Validate an ISO-8601 datetime string in Python?

Question

I want to write a function that takes a string and returns True if it is a valid ISO-8601 datetime--precise to microseconds, including a timezone offset--False otherwise.

I have found other questions that provide different ways of parsing datetime strings, but I want to return True in the case of ISO-8601 format only. Parsing doesn't help me unless I can get it to throw an error for formats that don't match ISO-8601.

(I am using the nice arrow library elsewhere in my code. A solution that uses arrow would be welcome.)

---

EDIT: It appears that a general solution to "is this string a valid ISO 8601 datetime" does not exist among the common Python datetime packages.

So, to make this question narrower, more concrete and answerable, I will settle for a format string that will validate a datetime string in this form:

'2016-12-13T21:20:37.593194+00:00'

Currently I am using:

format_string = '%Y-%m-%dT%H:%M:%S.%f%z'
datetime.datetime.strptime(my_timestamp, format_string)

This gives:

ValueError: time data '2016-12-13T21:20:37.593194+00:00' does not match format '%Y-%m-%dT%H:%M:%S.%f%z'

The problem seems to lie with the colon in the UTC offset (+00:00). If I use an offset without a colon (e.g. '2016-12-13T21:20:37.593194+0000'), this parses properly as expected. This is apparently because datetime's %z token does not respect the UTC offset form that has a colon, only the form without, even though both are valid per the spec.

Answer 1

https://www.safaribooksonline.com/library/view/regular-expressions-cookbook/9781449327453/ch04s07.html

give many variants for validating date and times in ISO8601 format (e.g., 2008-08-30T01:45:36 or 2008-08-30T01:45:36.123Z). The regex for the XML Schema dateTime type is given as:

>>> regex = r'^(-?(?:[1-9][0-9]*)?[0-9]{4})-(1[0-2]|0[1-9])-(3[01]|0[1-9]|[12][0-9])T(2[0-3]|[01][0-9]):([0-5][0-9]):([0-5][0-9])(\.[0-9]+)?(Z|[+-](?:2[0-3]|[01][0-9]):[0-5][0-9])?$'

So in order to validate you could do:

import re
match_iso8601 = re.compile(regex).match
def validate_iso8601(str_val):
    try:            
        if match_iso8601( str_val ) is not None:
            return True
    except:
        pass
    return False

Some examples:

>>> validate_iso8601('2017-01-01')
False

>>> validate_iso8601('2008-08-30T01:45:36.123Z')
True

>>> validate_iso8601('2016-12-13T21:20:37.593194+00:00')
True

Answer 2

Recent versions of Python (from 3.7 onwards) have a fromisoformat() function in the datetime standard library. See: https://docs.python.org/3.7/library/datetime.html

So this will do the trick:

from datetime import datetime

def datetime_valid(dt_str):
    try:
        datetime.fromisoformat(dt_str)
    except:
        return False
    return True

Update:

I learned that Python does not recognize the 'Z'-suffix as valid. As I wanted to support this in my API, I'm now using:

from datetime import datetime

def datetime_valid(dt_str):
    try:
        datetime.fromisoformat(dt_str)
    except:
        try:
            datetime.fromisoformat(dt_str.replace('Z', '+00:00'))
        except:
            return False
        return True
    return True