Linting, parameterizing test, whatsnew, raising

GH21220

Author: KalyanGokhale

doc/source/whatsnew/v0.24.0.txt

@@ -7,7 +7,7 @@ v0.24.0
 
 New features
 ~~~~~~~~~~~~
-- :func: merge now allows  a ``DataFrame`` and a ``Series`` with a name as inputs (:issue:`21220`)
+- :func:`merge` now directly allows merge between objects of type ``DataFrame`` and ``Series`` with a name, without need to convert the ``Series`` object into a ``DataFrame`` beforehand (:issue:`21220`)
 
 .. _whatsnew_0240.enhancements.other:
 

pandas/core/reshape/merge.py

@@ -1645,5 +1645,5 @@ def validate_operand(obj):
         else:
             return obj.to_frame()
     else:
-        raise ValueError('Cannot merge a DataFrame or a Series with '
-                         'instance of type {obj}'.format(obj=type(obj)))
+        raise TypeError('Can only merge Series or DataFrame objects, '
+                        'a {obj} was passed'.format(obj=type(obj)))

pandas/tests/reshape/merge/test_join.py

@@ -228,18 +228,19 @@ def test_join_on_fails_with_different_column_counts(self):
                             index=tm.makeCustomIndex(10, 2))
             merge(df, df2, right_on='a', left_on=['a', 'b'])
 
-    def test_join_on_fails_with_wrong_object_type(self):
-        # GH12081
-        wrongly_typed = [2, 'str', None, np.array([0, 1])]
+    @pytest.mark.parametrize("wrong_type", [2, 'str', None, np.array([0, 1])])
+    def test_join_on_fails_with_wrong_object_type(self, wrong_type):
+        # GH12081 - original issue
+
         # GH21220 - merging of Series and DataFrame is now allowed
-        # Edited the test to remove the Series object from 'wrongly_typed'
-        df = DataFrame({'a': [1, 1]})
+        # Edited the test to remove the Series object from test parameters
+        # Also, parameterized the original test
 
-        for obj in wrongly_typed:
-            with tm.assert_raises_regex(ValueError, str(type(obj))):
-                merge(obj, df, left_on='a', right_on='a')
-            with tm.assert_raises_regex(ValueError, str(type(obj))):
-                merge(df, obj, left_on='a', right_on='a')
+        df = DataFrame({'a': [1, 1]})
+        with tm.assert_raises_regex(ValueError, str(type(wrong_type))):
+            merge(wrong_type, df, left_on='a', right_on='a')
+        with tm.assert_raises_regex(ValueError, str(type(wrong_type))):
+            merge(df, wrong_type, left_on='a', right_on='a')
 
     def test_join_on_pass_vector(self):
         expected = self.target.join(self.source, on='C')

pandas/tests/reshape/merge/test_merge.py

@@ -1867,23 +1867,14 @@ def test_merge_index_types(index):
 
 
 @pytest.mark.parametrize("on,left_on,right_on,left_index,right_index,nms,nm", [
-                         (['outer', 'inner'], None, None, False, False,
-                          ['outer', 'inner'], 'B'),
-                         (None, None, None, True, True, ['outer', 'inner'],
-                          'B'),
-                         (None, ['outer', 'inner'], None, False, True, None,
-                          'B'),
-                         (None, None, ['outer', 'inner'], True, False, None,
-                          'B'),
-                         (['outer', 'inner'], None, None, False, False,
-                          ['outer', 'inner'], None),
-                         (None, None, None, True, True, ['outer', 'inner'],
-                          None),
-                         (None, ['outer', 'inner'], None, False, True, None,
-                          None),
-                         (None, None, ['outer', 'inner'], True, False, None,
-                          None),
-                         ])
+    (['outer', 'inner'], None, None, False, False, ['outer', 'inner'], 'B'),
+    (None, None, None, True, True, ['outer', 'inner'], 'B'),
+    (None, ['outer', 'inner'], None, False, True, None, 'B'),
+    (None, None, ['outer', 'inner'], True, False, None, 'B'),
+    (['outer', 'inner'], None, None, False, False, ['outer', 'inner'], None),
+    (None, None, None, True, True, ['outer', 'inner'], None),
+    (None, ['outer', 'inner'], None, False, True, None, None),
+    (None, None, ['outer', 'inner'], True, False, None, None)])
 def test_merge_series(on, left_on, right_on, left_index, right_index, nms, nm):
     # GH 21220
     a = pd.DataFrame({"A": [1, 2, 3, 4]},