Augmenting Python's strftime

The strftime function is the prescribed way to format dates and times in Python (and other languages). It has limitations, such as forcing a leading zero on days of the month, 01-31, and on 12-hour clock hours, 01-12.

Edit I noticed that we were repeatedly writing expressions like these

d.strftime('%A, %B ') + str(d.day)
t.strftime("%I:%M").lstrip('0') + ('a' if t.hour < 12 else 'p')

and realized that there had to be a better way.

Here's a straightforward way to augment the directives: preprocess the format string, replacing new directives with their values, then let the underlying strftime implementation take care of the rest.

import re

_re_aux_format = re.compile("%([DiP])")

def strftime_aux(d, format):
    """
    Augmented strftime that handles additional directives.

    %D  Day of the month as a decimal number [1,31] (no leading zero)
    %i  Hour (12-hour clock) as a decimal number [1,12] (no leading zero)
    %P  'a' for AM, 'p' for PM

    >>> import datetime
    >>> d = datetime.datetime(2009, 4, 1, 9+12, 37)
    >>> strftime_aux(d, '%A, %B %d, %I:%M %p')
    'Wednesday, April 01, 09:37 PM'
    >>> strftime_aux(d, '%A, %B %D, %i:%M%P')
    'Wednesday, April 1, 9:37p'
    """

    # Precompute the values of the augmented directives
    directive_map = {
        'D': str(d.day),
        'i': '12' if d.hour in (0, 12) else str(d.hour % 12),
        'P': 'a' if d.hour < 12 else 'p',
    }
    # Substitute those values into the format string
    new_format = _re_aux_format.sub(
        lambda match: directive_map.get( match.group(1), ''),
        format)
    # Let the stock implementation of strftime handle everything else
    return d.strftime(new_format)

if __name__ == "__main__":
    import doctest
    doctest.testmod()