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Reproducible Example

to_datetime(['19801212.0'], format='%Y%m%d', exact=True)

Issue Description

This should fail, there's no interpretation of format in which 19801212.0 matches %Y%m%d

Expected Behavior

Probably something like

ValueError: unconverted data remains: .0

but certainly not that it passes without an error

Installed Versions

INSTALLED VERSIONS ------------------ commit : 732ad9f281bf6efffa12b8a872df98e18c11e3b7 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+838.g732ad9f281.dirty 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 : 6.59.0 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 : 3.0.3 IPython : 8.7.0 pandas_datareader: 0.10.0 bs4 : 4.11.1 bottleneck : 1.3.5 brotli : fastparquet : 2022.11.0 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 : 0.17.9 pyarrow : 9.0.0 pyreadstat : 1.2.0 pyxlsb : 1.0.10 s3fs : 2021.11.0 scipy : 1.9.3 snappy : sqlalchemy : 1.4.44 tables : 3.7.0 tabulate : 0.9.0 xarray : 2022.11.0 xlrd : 2.0.1 zstandard : 0.19.0 tzdata : None qtpy : None pyqt5 : None None

Comment From: MarcoGorelli

We don't do this for any other format, why are we special-casing %Y%m%d?

In [1]: print(to_datetime(['198012.0'], format='%Y%m'))
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Cell In[1], line 1
----> 1 print(to_datetime(['198012.0'], format='%Y%m'))

File ~/pandas-dev/pandas/core/tools/datetimes.py:1115, in to_datetime(arg, errors, dayfirst, yearfirst, utc, format, exact, unit, infer_datetime_format, origin, cache)
   1113         result = _convert_and_box_cache(argc, cache_array)
   1114     else:
-> 1115         result = convert_listlike(argc, format)
   1116 else:
   1117     result = convert_listlike(np.array([arg]), format)[0]

File ~/pandas-dev/pandas/core/tools/datetimes.py:438, in _convert_listlike_datetimes(arg, format, name, utc, unit, errors, infer_datetime_format, dayfirst, yearfirst, exact)
    435         require_iso8601 = not infer_datetime_format
    437 if format is not None and not require_iso8601:
--> 438     res = _to_datetime_with_format(
    439         arg, orig_arg, name, utc, format, exact, errors, infer_datetime_format
    440     )
    441     if res is not None:
    442         return res

File ~/pandas-dev/pandas/core/tools/datetimes.py:544, in _to_datetime_with_format(arg, orig_arg, name, utc, fmt, exact, errors, infer_datetime_format)
    541         return _box_as_indexlike(result, utc=utc, name=name)
    543 # fallback
--> 544 res = _array_strptime_with_fallback(
    545     arg, name, utc, fmt, exact, errors, infer_datetime_format
    546 )
    547 return res

File ~/pandas-dev/pandas/core/tools/datetimes.py:478, in _array_strptime_with_fallback(arg, name, utc, fmt, exact, errors, infer_datetime_format)
    474 """
    475 Call array_strptime, with fallback behavior depending on 'errors'.
    476 """
    477 try:
--> 478     result, timezones = array_strptime(
    479         arg, fmt, exact=exact, errors=errors, utc=utc
    480     )
    481 except OutOfBoundsDatetime:
    482     if errors == "raise":

File ~/pandas-dev/pandas/_libs/tslibs/strptime.pyx:185, in pandas._libs.tslibs.strptime.array_strptime()
    183             iresult[i] = NPY_NAT
    184             continue
--> 185         raise ValueError(f"unconverted data remains: {val[found.end():]}")
    186 
    187 # search

ValueError: unconverted data remains: .0

Comment From: MarcoGorelli

Pretty sure I disagree with this test:

https://github.com/pandas-dev/pandas/blob/2804681c45b5f748d0977e3998bbfc91d4653b10/pandas/tests/tools/test_to_datetime.py#L119-L135

These should raise, the inputs don't match the given format

I don't think the %Y%m%d special path serves any purpose, we should just remove it

Comment From: MarcoGorelli

This doesn't look right:

https://github.com/pandas-dev/pandas/blob/073e48b7cfe85b7a1173a275296471b51276826d/pandas/tests/tools/test_to_datetime.py#L142-L148

If the input is invalid, errors='ignore' should return the input

Comment From: jorisvandenbossche

Resolved by https://github.com/pandas-dev/pandas/pull/50242