BUG: mixture of timezone-aware string and timezone-naive datetime treated differently for ISO vs non-ISO paths

[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

# ISO:
In [1]: pd.to_datetime(["2020-01-01 01:00:00-01:00", datetime(2020, 1, 1, 3, 0)], format='%Y-%m-%d %H:%M:%S%z')
Out[1]: DatetimeIndex(['2020-01-01 01:00:00-01:00', '2020-01-01 02:00:00-01:00'], dtype='datetime64[ns, UTC-01:00]', freq=None)

# non-ISO:
In [2]: pd.to_datetime(["2020-01-01 01:00:00-01:00", datetime(2020, 1, 1, 3, 0)], format='%Y-%d-%m %H:%M:%S%z')
Out[2]: Index([2020-01-01 01:00:00-01:00, 2020-01-01 03:00:00], dtype='object')

Issue Description

This is inconsistent. I think I prefer the latter - datetime(2020, 1, 1, 3, 0) is timezone-naive, so it seems right to do the same thing we'd do if there were mixed offsets, i.e. return an Index of dtype object

As far as I can tell, the only place this is tested is

pandas/pandas/core/tools/datetimes.py

Lines 993 to 1000 in 1d5f05c

	- A mix of timezone-aware and timezone-naive inputs is converted to
	a timezone-aware :class:`DatetimeIndex` if the offsets of the timezone-aware
	are constant:
	>>> from datetime import datetime
	>>> pd.to_datetime(["2020-01-01 01:00:00-01:00", datetime(2020, 1, 1, 3, 0)])
	DatetimeIndex(['2020-01-01 01:00:00-01:00', '2020-01-01 02:00:00-01:00'],
	dtype='datetime64[ns, UTC-01:00]', freq=None)

Timezone-naive and timezone-aware are not the same format, so if we now expect elements to be parsed with the same format, I don't think this should fly

Expected Behavior

Same output in both cases, I think this should be:

In [1]: pd.to_datetime(["2020-01-01 01:00:00-01:00", datetime(2020, 1, 1, 3, 0)], format='%Y-%m-%d %H:%M:%S%z')  # ISO: naive dt.datetime is converted
Out[1]: Index([2020-01-01 01:00:00-01:00, 2020-01-01 03:00:00], dtype='object')

In [2]: pd.to_datetime(["2020-01-01 01:00:00-01:00", datetime(2020, 1, 1, 3, 0)], format='%Y-%d-%m %H:%M:%S%z')  # non-ISO: dt.datetime is left as-is, object Index returned
Out[2]: Index([2020-01-01 01:00:00-01:00, 2020-01-01 03:00:00], dtype='object')

Installed Versions

INSTALLED VERSIONS

commit : 113bdb3
python : 3.8.15.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 : 2.0.0.dev0+918.g113bdb3769
numpy : 1.23.5
pytz : 2022.6
dateutil : 2.8.2
setuptools : 65.5.1
pip : 22.3.1
Cython : 0.29.32
pytest : 7.2.0
hypothesis : 6.61.0
sphinx : 4.5.0
blosc : None
feather : None
xlsxwriter : 3.0.3
lxml.etree : 4.9.1
html5lib : 1.1
pymysql : 1.0.2
psycopg2 : 2.9.3
jinja2 : 3.1.2
IPython : 8.7.0
pandas_datareader: None
bs4 : 4.11.1
bottleneck : 1.3.5
brotli :
fastparquet : 2022.12.0
fsspec : 2021.11.0
gcsfs : 2021.11.0
matplotlib : 3.6.2
numba : 0.56.4
numexpr : 2.8.3
odfpy : None
openpyxl : 3.0.10
pandas_gbq : None
pyarrow : 9.0.0
pyreadstat : 1.2.0
pyxlsb : 1.0.10
s3fs : 2021.11.0
scipy : 1.9.3
snappy :
sqlalchemy : 1.4.45
tables : 3.7.0
tabulate : 0.9.0
xarray : 2022.12.0
xlrd : 2.0.1
zstandard : 0.19.0
tzdata : None
qtpy : None
pyqt5 : None
None

[jorisvandenbossche]

Resolved by #50242