From 91a3f1663437293eb1ede6e8784bfd7c95fb9b9a Mon Sep 17 00:00:00 2001
From: KalyanGokhale <4734245+KalyanGokhale@users.noreply.github.com>
Date: Sun, 27 May 2018 20:25:41 +0530
Subject: [PATCH 01/17] Initial commit

GH21220
---
 pandas/core/reshape/merge.py             | 20 +++++++++++++-------
 pandas/tests/reshape/merge/test_merge.py | 21 +++++++++++++++++++++
 2 files changed, 34 insertions(+), 7 deletions(-)

diff --git a/pandas/core/reshape/merge.py b/pandas/core/reshape/merge.py
index 4d8897fb7c811..2ef97f49f4cdc 100644
--- a/pandas/core/reshape/merge.py
+++ b/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 +492,10 @@ 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):
+        if isinstance(left, Series):
+            left = left.to_frame()
+        if isinstance(right, Series):
+            right = right.to_frame()
         self.left = self.orig_left = left
         self.right = self.orig_right = right
         self.how = how
@@ -535,12 +539,14 @@ def __init__(self, left, right, how='inner', on=None,
                 '{right_index}'.format(right_index=type(right_index)))
 
         # warn user when merging between different levels
-        if left.columns.nlevels != right.columns.nlevels:
-            msg = ('merging between different levels can give an unintended '
-                   'result ({left} levels on the left, {right} on the right)'
-                   ).format(left=left.columns.nlevels,
-                            right=right.columns.nlevels)
-            warnings.warn(msg, UserWarning)
+        if isinstance(left, DataFrame) and isinstance(right, DataFrame):
+            if left.columns.nlevels != right.columns.nlevels:
+                msg = ('merging between different levels can give an '
+                       'unintended result ({left} levels on the left, '
+                       '{right} on the right)'
+                       ).format(left=left.columns.nlevels,
+                                right=right.columns.nlevels)
+                warnings.warn(msg, UserWarning)
 
         self._validate_specification()
 
diff --git a/pandas/tests/reshape/merge/test_merge.py b/pandas/tests/reshape/merge/test_merge.py
index 8e639edd34b18..42a39b282f53f 100644
--- a/pandas/tests/reshape/merge/test_merge.py
+++ b/pandas/tests/reshape/merge/test_merge.py
@@ -1864,3 +1864,24 @@ def test_merge_index_types(index):
         OrderedDict([('left_data', [1, 2]), ('right_data', [1.0, 2.0])]),
         index=index)
     assert_frame_equal(result, expected)
+
+
+def test_merge_series():
+    # 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='B')
+    expected = pd.DataFrame({"A": [2, 4], "B": [1, 3]},
+                            index=pd.MultiIndex.from_product([['a', 'b'], [1]],
+                            names=['outer', 'inner']))
+
+    # Testing current merge behvaior is as before
+    result = pd.merge(a, b.to_frame(), on=['outer', 'inner'])
+    tm.assert_frame_equal(result, expected)
+
+    # Testing changed merge behvaior is as expected
+    result = pd.merge(a, b, on=['outer', 'inner'])
+    tm.assert_frame_equal(result, expected)

From 64e6e66d0d28e6132c64b917fc95a5b271d46391 Mon Sep 17 00:00:00 2001
From: KalyanGokhale <4734245+KalyanGokhale@users.noreply.github.com>
Date: Sun, 27 May 2018 20:36:22 +0530
Subject: [PATCH 02/17] Removed some now unrequired edits left

GH21220
---
 pandas/core/reshape/merge.py | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/pandas/core/reshape/merge.py b/pandas/core/reshape/merge.py
index 2ef97f49f4cdc..ed1e184d9915a 100644
--- a/pandas/core/reshape/merge.py
+++ b/pandas/core/reshape/merge.py
@@ -539,14 +539,12 @@ def __init__(self, left, right, how='inner', on=None,
                 '{right_index}'.format(right_index=type(right_index)))
 
         # warn user when merging between different levels
-        if isinstance(left, DataFrame) and isinstance(right, DataFrame):
-            if left.columns.nlevels != right.columns.nlevels:
-                msg = ('merging between different levels can give an '
-                       'unintended result ({left} levels on the left, '
-                       '{right} on the right)'
-                       ).format(left=left.columns.nlevels,
-                                right=right.columns.nlevels)
-                warnings.warn(msg, UserWarning)
+        if left.columns.nlevels != right.columns.nlevels:
+            msg = ('merging between different levels can give an unintended '
+                   'result ({left} levels on the left, {right} on the right)'
+                   ).format(left=left.columns.nlevels,
+                            right=right.columns.nlevels)
+            warnings.warn(msg, UserWarning)
 
         self._validate_specification()
 

From 8939fad829fc692bad978a57ad70dbe8016c04b4 Mon Sep 17 00:00:00 2001
From: KalyanGokhale <4734245+KalyanGokhale@users.noreply.github.com>
Date: Mon, 28 May 2018 07:50:48 +0530
Subject: [PATCH 03/17] Edits to test_join

GH21220
---
 pandas/tests/reshape/merge/test_join.py | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/pandas/tests/reshape/merge/test_join.py b/pandas/tests/reshape/merge/test_join.py
index 1b8f3632d381c..52f5dcb8d5db2 100644
--- a/pandas/tests/reshape/merge/test_join.py
+++ b/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])]
+        """ GH 21220 - 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:

From b09ffb7d779321102fb2f94d0c046dd70bec1c41 Mon Sep 17 00:00:00 2001
From: KalyanGokhale <4734245+KalyanGokhale@users.noreply.github.com>
Date: Mon, 28 May 2018 18:35:09 +0530
Subject: [PATCH 04/17] Minor comment edit to force Travis CI rerun

GH21220
---
 pandas/tests/reshape/merge/test_join.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas/tests/reshape/merge/test_join.py b/pandas/tests/reshape/merge/test_join.py
index 52f5dcb8d5db2..b2ba93aa4813f 100644
--- a/pandas/tests/reshape/merge/test_join.py
+++ b/pandas/tests/reshape/merge/test_join.py
@@ -230,7 +230,7 @@ def test_join_on_fails_with_different_column_counts(self):
 
     def test_join_on_fails_with_wrong_object_type(self):
         # GH12081
-        """ GH 21220 - merging of Series and DataFrame is now allowed
+        """ 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])]

From 1a0e5a2f5c95c5af7966f4fc8f7b3bdaeb357213 Mon Sep 17 00:00:00 2001
From: KalyanGokhale <4734245+KalyanGokhale@users.noreply.github.com>
Date: Mon, 28 May 2018 21:59:21 +0530
Subject: [PATCH 05/17] Updated comments in test

GH21220
---
 pandas/tests/reshape/merge/test_merge.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/pandas/tests/reshape/merge/test_merge.py b/pandas/tests/reshape/merge/test_merge.py
index 42a39b282f53f..2040934b5cc2c 100644
--- a/pandas/tests/reshape/merge/test_merge.py
+++ b/pandas/tests/reshape/merge/test_merge.py
@@ -1878,10 +1878,10 @@ def test_merge_series():
                             index=pd.MultiIndex.from_product([['a', 'b'], [1]],
                             names=['outer', 'inner']))
 
-    # Testing current merge behvaior is as before
+    # Test merge with a DataFrame and a Series 'converted-to-DataFrame' object
     result = pd.merge(a, b.to_frame(), on=['outer', 'inner'])
     tm.assert_frame_equal(result, expected)
 
-    # Testing changed merge behvaior is as expected
+    # Test merge with a DataFrame and a Series object
     result = pd.merge(a, b, on=['outer', 'inner'])
     tm.assert_frame_equal(result, expected)

From 59b487dc13d54bb96f9904a45a659a1935e44824 Mon Sep 17 00:00:00 2001
From: KalyanGokhale <4734245+KalyanGokhale@users.noreply.github.com>
Date: Thu, 31 May 2018 22:57:22 +0530
Subject: [PATCH 06/17] Validate operand, parametrized tests, whatnew

GH21220
---
 doc/source/whatsnew/v0.23.1.txt          |  1 +
 pandas/core/reshape/merge.py             | 26 ++++++++-------
 pandas/tests/reshape/merge/test_merge.py | 40 +++++++++++++++++-------
 3 files changed, 45 insertions(+), 22 deletions(-)

diff --git a/doc/source/whatsnew/v0.23.1.txt b/doc/source/whatsnew/v0.23.1.txt
index 4876678baaa6e..15871163d2b34 100644
--- a/doc/source/whatsnew/v0.23.1.txt
+++ b/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:
diff --git a/pandas/core/reshape/merge.py b/pandas/core/reshape/merge.py
index ed1e184d9915a..3ea4b02ed3359 100644
--- a/pandas/core/reshape/merge.py
+++ b/pandas/core/reshape/merge.py
@@ -492,10 +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):
-        if isinstance(left, Series):
-            left = left.to_frame()
-        if isinstance(right, Series):
-            right = right.to_frame()
+        left = validate_operand(left)
+        right = validate_operand(right)
         self.left = self.orig_left = left
         self.right = self.orig_right = right
         self.how = how
@@ -522,13 +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 '
@@ -1643,3 +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)))
diff --git a/pandas/tests/reshape/merge/test_merge.py b/pandas/tests/reshape/merge/test_merge.py
index 2040934b5cc2c..29630ca827ee3 100644
--- a/pandas/tests/reshape/merge/test_merge.py
+++ b/pandas/tests/reshape/merge/test_merge.py
@@ -1866,22 +1866,40 @@ def test_merge_index_types(index):
     assert_frame_equal(result, expected)
 
 
-def test_merge_series():
+@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='B')
+                  names=['outer', 'inner']), name=nm)
     expected = pd.DataFrame({"A": [2, 4], "B": [1, 3]},
                             index=pd.MultiIndex.from_product([['a', 'b'], [1]],
-                            names=['outer', 'inner']))
-
-    # Test merge with a DataFrame and a Series 'converted-to-DataFrame' object
-    result = pd.merge(a, b.to_frame(), on=['outer', 'inner'])
-    tm.assert_frame_equal(result, expected)
-
-    # Test merge with a DataFrame and a Series object
-    result = pd.merge(a, b, on=['outer', 'inner'])
-    tm.assert_frame_equal(result, expected)
+                            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)

From 0d14cc26e570208e70d34f77bfd4cea3b5f1337d Mon Sep 17 00:00:00 2001
From: KalyanGokhale <4734245+KalyanGokhale@users.noreply.github.com>
Date: Thu, 7 Jun 2018 21:57:53 +0530
Subject: [PATCH 07/17] Moved whatsnew to 0.24.0

GH21220
---
 doc/source/whatsnew/v0.23.1.txt | 2 +-
 doc/source/whatsnew/v0.24.0.txt | 1 +
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/doc/source/whatsnew/v0.23.1.txt b/doc/source/whatsnew/v0.23.1.txt
index 15871163d2b34..9a1ee2b8a77de 100644
--- a/doc/source/whatsnew/v0.23.1.txt
+++ b/doc/source/whatsnew/v0.23.1.txt
@@ -16,7 +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:
diff --git a/doc/source/whatsnew/v0.24.0.txt b/doc/source/whatsnew/v0.24.0.txt
index 43e513c9d03f5..6a48e3d728c83 100644
--- a/doc/source/whatsnew/v0.24.0.txt
+++ b/doc/source/whatsnew/v0.24.0.txt
@@ -7,6 +7,7 @@ v0.24.0
 
 New features
 ~~~~~~~~~~~~
+- :func: merge now allows  a ``DataFrame`` and a ``Series`` with a name as inputs (:issue:`21220`)
 
 .. _whatsnew_0240.enhancements.other:
 

From 12da2ce64e1c81f257f978996fe2765198298f18 Mon Sep 17 00:00:00 2001
From: KalyanGokhale <4734245+KalyanGokhale@users.noreply.github.com>
Date: Thu, 7 Jun 2018 21:59:14 +0530
Subject: [PATCH 08/17] Moved whatsnew to 0.24.0

GH21220
---
 doc/source/whatsnew/v0.23.1.txt | 1 -
 1 file changed, 1 deletion(-)

diff --git a/doc/source/whatsnew/v0.23.1.txt b/doc/source/whatsnew/v0.23.1.txt
index 9a1ee2b8a77de..4876678baaa6e 100644
--- a/doc/source/whatsnew/v0.23.1.txt
+++ b/doc/source/whatsnew/v0.23.1.txt
@@ -18,7 +18,6 @@ New features
 - :meth:`Index.droplevel` is now implemented also for flat indexes, for compatibility with MultiIndex (:issue:`21115`)
 
 
-
 .. _whatsnew_0231.deprecations:
 
 Deprecations

From 2d689ad8c643df95a3e0479d2da70ce351fb1e6c Mon Sep 17 00:00:00 2001
From: KalyanGokhale <4734245+KalyanGokhale@users.noreply.github.com>
Date: Fri, 15 Jun 2018 14:36:13 +0530
Subject: [PATCH 09/17] Updated comment in test_join

GH21220
---
 pandas/tests/reshape/merge/test_join.py | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/pandas/tests/reshape/merge/test_join.py b/pandas/tests/reshape/merge/test_join.py
index b2ba93aa4813f..bf1ae23ea3405 100644
--- a/pandas/tests/reshape/merge/test_join.py
+++ b/pandas/tests/reshape/merge/test_join.py
@@ -230,10 +230,9 @@ def test_join_on_fails_with_different_column_counts(self):
 
     def test_join_on_fails_with_wrong_object_type(self):
         # GH12081
-        """ 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])]
+        # 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]})
 
         for obj in wrongly_typed:

From 687568b20b0a7baeaad1b1e87cc64ae0fbc07a0b Mon Sep 17 00:00:00 2001
From: KalyanGokhale <4734245+KalyanGokhale@users.noreply.github.com>
Date: Tue, 19 Jun 2018 20:39:39 +0530
Subject: [PATCH 10/17] Linting, parameterizing test, whatsnew, raising

GH21220
---
 doc/source/whatsnew/v0.24.0.txt          |  2 +-
 pandas/core/reshape/merge.py             |  4 ++--
 pandas/tests/reshape/merge/test_join.py  | 21 ++++++++++----------
 pandas/tests/reshape/merge/test_merge.py | 25 ++++++++----------------
 4 files changed, 22 insertions(+), 30 deletions(-)

diff --git a/doc/source/whatsnew/v0.24.0.txt b/doc/source/whatsnew/v0.24.0.txt
index 6a48e3d728c83..cde6072dab0d4 100644
--- a/doc/source/whatsnew/v0.24.0.txt
+++ b/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:
 
diff --git a/pandas/core/reshape/merge.py b/pandas/core/reshape/merge.py
index 3ea4b02ed3359..45b993983a9e0 100644
--- a/pandas/core/reshape/merge.py
+++ b/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)))
diff --git a/pandas/tests/reshape/merge/test_join.py b/pandas/tests/reshape/merge/test_join.py
index bf1ae23ea3405..d2be67b841f08 100644
--- a/pandas/tests/reshape/merge/test_join.py
+++ b/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')
diff --git a/pandas/tests/reshape/merge/test_merge.py b/pandas/tests/reshape/merge/test_merge.py
index 29630ca827ee3..97bb8fc32f45b 100644
--- a/pandas/tests/reshape/merge/test_merge.py
+++ b/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]},

From 7ed96889e73967ed6c46b20cc3d190a18bab9bd0 Mon Sep 17 00:00:00 2001
From: KalyanGokhale <4734245+KalyanGokhale@users.noreply.github.com>
Date: Tue, 19 Jun 2018 22:15:04 +0530
Subject: [PATCH 11/17] Updated test_join

GH21220
---
 pandas/tests/reshape/merge/test_join.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/pandas/tests/reshape/merge/test_join.py b/pandas/tests/reshape/merge/test_join.py
index d2be67b841f08..2fe5c006e240d 100644
--- a/pandas/tests/reshape/merge/test_join.py
+++ b/pandas/tests/reshape/merge/test_join.py
@@ -237,9 +237,9 @@ def test_join_on_fails_with_wrong_object_type(self, wrong_type):
         # Also, parameterized the original test
 
         df = DataFrame({'a': [1, 1]})
-        with tm.assert_raises_regex(ValueError, str(type(wrong_type))):
+        with tm.assert_raises_regex(TypeError, str(type(wrong_type))):
             merge(wrong_type, df, left_on='a', right_on='a')
-        with tm.assert_raises_regex(ValueError, str(type(wrong_type))):
+        with tm.assert_raises_regex(TypeError, str(type(wrong_type))):
             merge(df, wrong_type, left_on='a', right_on='a')
 
     def test_join_on_pass_vector(self):

From 2ce56e7d61c9f9853731a853f67524f451f9eea6 Mon Sep 17 00:00:00 2001
From: KalyanGokhale <4734245+KalyanGokhale@users.noreply.github.com>
Date: Wed, 20 Jun 2018 06:52:27 +0530
Subject: [PATCH 12/17] Minor update to whatsnew to force TravisCI rebuild

GH21220
---
 doc/source/whatsnew/v0.24.0.txt | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/doc/source/whatsnew/v0.24.0.txt b/doc/source/whatsnew/v0.24.0.txt
index cde6072dab0d4..44259e03ed87b 100644
--- a/doc/source/whatsnew/v0.24.0.txt
+++ b/doc/source/whatsnew/v0.24.0.txt
@@ -7,7 +7,7 @@ v0.24.0
 
 New features
 ~~~~~~~~~~~~
-- :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`)
+- :func:`merge` now directly allows merge between objects of type ``DataFrame`` and ``Series`` with a name, without the need to convert the ``Series`` object into a ``DataFrame`` beforehand (:issue:`21220`)
 
 .. _whatsnew_0240.enhancements.other:
 

From ac9d5a19b51a07ad535bca79f7c9508600968005 Mon Sep 17 00:00:00 2001
From: KalyanGokhale <4734245+KalyanGokhale@users.noreply.github.com>
Date: Wed, 20 Jun 2018 20:38:34 +0530
Subject: [PATCH 13/17] Updating documentation

GH21220
---
 doc/source/merging.rst | 33 +++++++++++++++++----------------
 pandas/core/frame.py   |  7 ++++---
 2 files changed, 21 insertions(+), 19 deletions(-)

diff --git a/doc/source/merging.rst b/doc/source/merging.rst
index 1161656731f88..f88739efbd27a 100644
--- a/doc/source/merging.rst
+++ b/doc/source/merging.rst
@@ -531,23 +531,23 @@ all standard database join operations between ``DataFrame`` objects:
              suffixes=('_x', '_y'), copy=True, indicator=False,
              validate=None)
 
-- ``left``: A DataFrame object.
-- ``right``: Another DataFrame object.
+- ``left``: A DataFrame or named Series object.
+- ``right``: Another DataFrame or named Series object.
 - ``on``: Column or index level names to join on. Must be found in both the left
-  and right DataFrame objects. If not passed and ``left_index`` and
+  and right DataFrame and/or Series objects. If not passed and ``left_index`` and
   ``right_index`` are ``False``, the intersection of the columns in the
-  DataFrames will be inferred to be the join keys.
-- ``left_on``: Columns or index levels from the left DataFrame to use as
+  DataFrames and/or Series will be inferred to be the join keys.
+- ``left_on``: Columns or index levels from the left DataFrame or Series to use as
   keys. Can either be column names, index level names, or arrays with length
-  equal to the length of the DataFrame.
-- ``right_on``: Columns or index levels from the right DataFrame to use as
+  equal to the length of the DataFrame or Series.
+- ``right_on``: Columns or index levels from the right DataFrame or Series to use as
   keys. Can either be column names, index level names, or arrays with length
-  equal to the length of the DataFrame.
+  equal to the length of the DataFrame or Series.
 - ``left_index``: If ``True``, use the index (row labels) from the left
-  DataFrame as its join key(s). In the case of a DataFrame with a MultiIndex
+  DataFrame or Series as its join key(s). In the case of a DataFrame or Series with a MultiIndex
   (hierarchical), the number of levels must match the number of join keys
-  from the right DataFrame.
-- ``right_index``: Same usage as ``left_index`` for the right DataFrame
+  from the right DataFrame or Series.
+- ``right_index``: Same usage as ``left_index`` for the right DataFrame or Series
 - ``how``: One of ``'left'``, ``'right'``, ``'outer'``, ``'inner'``. Defaults
   to ``inner``. See below for more detailed description of each method.
 - ``sort``: Sort the result DataFrame by the join keys in lexicographical
@@ -555,7 +555,7 @@ all standard database join operations between ``DataFrame`` objects:
   substantially in many cases.
 - ``suffixes``: A tuple of string suffixes to apply to overlapping
   columns. Defaults to ``('_x', '_y')``.
-- ``copy``: Always copy data (default ``True``) from the passed DataFrame
+- ``copy``: Always copy data (default ``True``) from the passed DataFrame or named Series
   objects, even when reindexing is not necessary. Cannot be avoided in many
   cases but may improve performance / memory usage. The cases where copying
   can be avoided are somewhat pathological but this option is provided
@@ -563,8 +563,8 @@ all standard database join operations between ``DataFrame`` objects:
 - ``indicator``: Add a column to the output DataFrame called ``_merge``
   with information on the source of each row. ``_merge`` is Categorical-type
   and takes on a value of ``left_only`` for observations whose merge key
-  only appears in ``'left'`` DataFrame, ``right_only`` for observations whose
-  merge key only appears in ``'right'`` DataFrame, and ``both`` if the
+  only appears in ``'left'`` DataFrame or Series, ``right_only`` for observations whose
+  merge key only appears in ``'right'`` DataFrame or Series, and ``both`` if the
   observation's merge key is found in both.
 
 - ``validate`` : string, default None.
@@ -584,9 +584,10 @@ all standard database join operations between ``DataFrame`` objects:
 
    Support for specifying index levels as the ``on``, ``left_on``, and
    ``right_on`` parameters was added in version 0.23.0.
+   Support for merging named ``Series`` objects was added in version 0.24.0.
 
-The return type will be the same as ``left``. If ``left`` is a ``DataFrame``
-and ``right`` is a subclass of DataFrame, the return type will still be
+The return type will be the same as ``left``. If ``left`` is a ``DataFrame`` or named ``Series``
+and ``right`` is a subclass of ``DataFrame``, the return type will still be
 ``DataFrame``.
 
 ``merge`` is a function in the pandas namespace, and it is also available as a
diff --git a/pandas/core/frame.py b/pandas/core/frame.py
index d4ce8dc166b09..3c349d65e69ad 100644
--- a/pandas/core/frame.py
+++ b/pandas/core/frame.py
@@ -135,8 +135,8 @@
 """
 
 _merge_doc = """
-Merge DataFrame objects by performing a database-style join operation by
-columns or indexes.
+Merge DataFrame or named Series objects by performing a database-style join
+operation by columns or indexes.
 
 If joining columns on columns, the DataFrame indexes *will be
 ignored*. Otherwise if joining indexes on indexes or indexes on a column or
@@ -144,7 +144,7 @@
 
 Parameters
 ----------%s
-right : DataFrame
+right : DataFrame or named Series
 how : {'left', 'right', 'outer', 'inner'}, default 'inner'
     * left: use only keys from left frame, similar to a SQL left outer join;
       preserve key order
@@ -208,6 +208,7 @@
 -----
 Support for specifying index levels as the `on`, `left_on`, and
 `right_on` parameters was added in version 0.23.0
+Support for merging named Series objects was added in version 0.24.0
 
 Examples
 --------

From e896a917fecd5779d4a0a574747f667272684d46 Mon Sep 17 00:00:00 2001
From: KalyanGokhale <4734245+KalyanGokhale@users.noreply.github.com>
Date: Wed, 20 Jun 2018 20:53:29 +0530
Subject: [PATCH 14/17] Edits to rst

GH21220
---
 doc/source/merging.rst | 33 ++++++++++++++++-----------------
 1 file changed, 16 insertions(+), 17 deletions(-)

diff --git a/doc/source/merging.rst b/doc/source/merging.rst
index fdb347a7f347b..5ee28746c0956 100644
--- a/doc/source/merging.rst
+++ b/doc/source/merging.rst
@@ -506,8 +506,8 @@ You can also pass a list of dicts or Series:
 
 .. _merging.join:
 
-Database-style DataFrame joining/merging
-----------------------------------------
+Database-style DataFrame or named Series joining/merging
+--------------------------------------------------------
 
 pandas has full-featured, **high performance** in-memory join operations
 idiomatically very similar to relational databases like SQL. These methods
@@ -522,7 +522,7 @@ Users who are familiar with SQL but new to pandas might be interested in a
 :ref:`comparison with SQL<compare_with_sql.join>`.
 
 pandas provides a single function, :func:`~pandas.merge`, as the entry point for 
-all standard database join operations between ``DataFrame`` objects:
+all standard database join operations between ``DataFrame`` or named ``Series`` objects:
 
 ::
 
@@ -531,23 +531,23 @@ all standard database join operations between ``DataFrame`` objects:
              suffixes=('_x', '_y'), copy=True, indicator=False,
              validate=None)
 
-* ``left``: A DataFrame object.
-* ``right``: Another DataFrame object.
+* ``left``: A DataFrame or named Series object.
+* ``right``: Another DataFrame or named Series object.
 * ``on``: Column or index level names to join on. Must be found in both the left
-  and right DataFrame objects. If not passed and ``left_index`` and
+  and right DataFrame and/or Series objects. If not passed and ``left_index`` and
   ``right_index`` are ``False``, the intersection of the columns in the
-  DataFrames will be inferred to be the join keys.
-* ``left_on``: Columns or index levels from the left DataFrame to use as
+  DataFrames and/or Series will be inferred to be the join keys.
+* ``left_on``: Columns or index levels from the left DataFrame or Series to use as
   keys. Can either be column names, index level names, or arrays with length
-  equal to the length of the DataFrame.
-* ``right_on``: Columns or index levels from the right DataFrame to use as
+  equal to the length of the DataFrame or Series.
+* ``right_on``: Columns or index levels from the right DataFrame or Series to use as
   keys. Can either be column names, index level names, or arrays with length
-  equal to the length of the DataFrame.
+  equal to the length of the DataFrame or Series.
 * ``left_index``: If ``True``, use the index (row labels) from the left
-  DataFrame as its join key(s). In the case of a DataFrame with a MultiIndex
+  DataFrame or Series as its join key(s). In the case of a DataFrame or Series with a MultiIndex
   (hierarchical), the number of levels must match the number of join keys
-  from the right DataFrame.
-* ``right_index``: Same usage as ``left_index`` for the right DataFrame
+  from the right DataFrame or Series.
+* ``right_index``: Same usage as ``left_index`` for the right DataFrame or Series
 * ``how``: One of ``'left'``, ``'right'``, ``'outer'``, ``'inner'``. Defaults
   to ``inner``. See below for more detailed description of each method.
 * ``sort``: Sort the result DataFrame by the join keys in lexicographical
@@ -555,7 +555,7 @@ all standard database join operations between ``DataFrame`` objects:
   substantially in many cases.
 * ``suffixes``: A tuple of string suffixes to apply to overlapping
   columns. Defaults to ``('_x', '_y')``.
-* ``copy``: Always copy data (default ``True``) from the passed DataFrame
+* ``copy``: Always copy data (default ``True``) from the passed DataFrame or named Series
   objects, even when reindexing is not necessary. Cannot be avoided in many
   cases but may improve performance / memory usage. The cases where copying
   can be avoided are somewhat pathological but this option is provided
@@ -587,8 +587,7 @@ all standard database join operations between ``DataFrame`` objects:
    Support for merging named ``Series`` objects was added in version 0.24.0.
 
 The return type will be the same as ``left``. If ``left`` is a ``DataFrame`` or named ``Series``
-and ``right`` is a subclass of ``DataFrame``, the return type will still be
-``DataFrame``.
+and ``right`` is a subclass of ``DataFrame``, the return type will still be ``DataFrame``.
 
 ``merge`` is a function in the pandas namespace, and it is also available as a
 ``DataFrame`` instance method :meth:`~DataFrame.merge`, with the calling 

From 69d8bda85dc090db693064455df762e063f533ca Mon Sep 17 00:00:00 2001
From: KalyanGokhale <4734245+KalyanGokhale@users.noreply.github.com>
Date: Wed, 20 Jun 2018 20:57:19 +0530
Subject: [PATCH 15/17] Whatsnew

GH21220
---
 doc/source/whatsnew/v0.24.0.txt | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/doc/source/whatsnew/v0.24.0.txt b/doc/source/whatsnew/v0.24.0.txt
index 515a9cfecf91e..f44c2f49836de 100644
--- a/doc/source/whatsnew/v0.24.0.txt
+++ b/doc/source/whatsnew/v0.24.0.txt
@@ -7,7 +7,7 @@ v0.24.0
 
 New features
 ~~~~~~~~~~~~
-- :func:`merge` now directly allows merge between objects of type ``DataFrame`` and ``Series`` with a name, without the need to convert the ``Series`` object into a ``DataFrame`` beforehand (:issue:`21220`)
+- :func:`merge` now directly allows merge between objects of type ``DataFrame`` and named ``Series``, without the need to convert the ``Series`` object into a ``DataFrame`` beforehand (:issue:`21220`)
 
 - ``ExcelWriter`` now accepts ``mode`` as a keyword argument, enabling append to existing workbooks when using the ``openpyxl`` engine (:issue:`3441`)
 

From c4c988c91dc5caa337f96d8360511f64866eefd2 Mon Sep 17 00:00:00 2001
From: Kalyan Gokhale <4734245+KalyanGokhale@users.noreply.github.com>
Date: Mon, 23 Jul 2018 19:29:03 +0530
Subject: [PATCH 16/17] Update test_join.py

---
 pandas/tests/reshape/merge/test_join.py | 1 -
 1 file changed, 1 deletion(-)

diff --git a/pandas/tests/reshape/merge/test_join.py b/pandas/tests/reshape/merge/test_join.py
index 2fe5c006e240d..7e8ddd7eabffe 100644
--- a/pandas/tests/reshape/merge/test_join.py
+++ b/pandas/tests/reshape/merge/test_join.py
@@ -234,7 +234,6 @@ def test_join_on_fails_with_wrong_object_type(self, wrong_type):
 
         # GH21220 - merging of Series and DataFrame is now allowed
         # Edited the test to remove the Series object from test parameters
-        # Also, parameterized the original test
 
         df = DataFrame({'a': [1, 1]})
         with tm.assert_raises_regex(TypeError, str(type(wrong_type))):

From d099fd6c78db0739d9abe04bfb5c89720d234a7b Mon Sep 17 00:00:00 2001
From: Kalyan Gokhale <4734245+KalyanGokhale@users.noreply.github.com>
Date: Mon, 23 Jul 2018 20:15:59 +0530
Subject: [PATCH 17/17] Force CI tests

---
 pandas/tests/reshape/merge/test_join.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas/tests/reshape/merge/test_join.py b/pandas/tests/reshape/merge/test_join.py
index 7e8ddd7eabffe..09f511886583c 100644
--- a/pandas/tests/reshape/merge/test_join.py
+++ b/pandas/tests/reshape/merge/test_join.py
@@ -233,7 +233,7 @@ 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 test parameters
+        # Edited test to remove the Series object from test parameters
 
         df = DataFrame({'a': [1, 1]})
         with tm.assert_raises_regex(TypeError, str(type(wrong_type))):