Python 3.15 datetime.strptime() %z behaviour change

[suve]

Bug report

Bug description:

If you use %z in the format string for datetime.strptime(), Python 3.14.4 requires offset information to be present and will produce a ValueError if it's missing...

# Python 3.14
>>> import datetime
>>> datetime.datetime.strptime("2026-05-19T20:21:22Z", "%Y-%m-%dT%H:%M:%S%z")
datetime.datetime(2026, 5, 19, 20, 21, 22, tzinfo=datetime.timezone.utc)
>>> datetime.datetime.strptime("2026-05-19T20:21:22", "%Y-%m-%dT%H:%M:%S%z")
Traceback (most recent call last):
  File "<python-input-4>", line 1, in <module>
    datetime.datetime.strptime("2026-05-19T20:21:22", "%Y-%m-%dT%H:%M:%S%z")
    ~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.14/_strptime.py", line 815, in _strptime_datetime_datetime
    tt, fraction, gmtoff_fraction = _strptime(data_string, format)
                                    ~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.14/_strptime.py", line 555, in _strptime
    raise ValueError("time data %r does not match format %r" %
                     (data_string, format))
ValueError: time data '2026-05-19T20:21:22' does not match format '%Y-%m-%dT%H:%M:%S%z'

...whereas Python 3.15-beta1 treats it as optional and will create a naive datetime object, instead.

# Python 3.15-beta1
>>> import datetime
>>> datetime.datetime.strptime("2026-05-19T20:21:22Z", "%Y-%m-%dT%H:%M:%S%z")
datetime.datetime(2026, 5, 19, 20, 21, 22, tzinfo=datetime.timezone.utc)
>>> datetime.datetime.strptime("2026-05-19T20:21:22", "%Y-%m-%dT%H:%M:%S%z")
datetime.datetime(2026, 5, 19, 20, 21, 22)

Looking through the documentation for Python 3.15.0b1 for mentions of "3.15" or "%z" yields only this single note:

Added in version 3.15: %D, %F, %n, %t, and %:z were added for strptime().

This mentions %:z, but not %z. Thus, I'm not sure if this behaviour change is intentional and just needs documenting, or if it's an actual bug.

Operating System: Fedora Linux 44
CPython versions: 3.15.0b1, 3.14.4

CPython versions tested on:

3.15

Operating systems tested on:

Linux

[StanFromIreland]

This was intentionally changed in 99b5808. It is however already documented, in the specification for %z we explicitly list "(empty)".