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ENH Guess %Y-%m format #49389

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Merged
merged 1 commit into from
Oct 29, 2022
Merged

ENH Guess %Y-%m format #49389

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5 changes: 3 additions & 2 deletions pandas/_libs/tslibs/parsing.pyx
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Original file line number Diff line number Diff line change
Expand Up @@ -1011,10 +1011,11 @@ def guess_datetime_format(dt_str: str, bint dayfirst=False) -> str | None:
break

# Only consider it a valid guess if we have a year, month and day,
# unless it's %Y which is both common and unambiguous.
# 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.
if (
len({'year', 'month', 'day'} & found_attrs) != 3
and format_guess != ['%Y']
and format_guess not in (['%Y'], ['%Y', None, '%m'])
):
return None

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3 changes: 3 additions & 0 deletions pandas/tests/tslibs/test_parsing.py
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Expand Up @@ -148,6 +148,8 @@ def test_parsers_month_freq(date_str, expected):
("20111230", "%Y%m%d"),
("2011-12-30", "%Y-%m-%d"),
("2011", "%Y"),
("2011-01", "%Y-%m"),
("2011/01", "%Y/%m"),
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So this is ISO 8601 conformant or is it just a byproduct of the implementation? Don't have the actual spec just looking at the Wikipedia. Didn't see anything for this

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It's not strictly ISO8601 (which wants a hyphen), but in pandas it's still treated as if it were, so long as the separator is consistent

pandas/pandas/_libs/tslibs/parsing.pyx

Lines 883 to 892 in d3adfe5

def format_is_iso(f: str) -> bint:
"""
Does format match the iso8601 set that can be handled by the C parser?
Generally of form YYYY-MM-DDTHH:MM:SS - date separator can be different
but must be consistent. Leading 0s in dates and times are optional.
"""
iso_template = '%Y{date_sep}%m{date_sep}%d{time_sep}%H:%M:%S{micro_or_tz}'.format
excluded_formats = ['%Y%m%d', '%Y%m', '%Y']
for date_sep in [' ', '/', '\\', '-', '.', '']:

pandas/pandas/_libs/tslibs/src/datetime/np_datetime_strings.c

Line 87 in 009c4c6

char valid_ymd_sep[] = {'-', '.', '/', '\\', ' '};

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Ah interesting...might be worth reconsidering but definitely out of scope for this PR

("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 @@ -215,6 +217,7 @@ def test_guess_datetime_format_with_locale_specific_formats(string, fmt):
"this_is_not_a_datetime",
"51a",
"13/2019",
"202001", # YYYYMM isn't ISO8601
],
)
def test_guess_datetime_format_invalid_inputs(invalid_dt):
Expand Down