How to require a timestamp to be zero-padded during validation in Python?

There is already an answer that parsing ISO8601 or RFC3339 date/time with Python strptime() is impossible: How to parse an ISO 8601-formatted date? So, to answer you question, no there is no way in the standard Python library to reliable parse such a date. Regarding the regex suggestions, a date string like

2020-14-32T45:33:44.123

would result in a valid date. There are lots of Python modules (if you search for "iso8601" on https://pypi.python.org), but building a complete ISO8601 Validator would require things like leap seconds, the list of possible time zone offset values and many more.

To enforce strptime to validate leading zeros for you you'll have to add your own literals to Python's _strptime._TimeRE_cache. The solution is very hacky, most likely not very portable, and requires writing a RegEx - although only for the hour part of a timestamp.

Another solution to the problem would be to write your own function that uses strptime and also converts the parsed date back to a string and compares the two strings. This solution is portable, but it lacks for the clear error messages - you won't be able to distinguish between missing leading zeros in hours, minutes, seconds.

You said you want to avoid a regex, but this is actually the type of problem where a regex is appropriate. As you discovered, strptime is very flexible about the input it will accept. However, the regex for this problem is relatively easy to compose:

import re

date_pattern = re.compile(r'\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}.\d{3}')
s_list = [
    '1985-08-23T3:00:00.000',
    '1985-08-23T03:00:00.000'
]
for s in s_list:
    if date_pattern.match(s):
        print "%s is valid" % s
    else:
        print "%s is invalid" % s

Output

1985-08-23T3:00:00.000 is invalid
1985-08-23T03:00:00.000 is valid

Try it on repl.it