added warnings when parse inconsistent with dayfirst arg

Author: arw2019

pandas/_libs/tslibs/parsing.pyx

@@ -3,6 +3,7 @@ Parsing functions for datetime and datetime-like strings.
 """
 import re
 import time
+import warnings
 
 from libc.string cimport strchr
 
@@ -149,14 +150,28 @@ cdef inline object _parse_delimited_date(str date_string, bint dayfirst):
         # date_string can't be converted to date, above format
         return None, None
 
+    swapped_day_and_month = False
     if 1 <= month <= MAX_DAYS_IN_MONTH and 1 <= day <= MAX_DAYS_IN_MONTH \
             and (month <= MAX_MONTH or day <= MAX_MONTH):
         if (month > MAX_MONTH or (day <= MAX_MONTH and dayfirst)) and can_swap:
             day, month = month, day
+            swapped_day_and_month = True
         if PY_VERSION_HEX >= 0x03060100:
             # In Python <= 3.6.0 there is no range checking for invalid dates
             # in C api, thus we call faster C version for 3.6.1 or newer
+        
+            if dayfirst and not swapped_day_and_month:
+                warnings.warn(f"Parsing {date_string} MM/DD format.")
+            elif not dayfirst and swapped_day_and_month:
+                warnings.warn(f"Parsing {date_string} DD/MM format.")
+        
             return datetime_new(year, month, day, 0, 0, 0, 0, None), reso
+        
+        if dayfirst and not swapped_day_and_month:
+            warnings.warn(f"Parsing {date_string} MM/DD format.")
+        elif not dayfirst and swapped_day_and_month:
+            warnings.warn(f"Parsing {date_string} DD/MM format.")
+        
         return datetime(year, month, day, 0, 0, 0, 0, None), reso
 
     raise DateParseError(f"Invalid date specified ({month}/{day})")