ENH: guess ISO8601 formats without separators

[MarcoGorelli]

Feature Type

• Adding new functionality to pandas

• Changing existing functionality in pandas

• Removing existing functionality in pandas

Problem Description

stdlib accepts these as ISO8601

$ python3.11
Python 3.11.1 (main, Dec  7 2022, 01:11:34) [GCC 11.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import datetime as dt
>>> dt.datetime.fromisoformat
<built-in method fromisoformat of type object at 0xa31060>
>>> dt.datetime.fromisoformat('20110519 010000')
datetime.datetime(2011, 5, 19, 1, 0)
>>> dt.datetime.fromisoformat('20110519T010000')
datetime.datetime(2011, 5, 19, 1, 0)

but pandas doesn't guess them:

In [5]: assert guess_datetime_format("20110519 010000") is None

Feature Description

This probably needs amending to guess more iso8601 formats:

pandas/pandas/_libs/tslibs/parsing.pyx

Lines 843 to 859 in 502919e

	datetime_attrs_to_format = [
	(("year", "month", "day"), "%Y%m%d", 0),
	(("year",), "%Y", 0),
	(("month",), "%B", 0),
	(("month",), "%b", 0),
	(("month",), "%m", 2),
	day_attribute_and_format,
	(("hour",), "%H", 2),
	(("minute",), "%M", 2),
	(("second",), "%S", 2),
	(("second", "microsecond"), "%S.%f", 0),
	(("tzinfo",), "%z", 0),
	(("tzinfo",), "%Z", 0),
	(("day_of_week",), "%a", 0),
	(("day_of_week",), "%A", 0),
	(("meridiem",), "%p", 0),
	]

Expected output:

>>> guess_datetime_format("20110519 010000")
'%Y%m%d %H%M%S'

Alternative Solutions

If dateutil/dateutil#1138 was implemented, then that would probably be the best solution. Though, outside of pandas' control

Additional Context

No response

[ramvikrams]

The dateutil issue was still open so I appended our parsing.pyx and made a pr