ENH Guess %Y-%m format (#49389)

guess %Y-%m format Co-authored-by: MarcoGorelli <>

Author: MarcoGorelli

pandas/_libs/tslibs/parsing.pyx

@@ -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
 

pandas/tests/tslibs/test_parsing.py

@@ -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"),
  ("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"),
@@ -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):