Specify date format for Python argparse input arguments

Question

I have a Python script that requires some command line inputs and I am using argparse for parsing them. I found the documentation a bit confusing and couldn't find a way to check for a format in the input parameters. What I mean by checking format is explained with this example script:

parser.add_argument('-s', "--startdate", help="The Start Date - format YYYY-MM-DD ", required=True)
parser.add_argument('-e', "--enddate", help="The End Date format YYYY-MM-DD (Inclusive)", required=True)
parser.add_argument('-a', "--accountid", type=int, help='Account ID for the account for which data is required (Default: 570)')
parser.add_argument('-o', "--outputpath", help='Directory where output needs to be stored (Default: ' + os.path.dirname(os.path.abspath(__file__)))

I need to check for option -s and -e that the input by the user is in the format YYYY-MM-DD. Is there an option in argparse that I do not know of which accomplishes this?

jonrsharpe · Accepted Answer · 2024-06-15 10:10:13Z

297

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:

import argparse
import datetime


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

Then use that as type:

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

If the user supplies an invalid value, the feedback will look like:

error: argument -s/--startdate: not a valid date: 'foo'

edited Jun 15, 2024 at 10:10

answered Aug 24, 2014 at 10:53

jonrsharpe

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5 Comments

Sohaib Over a year ago

About the other question that you edited out can you point me to the documentation (if any) for that?

Stevoisiak Over a year ago

Does valid_date() assume the given argument is a string?

jonrsharpe Over a year ago

@StevenVascellaro yes, but that's what the API defines; "takes a single string argument" per the quote from the docs.

Georg Pfolz Over a year ago

I get NameError: name 's' is not defined with this code.

jonrsharpe Over a year ago

@GeorgPfolz s is defined in the parameter list for valid_date, so that seems unlikely.

jonrsharpe · Accepted Answer · 2024-01-01 12:31:23Z

85

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'))

If the user supplies an invalid value, the feedback will look like:

error: argument -e/--enddate: invalid <lambda> value: 'foo'

edited Jan 1, 2024 at 12:31

jonrsharpe

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answered Oct 17, 2016 at 20:48

Evan V

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2 Comments

Brad Solomon Over a year ago

This is a nice trick, although if you pass something invalid, the error message will just be invalid <lambda> value

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jonrsharpe · Accepted Answer · 2024-01-01 12:29:35Z

79

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)

If the user supplies an invalid valid, the feedback will look like:

error: argument -e/--enddate: invalid fromisoformat value: 'foo'

edited Jan 1, 2024 at 12:29

jonrsharpe

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answered Aug 6, 2018 at 7:13

Michał Górny

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3 Comments

Brad Solomon Over a year ago

In addition, dateutil's ISO parser takes a number of additional fmts that are still ISO 8601-2004 compliant. You can use import dateutil.parser --> type=dateutil.parser.isoparse or type=dateutil.parser.parse

András Aszódi Over a year ago

@BradSolomon Indeed a good suggestion (hence my +1), but dateutil is a third-party package. One would have to judge its benefits against the (small) hassle of installing it extra. YMMV.

Paul P Over a year ago

Very elegant solution!

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