datetime.strptime no longer raises ValueError: stray % in format '%Y %'

[westandskif]

Bug report

Bug description:

in 3.13.2 and 3.13.1 datetime.strptime no longer raises ValueError: stray % in format '%Y %'

Python 3.13.2 (main, Mar 19 2025, 01:28:14) [Clang 16.0.0 (clang-1600.0.26.6)] on Darwin
>>> from datetime import datetime
>>> datetime.strptime("2000 %", "%Y %")
datetime.datetime(2000, 1, 1, 0, 0)

as it used to do in 3.13.0:

Python 3.13.0 (main, Mar 19 2025, 01:42:48) [Clang 16.0.0 (clang-1600.0.26.6)] on darwin
>>> from datetime import datetime
>>> datetime.strptime("2000 %", "%Y %")
Traceback (most recent call last):
  File "<python-input-1>", line 1, in <module>
    datetime.strptime("2000 %", "%Y %")
    ~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^
  File "/Users/nik/.pyenv/versions/3.13.0/lib/python3.13/_strptime.py", line 573, in _strptime_datetime
    tt, fraction, gmtoff_fraction = _strptime(data_string, format)
                                    ~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^
  File "/Users/nik/.pyenv/versions/3.13.0/lib/python3.13/_strptime.py", line 348, in _strptime
    raise ValueError("stray %% in format '%s'" % format) from None
ValueError: stray % in format '%Y %'

and e.g. 3.10:

Python 3.10.15 (main, Nov 28 2024, 23:39:17) [Clang 16.0.0 (clang-1600.0.26.4)] on Darwin
>>> from datetime import datetime
>>> datetime.strptime("2000 %", "%Y %")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/nik/.pyenv/versions/3.10.15/lib/python3.10/_strptime.py", line 568, in _strptime_datetime
    tt, fraction, gmtoff_fraction = _strptime(data_string, format)
  File "/Users/nik/.pyenv/versions/3.10.15/lib/python3.10/_strptime.py", line 345, in _strptime
    raise ValueError("stray %% in format '%s'" % format) from None
ValueError: stray % in format '%Y %'

edit 1:
got the same in the latest 3.12.9 (used to work in older 3.12)

CPython versions tested on:

3.13

Operating systems tested on:

macOS

Linked PRs

• gh-131434: Improve error reporting for incorrect format in strptime() #131568

[StanFromIreland]

@picnixz This is in _strptime.py

The error does not execute, I will look into this.

cpython/Lib/_strptime.py

Lines 448 to 449 in b8367e7

	except IndexError:
	raise ValueError("stray %% in format '%s'" % format) from None

I presume it was because of this commit, 5f4e5b5 I will revert it and see if it fixes it.

Edit: I was correct, it was Serhiys commit. It will not be as simple as I though to fix as we no longer iterate over the format. @serhiy-storchaka any ideas for a fix for your old commit?

[serhiy-storchaka]

Good catch! Indeed, the regression was introduced in #125406. There was only one test that indirectly tested this error, but it was broken, so the regression has not been noticed.

#131568 fixes the regression for stray % at the end of the format string, and also improves error reporting for some other cases.

[duaneg]

Sorry, I was looking at this and didn't refresh the page and notice you already had a fix :-)