ENH: Add support for more placeholders in guess_datetime_format (#43901)

Add support for day of week and meridiem placeholders and any
combination of placeholders supported by `strftime` that do not
correspond to a datetime attribute.

Author: davesque

doc/source/whatsnew/v1.4.0.rst

@@ -129,6 +129,7 @@ Other enhancements
 - :meth:`DataFrame.__pos__`, :meth:`DataFrame.__neg__` now retain ``ExtensionDtype`` dtypes (:issue:`43883`)
 - The error raised when an optional dependency can't be imported now includes the original exception, for easier investigation (:issue:`43882`)
 - Added :meth:`.ExponentialMovingWindow.sum` (:issue:`13297`)
+- :func:`guess_datetime_format` now correctly guesses the format of datetime strings with day of week and AM/PM placeholders (:issue:`43901`)
 
 .. ---------------------------------------------------------------------------
 

pandas/_libs/tslibs/parsing.pyx

@@ -911,6 +911,9 @@ def guess_datetime_format(
         (('second', 'microsecond'), '%S.%f', 0),
         (('tzinfo',), '%z', 0),
         (('tzinfo',), '%Z', 0),
+        (('day_of_week',), '%a', 0),
+        (('day_of_week',), '%A', 0),
+        (('meridiem',), '%p', 0),
     ]
 
     if dayfirst:
@@ -967,15 +970,18 @@ def guess_datetime_format(
         if set(attrs) & found_attrs:
             continue
 
-        if all(getattr(parsed_datetime, attr) is not None for attr in attrs):
-            for i, token_format in enumerate(format_guess):
-                token_filled = tokens[i].zfill(padding)
-                if (token_format is None and
-                        token_filled == parsed_datetime.strftime(attr_format)):
-                    format_guess[i] = attr_format
-                    tokens[i] = token_filled
-                    found_attrs.update(attrs)
-                    break
+        parsed_formatted = parsed_datetime.strftime(attr_format)
+        # The result of `.strftime("%Z")` on a dt with no timezone is ""
+        if len(parsed_formatted) == 0:
+            continue
+
+        for i, token_format in enumerate(format_guess):
+            token_filled = tokens[i].zfill(padding)
+            if token_format is None and token_filled == parsed_formatted:
+                format_guess[i] = attr_format
+                tokens[i] = token_filled
+                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:

pandas/tests/tslibs/test_parsing.py

@@ -167,6 +167,7 @@ def test_parsers_month_freq(date_str, expected):
         ("2011-12-30T00:00:00.000000+9:0", "%Y-%m-%dT%H:%M:%S.%f%z"),
         ("2011-12-30T00:00:00.000000+09:", None),
         ("2011-12-30 00:00:00.000000", "%Y-%m-%d %H:%M:%S.%f"),
+        ("Tue 24 Aug 2021 01:30:48 AM", "%a %d %b %Y %H:%M:%S %p"),
     ],
 )
 def test_guess_datetime_format_with_parseable_formats(string, fmt):