TST: added tests

Author: arw2019

pandas/tests/tools/test_to_datetime.py

@@ -1839,6 +1839,60 @@ def test_dayfirst(self, cache):
         tm.assert_index_equal(expected, idx5)
         tm.assert_index_equal(expected, idx6)
 
+    def test_dayfirst_warnings(self):
+        # GH 12585
+
+        # CASE 1: valid input
+        arr = ["31/12/2014", "10/03/2011"]
+        expected = DatetimeIndex(
+            ["2014-12-31", "2011-03-10"], dtype="datetime64[ns]", freq=None
+        )
+
+        # A. dayfirst arg correct, no warning
+        res1 = to_datetime(arr, dayfirst=True)
+        tm.assert_index_equal(expected, res1)
+
+        # B. dayfirst arg incorrect, warning + incorrect output
+        msg = r"Parsing '31/12/2014' in DD/MM/YYYY format."
+        with pytest.warns(UserWarning, match=msg):
+            res2 = to_datetime(arr, dayfirst=False)
+        with pytest.raises(AssertionError):
+            tm.assert_index_equal(expected, res2)
+
+        # C. dayfirst default arg, same as B
+        msg = r"Parsing '31/12/2014' in DD/MM/YYYY format."
+        with pytest.warns(UserWarning, match=msg):
+            res3 = to_datetime(arr, dayfirst=False)
+        with pytest.raises(AssertionError):
+            tm.assert_index_equal(expected, res3)
+
+        # D. infer_datetime_format=True overrides dayfirst default
+        # no warning + correct result
+        res4 = to_datetime(arr, infer_datetime_format=True)
+        tm.assert_index_equal(expected, res4)
+
+        # CASE 2: invalid input
+        # cannot consistently process with single format
+        # warnings *always* raised
+
+        arr = ["31/12/2014", "03/30/2011"]
+
+        msg = r"Parsing '03/30/2011' in MM/DD/YYYY format."
+        with pytest.warns(UserWarning, match=msg):
+            to_datetime(arr, dayfirst=True)
+
+        msg = r"Parsing '31/12/2014' in DD/MM/YYYY format."
+        with pytest.warns(UserWarning, match=msg):
+            to_datetime(arr, dayfirst=False)
+
+        msg = r"Parsing '31/12/2014' in DD/MM/YYYY format."
+        with pytest.warns(UserWarning, match=msg):
+            to_datetime(arr)
+
+        msg = r"Parsing '31/12/2014' in DD/MM/YYYY format."
+        with pytest.warns(UserWarning, match=msg):
+            to_datetime(arr, infer_datetime_format=True)
+
     @pytest.mark.parametrize("klass", [DatetimeIndex, DatetimeArray])
     def test_to_datetime_dta_tz(self, klass):
         # GH#27733