Change join type only for concat() case

Prevents changing the join type from 'left' to 'outer' when merge() is used. Fixes pandas-dev#19607.

Author: elrubio

pandas/core/frame.py

@@ -5328,18 +5328,18 @@ 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)