BUG: guess_datetime_format doesn't guess just year (#49127)

* guess %Y format

* fixup

Co-authored-by: MarcoGorelli <>

Author: MarcoGorelli

pandas/_libs/tslibs/parsing.pyx

@@ -1053,8 +1053,12 @@ 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
-    if len({'year', 'month', 'day'} & found_attrs) != 3:
+    # Only consider it a valid guess if we have a year, month and day,
+    # unless it's %Y which is both common and unambiguous.
+    if (
+        len({'year', 'month', 'day'} & found_attrs) != 3
+        and format_guess != ['%Y']
+    ):
         return None
 
     output_format = []

pandas/tests/tslibs/test_parsing.py

@@ -147,6 +147,7 @@ def test_parsers_month_freq(date_str, expected):
     [
         ("20111230", "%Y%m%d"),
         ("2011-12-30", "%Y-%m-%d"),
+        ("2011", "%Y"),
         ("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"),
@@ -208,7 +209,6 @@ def test_guess_datetime_format_with_locale_specific_formats(string, fmt):
 @pytest.mark.parametrize(
     "invalid_dt",
     [
-        "2013",
         "01/2013",
         "12:00:00",
         "1/1/1/1",