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ENH: dont guess %Y/%m #49711

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Nov 17, 2022
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11 changes: 7 additions & 4 deletions pandas/_libs/tslibs/parsing.pyx
Original file line number Diff line number Diff line change
Expand Up @@ -1013,12 +1013,15 @@ def guess_datetime_format(dt_str: str, bint dayfirst=False) -> str | None:
found_attrs.update(attrs)
break

# Only consider it a valid guess if we have a year, month and day,
# unless it's %Y or %Y-%m which conform with ISO8601. Note that we don't
# make an exception for %Y%m because it's explicitly not considered ISO8601.
# Only consider it a valid guess if we have a year, month and day.
# We make exceptions for %Y and %Y-%m (only with the `-` separator)
# as they conform with ISO8601.
if (
len({'year', 'month', 'day'} & found_attrs) != 3
and format_guess not in (['%Y'], ['%Y', None, '%m'])
and format_guess != ['%Y']
and not (
format_guess == ['%Y', None, '%m'] and tokens[1] == '-'
)
):
return None

Expand Down
2 changes: 1 addition & 1 deletion pandas/tests/tslibs/test_parsing.py
Original file line number Diff line number Diff line change
Expand Up @@ -149,7 +149,6 @@ def test_parsers_month_freq(date_str, expected):
("2011-12-30", "%Y-%m-%d"),
("2011", "%Y"),
("2011-01", "%Y-%m"),
("2011/01", "%Y/%m"),
("30-12-2011", "%d-%m-%Y"),
("2011-12-30 00:00:00", "%Y-%m-%d %H:%M:%S"),
("2011-12-30T00:00:00", "%Y-%m-%dT%H:%M:%S"),
Expand Down Expand Up @@ -218,6 +217,7 @@ def test_guess_datetime_format_with_locale_specific_formats(string, fmt):
"51a",
"13/2019",
"202001", # YYYYMM isn't ISO8601
"2020/01", # YYYY/MM isn't ISO8601 either
],
)
def test_guess_datetime_format_invalid_inputs(invalid_dt):
Expand Down