Specify date format for Python argparse input arguments

Question

Per the documentation:

The type keyword argument of add_argument() allows any necessary type-checking and type conversions to be performed ... The argument to type can be any callable that accepts a single string.

You could do something like:

def valid_date(s):
    try:
        return datetime.strptime(s, "%Y-%m-%d")
    except ValueError:
        msg = "not a valid date: {0!r}".format(s)
        raise argparse.ArgumentTypeError(msg)

Then use that as type:

parser.add_argument(
    "-s", 
    "--startdate", 
    help="The Start Date - format YYYY-MM-DD", 
    required=True, 
    type=valid_date
)

Just to add on to the answer above, you can use a lambda function if you want to keep it to a one-liner. For example:

parser.add_argument('--date', type=lambda d: datetime.strptime(d, '%Y%m%d'))

Old thread but the question was still relevant for me at least!

For others who hit this via search engines: in Python 3.7, you can use the standard .fromisoformat class method instead of reinventing the wheel for ISO-8601 compliant dates, e.g.:

import datetime

parser.add_argument('-s', "--startdate",
    help="The Start Date - format YYYY-MM-DD",
    required=True,
    type=datetime.date.fromisoformat)
parser.add_argument('-e', "--enddate",
    help="The End Date format YYYY-MM-DD (Inclusive)",
    required=True,
    type=datetime.date.fromisoformat)