From 01355d59fc22860de0d7f04b86318944643ba0f6 Mon Sep 17 00:00:00 2001
From: Alexander Haupt <1485187+elrubio@users.noreply.github.com>
Date: Fri, 9 Feb 2018 18:39:20 +0100
Subject: [PATCH 1/4] BUG: Fix 'left' join turned into 'outer' join when
 joining with a sequence of dataframes (#19607)

---
 pandas/core/frame.py | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/pandas/core/frame.py b/pandas/core/frame.py
index 6d8dcb8a1ca89..a6417f821a4e6 100644
--- a/pandas/core/frame.py
+++ b/pandas/core/frame.py
@@ -5328,18 +5328,17 @@ def _join_compat(self, other, on=None, how='left', lsuffix='', rsuffix='',
                 raise ValueError('Joining multiple DataFrames only supported'
                                  ' for joining on index')
 
-            # join indexes only using concat
-            if how == 'left':
-                how = 'outer'
-                join_axes = [self.index]
-            else:
-                join_axes = None
-
             frames = [self] + list(other)
 
             can_concat = all(df.index.is_unique for df in frames)
 
+            # join indexes only using concat
             if can_concat:
+                if how == 'left':
+                    how = 'outer'
+                    join_axes = [self.index]
+                else:
+                    join_axes = None
                 return concat(frames, axis=1, join=how, join_axes=join_axes,
                               verify_integrity=True)
 

From d1a85e4b19c61a58179a17757090aaf807d41587 Mon Sep 17 00:00:00 2001
From: Alexander Haupt <1485187+elrubio@users.noreply.github.com>
Date: Fri, 9 Feb 2018 18:43:22 +0100
Subject: [PATCH 2/4] TST: Check for correct index after left-joining a
 sequence of dataframes (#19607)

---
 pandas/tests/frame/test_join.py | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/pandas/tests/frame/test_join.py b/pandas/tests/frame/test_join.py
index afecba2026dd7..5a3074716494a 100644
--- a/pandas/tests/frame/test_join.py
+++ b/pandas/tests/frame/test_join.py
@@ -31,6 +31,11 @@ def right():
     return DataFrame({'b': [300, 100, 200]}, index=[3, 1, 2])
 
 
+@pytest.fixture
+def right_non_unique():
+    return DataFrame({'c': [400, 500, 600]}, index=[2, 2, 4])
+
+
 @pytest.mark.parametrize(
     "how, sort, expected",
     [('inner', False, DataFrame({'a': [20, 10],
@@ -165,3 +170,11 @@ def test_join_period_index(frame_with_period_index):
         index=frame_with_period_index.index)
 
     tm.assert_frame_equal(joined, expected)
+
+
+def test_join_left_sequence_non_unique_index(left, right, right_non_unique):
+    # left join sequence of dataframes with non-unique indices (issue #19607)
+    joined = left.join([right_non_unique], how='left')
+    tm.assert_index_equal(
+        joined.index.unique().sort_values(),
+        left.index.sort_values())

From 6eb27ab4725922d15ef65948ebf0c245f6706d03 Mon Sep 17 00:00:00 2001
From: Alexander Haupt <1485187+elrubio@users.noreply.github.com>
Date: Sat, 10 Feb 2018 00:42:56 +0100
Subject: [PATCH 3/4] Test fixture removed, using assert_frame_equal

---
 pandas/tests/frame/test_join.py | 26 +++++++++++++++-----------
 1 file changed, 15 insertions(+), 11 deletions(-)

diff --git a/pandas/tests/frame/test_join.py b/pandas/tests/frame/test_join.py
index 5a3074716494a..ccdba6df2521a 100644
--- a/pandas/tests/frame/test_join.py
+++ b/pandas/tests/frame/test_join.py
@@ -31,11 +31,6 @@ def right():
     return DataFrame({'b': [300, 100, 200]}, index=[3, 1, 2])
 
 
-@pytest.fixture
-def right_non_unique():
-    return DataFrame({'c': [400, 500, 600]}, index=[2, 2, 4])
-
-
 @pytest.mark.parametrize(
     "how, sort, expected",
     [('inner', False, DataFrame({'a': [20, 10],
@@ -172,9 +167,18 @@ def test_join_period_index(frame_with_period_index):
     tm.assert_frame_equal(joined, expected)
 
 
-def test_join_left_sequence_non_unique_index(left, right, right_non_unique):
-    # left join sequence of dataframes with non-unique indices (issue #19607)
-    joined = left.join([right_non_unique], how='left')
-    tm.assert_index_equal(
-        joined.index.unique().sort_values(),
-        left.index.sort_values())
+def test_join_left_sequence_non_unique_index():
+    # https://github.com/pandas-dev/pandas/issues/19607
+    df1 = DataFrame({'a': [0, 10, 20]}, index=[1, 2, 3])
+    df2 = DataFrame({'b': [100, 200, 300]}, index=[4, 3, 2])
+    df3 = DataFrame({'c': [400, 500, 600]}, index=[2, 2, 4])
+
+    joined = df1.join([df2, df3], how='left')
+
+    expected = DataFrame({
+        'a': [0, 10, 10, 20],
+        'b': [np.nan, 300, 300, 200],
+        'c': [np.nan, 400, 500, np.nan]
+    }, index=[1, 2, 2, 3])
+
+    tm.assert_frame_equal(joined, expected)

From 0a068a09e45f03ab831a125c7e96cfb2e68ee28e Mon Sep 17 00:00:00 2001
From: Alexander Haupt <1485187+elrubio@users.noreply.github.com>
Date: Sat, 10 Feb 2018 01:21:33 +0100
Subject: [PATCH 4/4] Mention bug in release notes

---
 doc/source/whatsnew/v0.23.0.txt | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/doc/source/whatsnew/v0.23.0.txt b/doc/source/whatsnew/v0.23.0.txt
index 083242cd69b74..3b626ddced5bf 100644
--- a/doc/source/whatsnew/v0.23.0.txt
+++ b/doc/source/whatsnew/v0.23.0.txt
@@ -772,7 +772,7 @@ Reshaping
 - Bug in timezone comparisons, manifesting as a conversion of the index to UTC in ``.concat()`` (:issue:`18523`)
 - Bug in :func:`concat` when concatting sparse and dense series it returns only a ``SparseDataFrame``. Should be a ``DataFrame``. (:issue:`18914`, :issue:`18686`, and :issue:`16874`)
 - Improved error message for :func:`DataFrame.merge` when there is no common merge key (:issue:`19427`)
--
+- Bug in :func:`DataFrame.join` which does an *outer* instead of a *left* join when being called with multiple DataFrames and some have non-unique indices (:issue:`19624`)
 
 Other
 ^^^^^