BUG: to_datetime - confusion with timeawarness

[PrimozGodec]

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

# passes
>>> pd.to_datetime([pd.Timestamp("2022-01-01 00:00:00"), pd.Timestamp("1724-12-20 20:20:20+01:00")], utc=True)
# fails
>>> pd.to_datetime([pd.Timestamp("1724-12-20 20:20:20+01:00"), pd.Timestamp("2022-01-01 00:00:00")], utc=True)
Traceback (most recent call last):
  File "/Users/primoz/miniconda3/envs/orange3/lib/python3.10/site-packages/IPython/core/interactiveshell.py", line 3398, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-7-69a49866a765>", line 1, in <cell line: 1>
    pd.to_datetime([pd.Timestamp("1724-12-20 20:20:20+01:00"), pd.Timestamp("2022-01-01 00:00:00")], utc=True)
  File "/Users/primoz/python-projects/pandas/pandas/core/tools/datetimes.py", line 1124, in to_datetime
    result = convert_listlike(argc, format)
  File "/Users/primoz/python-projects/pandas/pandas/core/tools/datetimes.py", line 439, in _convert_listlike_datetimes
    result, tz_parsed = objects_to_datetime64ns(
  File "/Users/primoz/python-projects/pandas/pandas/core/arrays/datetimes.py", line 2182, in objects_to_datetime64ns
    result, tz_parsed = tslib.array_to_datetime(
  File "pandas/_libs/tslib.pyx", line 428, in pandas._libs.tslib.array_to_datetime
  File "pandas/_libs/tslib.pyx", line 535, in pandas._libs.tslib.array_to_datetime
ValueError: Cannot mix tz-aware with tz-naive values

# the same examples with strings only also pass
>>> pd.to_datetime(["1724-12-20 20:20:20+01:00", "2022-01-01 00:00:00"], utc=True)

Issue Description

When calling to_datetime with pd.Timestamps in the list/series conversion fail when the datetime at position 0 is tz-aware and the other datetime is tz-naive. When datetimes are reordered the same example pass. The same example also pass when datetimes are strings.

So I am a bit confused about what is supported now. Should a case that fails work?
The error is only present in pandas==1.5.0 and in the main branch. I think it was introduced with this PR #47018.

Expected Behavior

I think to_datetime should have the same behavior in all cases.

Installed Versions

INSTALLED VERSIONS ------------------ commit : 87cfe4e python : 3.10.4.final.0 python-bits : 64 OS : Darwin OS-release : 21.6.0 Version : Darwin Kernel Version 21.6.0: Wed Aug 10 14:28:23 PDT 2022; root:xnu-8020.141.5~2/RELEASE_ARM64_T6000 machine : arm64 processor : arm byteorder : little LC_ALL : None LANG : None LOCALE : None.UTF-8 pandas : 1.5.0 numpy : 1.22.4 pytz : 2022.1 dateutil : 2.8.2 setuptools : 61.2.0 pip : 22.1.2 Cython : None pytest : None hypothesis : None sphinx : 5.1.1 blosc : None feather : None xlsxwriter : 3.0.3 lxml.etree : 4.9.1 html5lib : None pymysql : None psycopg2 : None jinja2 : 3.1.2 IPython : 8.4.0 pandas_datareader: None bs4 : 4.11.1 bottleneck : 1.3.5 brotli : None fastparquet : None fsspec : None gcsfs : None matplotlib : 3.5.2 numba : 0.56.0 numexpr : None odfpy : None openpyxl : 3.0.10 pandas_gbq : None pyarrow : None pyreadstat : None pyxlsb : None s3fs : None scipy : 1.9.1 snappy : None sqlalchemy : 1.4.39 tables : None tabulate : 0.8.10 xarray : None xlrd : 2.0.1 xlwt : None zstandard : None tzdata : 2022.2

[MarcoGorelli]

I think it was introduced with this PR #47018.

Can confirm (though haven't looked any more closely than that yet), thanks for the report

0277a9c49f5efb1edcb0ebdba48123e2fda2a590 is the first bad commit
commit 0277a9c49f5efb1edcb0ebdba48123e2fda2a590
Author: jbrockmendel <jbrockmendel@gmail.com>
Date:   Sat May 21 12:47:14 2022 -0700

    REF: merge datetime_to_datetime64 into array_to_datetime (#47018)

 pandas/_libs/tslib.pyx                 | 28 ++++++++++---
 pandas/_libs/tslibs/conversion.pyi     |  3 --
 pandas/_libs/tslibs/conversion.pyx     | 74 ----------------------------------
 pandas/core/arrays/datetimes.py        |  8 ----
 pandas/core/tools/datetimes.py         | 72 ++++++++++++---------------------
 pandas/tests/tools/test_to_datetime.py |  9 +++++
 6 files changed, 58 insertions(+), 136 deletions(-)
bisect run success

[MarcoGorelli]

Hey @PrimozGodec

Thanks for having reported this - there's now a release candidate for pandas 2.0.0, which you can install with

mamba install -c conda-forge/label/pandas_rc pandas==2.0.0rc0

or

python -m pip install --upgrade --pre pandas==2.0.0rc0

If you try it out and report bugs, then we can fix them before the final 2.0.0 release

[PrimozGodec]

@MarcoGorelli, thank you for working on 2.0. For now I don't find any issue.