How to parse str(my_datetime) using strptime?

Question

This is the default string representation of a datetime:

>>> from datetime import datetime, timezone
>>> dt = datetime(2017, 1, 1, tzinfo=timezone.utc)
>>> print(dt)
2017-01-01 00:00:00+00:00

What is the correct format string to parse that with datetime.strptime? That is, what format goes in place of the "???" to consistently have the following invariant:

>>> dt == datetime.strptime(str(dt), "???")
True

Answer 1

Note that str(d) is documented as being equivalent to d.isoformat(' '). This starts with %Y-%m-%d %H:%M:%S (2017-01-01 00:00:00), but then:

• Either has nothing or .%f, depending whether the microseconds part is nonzero.
• Either has nothing or an offset like +HH:MM, depending whether the instance is timezone-aware.

datetime.strptime doesn't have support for optional parts, therefore there isn't a single format parameter that can match all of the possible outputs.

In Python 3.7+, you can use datetime.fromisoformat to parse datetime.isoformat output. Contributed by Paul Ganssle in issue15873.