guess nanoseconds

Author: MarcoGorelli

pandas/_libs/tslibs/parsing.pyx

@@ -1051,10 +1051,13 @@ def guess_datetime_format(dt_str: str, bint dayfirst=False) -> str | None:
             if '.' not in tokens[i]:
                 token_filled = tokens[i].zfill(padding)
             else:
-                seconds, milliseconds = tokens[i].split('.')
-                seconds = seconds.zfill(2)
-                millisecond = milliseconds.zfill(6)
-                token_filled = f'{seconds}.{milliseconds}'
+                seconds, nanoseconds = tokens[i].split('.')
+                seconds = f'{int(seconds):02d}'
+                # right-pad so we get nanoseconds, then only take
+                # first 6 digits (microseconds) as stdlib datetime
+                # doesn't support nanoseconds
+                nanoseconds = nanoseconds.ljust(9, '0')[:6]
+                token_filled = f'{seconds}.{nanoseconds}'
             if token_format is None and token_filled == parsed_formatted:
                 format_guess[i] = attr_format
                 tokens[i] = token_filled