setuptools : 41.2.0
Cython : None
pytest : None
hypothesis : None
sphinx : 2.4.4
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 2.11.1
IPython : 7.13.0
pandas_datareader: None
bs4 : None
bottleneck : None
fastparquet : None
gcsfs : None
lxml.etree : None
matplotlib : None
numexpr : None
odfpy : None
openpyxl : 3.0.3
pandas_gbq : None
pyarrow : None
pytables : None
pytest : None
pyxlsb : None
s3fs : None
scipy : None
sqlalchemy : 1.3.13
tables : None
tabulate : None
xarray : None
xlrd : None
xlwt : None
xlsxwriter : None
numba : None

[TomAugspurger]

Thanks for the report. Confirmed on master.

@eyjay-ok do you know if this happened in pandas 0.25?

[eyjay-ok]

Yes, it was the same behavior in Pandas 0.25.1
I also tested it in pandas 0.24.0 and it was also the same (the output was a bit different but still the timezone was changed to UTC)

[mroeschke]

Thanks for the report.

This is the expected behavior. Please see https://pandas.pydata.org/pandas-docs/stable/user_guide/timeseries.html#working-with-time-zones

Operations between Series in different time zones will yield UTC Series, aligning the data on the UTC timestamps:

[TomAugspurger]

I think the surprising behavior is that one of the operands (cet_series) is mutated so that it's index changes from CET to UTC.

[mroeschke]

Ah that is really weird behavior. Yes, that definitely looks like a bug. Sorry for reading the issue too quickly.

[mroeschke]

This fixes the issue locally:

(pandas-dev) matthewroeschke:pandas-mroeschke matthewroeschke$ git diff
diff --git a/pandas/core/ops/__init__.py b/pandas/core/ops/__init__.py
index 9a7c9fdad..048eb6cc5 100644
--- a/pandas/core/ops/__init__.py
+++ b/pandas/core/ops/__init__.py
@@ -380,7 +380,7 @@ def _align_method_SERIES(left, right, align_asobject=False):
                 left = left.astype(object)
                 right = right.astype(object)

-            left, right = left.align(right, copy=False)
+            left, right = left.align(right, copy=True)

     return left, right

Not sure if always copying is necessary though. Maybe we should set copy=True for Extension Array indexes?

[phofl]

Not sure if always copying is necessary though. Maybe we should set copy=True for Extension Array indexes?

We could set copy=True if the indices at this point:

https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L8486-8491

are not equal. (else block). This works locally.