pythonpython-datetime

Why does `strftime("%Y")` not yield a 4-digit year for dates < 1000 AD in Python's datetime module on Linux?

I am puzzled by an inconsistency when calling .strftime() for dates which are pre-1000 AD, using Python's datetime module.

Take the following example:

import datetime
old_date = datetime.date(year=33, month=3, day=28)  # 28th March 33AD

old_date.isoformat()
>>> "0033-03-28"  # Fine!

old_date.strftime("%Y-%m-%d")
>>> "33-03-28"  # Woah - where did my leading zeros go?

# And even worse
datetime.datetime.strptime(old_date.strftime("%Y-%m-%d"), "%Y-%m-%d")
>>>
  ...
  File "<input>", line 1, in <module>
  File "/usr/lib/python3.12/_strptime.py", line 554, in _strptime_datetime
    tt, fraction, gmtoff_fraction = _strptime(data_string, format)
                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/_strptime.py", line 333, in _strptime
    raise ValueError("time data %r does not match format %r" %
ValueError: time data '33-03-28' does not match format '%Y-%m-%d'

The documentation shows examples of %Y yielding zero-padded years. Even using %G, which is documented to be an ISO-8601 4-digit year, is showing only two digits.

This caused a problem in an application where a user can enter a date, and if they type in an old date the exception above would arise when trying to convert a date-string back into a date.

Presumably there is something in my local configuration which is causing this, as this seems too obvious to be a bug in Python. I'm using Python 3.12 on Ubuntu 24.04.

Solution

  • This is caused by the implementation of .strftime() in the C library in Linux omitting any leading zeros from %Y and %G. The related issue in CPython's issue tracker is here.

    Thanks to jonrsharpe's comment for the answer, and highlighting this section of the documentation:

    "The full set of format codes supported varies across platforms, because Python calls the platform C library’s strftime() function, and platform variations are common."