PERF: speed up concat on Series by skipping unnecessary DataFrame creation

Author: qwhelan

doc/source/whatsnew/v0.24.0.txt

@@ -981,6 +981,7 @@ Performance Improvements
 - Improved the performance of :func:`pandas.get_dummies` with ``sparse=True`` (:issue:`21997`)
 - Improved performance of :func:`IndexEngine.get_indexer_non_unique` for sorted, non-unique indexes (:issue:`9466`)
 - Improved performance of :func:`PeriodIndex.unique` (:issue:`23083`)
+- Improved performance of :func:`pd.concat` for `Series` objects (:issue:`23404`)
 
 
 .. _whatsnew_0240.docs:

pandas/core/reshape/concat.py

@@ -320,7 +320,10 @@ def __init__(self, objs, axis=0, join='outer', join_axes=None,
 
         # Standardize axis parameter to int
         if isinstance(sample, Series):
-            axis = DataFrame()._get_axis_number(axis)
+            if compat.PY2:
+                axis = DataFrame()._get_axis_number(axis)
+            else:
+                axis = DataFrame._get_axis_number(DataFrame, axis)
         else:
             axis = sample._get_axis_number(axis)