WARN: add stacklevel to to_dict() UserWarning (#16927) (#16936)

* ERR: add stacklevel to to_dict() UserWarning (#16927)

* TST: Add warning testing to to_dict()

* Fix warning assertion on to_dict() test

* Add github issue to documentation on to_dict() warning test

pandas/core/frame.py

@@ -972,7 +972,8 @@ def to_dict(self, orient='dict', into=dict):
         """
         if not self.columns.is_unique:
             warnings.warn("DataFrame columns are not unique, some "
-                          "columns will be omitted.", UserWarning)
+                          "columns will be omitted.", UserWarning,
+                          stacklevel=2)
         # GH16122
         into_c = standardize_mapping(into)
         if orient.lower().startswith('d'):

pandas/tests/frame/test_convert_to.py

@@ -216,6 +216,13 @@ def test_to_dict_errors(self, mapping):
         with pytest.raises(TypeError):
             df.to_dict(into=mapping)
 
+    def test_to_dict_not_unique_warning(self):
+        # GH16927: When converting to a dict, if a column has a non-unique name
+        # it will be dropped, throwing a warning.
+        df = DataFrame([[1, 2, 3]], columns=['a', 'a', 'b'])
+        with tm.assert_produces_warning(UserWarning):
+            df.to_dict()
+
     @pytest.mark.parametrize('tz', ['UTC', 'GMT', 'US/Eastern'])
     def test_to_records_datetimeindex_with_tz(self, tz):
         # GH13937