BUG: enlarging a DataFrame by adding an extra column with a tz-aware datetime results in object dtype

[MarcoGorelli]

Pandas version checks

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• I have confirmed this bug exists on the main branch of pandas.

Reproducible Example

import pandas as pd

import datetime as dt

df = pd.DataFrame([{'id': 1},{'id': 2},{'id': 3}])
_time = dt.datetime.utcfromtimestamp(1695887042)
_time = _time.replace(tzinfo=dt.timezone.utc)
df.loc[df.id>=2, 'time'] = _time
print(df)
print(df.dtypes)

Issue Description

the above outputs

   id                       time
0   1                        NaT
1   2  2023-09-28 07:44:02+00:00
2   3  2023-09-28 07:44:02+00:00
id       int64
time    object
dtype: object

Expected Behavior

it would be nice to get to

   id                      time
0   1                       NaT
1   2 2023-09-28 07:44:02+00:00
2   3 2023-09-28 07:44:02+00:00
id                    int64
time    datetime64[ns, UTC]
dtype: object

right away

Installed Versions

[1/1] Generating write_version_file with a custom command

• /home/marcogorelli/.virtualenvs/pandas/bin/ninja

INSTALLED VERSIONS

commit : 99efe62
python : 3.10.12.final.0
python-bits : 64
OS : Linux
OS-release : 5.10.102.1-microsoft-standard-WSL2
Version : #1 SMP Wed Mar 2 00:30:59 UTC 2022
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : en_GB.UTF-8
LOCALE : en_GB.UTF-8

pandas : 2.1.0rc0+99.g99efe62afd.dirty
numpy : 1.24.4
pytz : 2023.3.post1
dateutil : 2.8.2
setuptools : 59.6.0
pip : 23.2.1
Cython : 0.29.33
pytest : 7.4.2
hypothesis : 6.86.2
sphinx : 6.2.1
blosc : 1.11.1
feather : None
xlsxwriter : 3.1.4
lxml.etree : 4.9.3
html5lib : 1.1
pymysql : 1.4.6
psycopg2 : 2.9.7
jinja2 : 3.1.2
IPython : 8.15.0
pandas_datareader : None
bs4 : 4.12.2
bottleneck : 1.3.7
dataframe-api-compat: None
fastparquet : 2023.8.0
fsspec : 2023.9.1
gcsfs : 2023.9.1
matplotlib : 3.7.3
numba : 0.57.1
numexpr : 2.8.6
odfpy : None
openpyxl : 3.1.2
pandas_gbq : None
pyarrow : 13.0.0
pyreadstat : 1.2.3
python-calamine : None
pyxlsb : 1.0.10
s3fs : 2023.9.1
scipy : 1.11.2
sqlalchemy : 2.0.21
tables : 3.8.0
tabulate : 0.9.0
xarray : 2023.8.0
xlrd : 2.0.1
zstandard : 0.21.0
tzdata : 2023.3
qtpy : None
pyqt5 : None
None

[jbrockmendel]

the title suggests the issue is about the Series constructor, but the example seems to be about setitem-with-expansion. Is the example minimal?

[MarcoGorelli]

thanks, have updated