ENH: parse 8 or 9 digit delimited dates

[MarcoGorelli]

Feature Type

• Adding new functionality to pandas

• Changing existing functionality in pandas

• Removing existing functionality in pandas

Problem Description

Currently, a string such as 01-01-2020 is parsed as a delimited date, whereas 1-1-2020 is parsed by dateutil

One consequence of this is that warnings about e.g. dayfirst aren't shown in the latter case, e.g.:

>>> pd.to_datetime(['13-01-2020'], dayfirst=False)
<stdin>:1: UserWarning: Parsing dates in DD/MM/YYYY format when dayfirst=False (the default) was specified. This may lead to inconsistently parsed dates! Specify a format to ensure consistent parsing.
DatetimeIndex(['2020-01-13'], dtype='datetime64[ns]', freq=None)
>>> pd.to_datetime(['13-1-2020'], dayfirst=False)
DatetimeIndex(['2020-01-13'], dtype='datetime64[ns]', freq=None)

Feature Description

In

pandas/pandas/_libs/tslibs/parsing.pyx

Lines 122 to 173 in 9a2276f

	cdef inline object _parse_delimited_date(str date_string, bint dayfirst):
	"""
	Parse special cases of dates: MM/DD/YYYY, DD/MM/YYYY, MM/YYYY.
	At the beginning function tries to parse date in MM/DD/YYYY format, but
	if month > 12 - in DD/MM/YYYY (`dayfirst == False`).
	With `dayfirst == True` function makes an attempt to parse date in
	DD/MM/YYYY, if an attempt is wrong - in DD/MM/YYYY
	For MM/DD/YYYY, DD/MM/YYYY: delimiter can be a space or one of /-.
	For MM/YYYY: delimiter can be a space or one of /-
	If `date_string` can't be converted to date, then function returns
	None, None
	Parameters
	----------
	date_string : str
	dayfirst : bool
	Returns:
	--------
	datetime or None
	str or None
	Describing resolution of the parsed string.
	"""
	cdef:
	const char* buf
	Py_ssize_t length
	int day = 1, month = 1, year
	bint can_swap = 0
	buf = get_c_string_buf_and_size(date_string, &length)
	if length == 10:
	# parsing MM?DD?YYYY and DD?MM?YYYY dates
	if _is_not_delimiter(buf[2]) or _is_not_delimiter(buf[5]):
	return None, None
	month = _parse_2digit(buf)
	day = _parse_2digit(buf + 3)
	year = _parse_4digit(buf + 6)
	reso = 'day'
	can_swap = 1
	elif length == 7:
	# parsing MM?YYYY dates
	if buf[2] == b'.' or _is_not_delimiter(buf[2]):
	# we cannot reliably tell whether e.g. 10.2010 is a float
	# or a date, thus we refuse to parse it here
	return None, None
	month = _parse_2digit(buf)
	year = _parse_4digit(buf + 3)
	reso = 'month'
	else:
	return None, None

some code could be added to deal with cases where buf is of length 8 or 9, and either the date or the month are of length 1

Alternative Solutions

Always warn when using dateutil, but I don't a warning should be necessary here

Additional Context

If we wanted to warn whenever dateutil is called (e.g. #47828), then this'd really simplify the adjustments necessary to the test suite, as a lot of tests could be kept as they are