ENH: Merge DataFrame and Series using on (GH21220)

doc/source/whatsnew/v0.23.1.txt

@@ -16,6 +16,7 @@ New features
 ~~~~~~~~~~~~
 
 - :meth:`Index.droplevel` is now implemented also for flat indexes, for compatibility with MultiIndex (:issue:`21115`)
+- :func: merge now allows  a ``DataFrame`` and a ``Series`` with a name as inputs (:issue:`21220`)
 
 
 .. _whatsnew_0231.deprecations:

pandas/core/reshape/merge.py

@@ -11,7 +11,7 @@
 import pandas.compat as compat
 
 from pandas import (Categorical, DataFrame,
-                    Index, MultiIndex, Timedelta)
+                    Index, MultiIndex, Timedelta, Series)
 from pandas.core.arrays.categorical import _recode_for_categories
 from pandas.core.frame import _merge_doc
 from pandas.core.dtypes.common import (
@@ -492,6 +492,8 @@ def __init__(self, left, right, how='inner', on=None,
                  left_index=False, right_index=False, sort=True,
                  suffixes=('_x', '_y'), copy=True, indicator=False,
                  validate=None):
+        left = validate_operand(left)
+        right = validate_operand(right)
         self.left = self.orig_left = left
         self.right = self.orig_right = right
         self.how = how
@@ -518,13 +520,6 @@ def __init__(self, left, right, how='inner', on=None,
             raise ValueError(
                 'indicator option can only accept boolean or string arguments')
 
-        if not isinstance(left, DataFrame):
-            raise ValueError('can not merge DataFrame with instance of '
-                             'type {left}'.format(left=type(left)))
-        if not isinstance(right, DataFrame):
-            raise ValueError('can not merge DataFrame with instance of '
-                             'type {right}'.format(right=type(right)))
-
         if not is_bool(left_index):
             raise ValueError(
                 'left_index parameter must be of type bool, not '
@@ -1639,3 +1634,16 @@ def _should_fill(lname, rname):
 
 def _any(x):
     return x is not None and com._any_not_none(*x)
+
+
+def validate_operand(obj):
+    if isinstance(obj, DataFrame):
+        return obj
+    elif isinstance(obj, Series):
+        if obj.name is None:
+            raise ValueError('Cannot merge a Series without a name')
+        else:
+            return obj.to_frame()
+    else:
+        raise ValueError('Cannot merge a DataFrame or a Series with '
+                         'instance of type {obj}'.format(obj=type(obj)))

pandas/tests/reshape/merge/test_join.py

@@ -230,7 +230,10 @@ def test_join_on_fails_with_different_column_counts(self):
 
     def test_join_on_fails_with_wrong_object_type(self):
         # GH12081
-        wrongly_typed = [Series([0, 1]), 2, 'str', None, np.array([0, 1])]
+        """ GH21220 - merging of Series and DataFrame is now allowed
+            Edited the test to remove the Series object from 'wrongly_typed'
+        """
+        wrongly_typed = [2, 'str', None, np.array([0, 1])]
         df = DataFrame({'a': [1, 1]})
 
         for obj in wrongly_typed:

pandas/tests/reshape/merge/test_merge.py

@@ -1864,3 +1864,42 @@ def test_merge_index_types(index):
         OrderedDict([('left_data', [1, 2]), ('right_data', [1.0, 2.0])]),
         index=index)
     assert_frame_equal(result, expected)
+
+
+@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),
+                         ])
+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]},
+                     index=pd.MultiIndex.from_product([['a', 'b'], [0, 1]],
+                     names=['outer', 'inner']))
+    b = pd.Series([1, 2, 3, 4],
+                  index=pd.MultiIndex.from_product([['a', 'b'], [1, 2]],
+                  names=['outer', 'inner']), name=nm)
+    expected = pd.DataFrame({"A": [2, 4], "B": [1, 3]},
+                            index=pd.MultiIndex.from_product([['a', 'b'], [1]],
+                            names=nms))
+    if nm is not None:
+        result = pd.merge(a, b, on=on, left_on=left_on, right_on=right_on,
+                          left_index=left_index, right_index=right_index)
+        tm.assert_frame_equal(result, expected)
+    else:
+        with tm.assert_raises_regex(ValueError, 'a Series without a name'):
+            result = pd.merge(a, b, on=on, left_on=left_on, right_on=right_on,
+                              left_index=left_index, right_index=right_index)