REF series concat: concat blocks if all of same type

Author: jorisvandenbossche

pandas/core/internals.py

@@ -312,6 +312,14 @@ def ftype(self):
     def merge(self, other):
         return _merge_blocks([self, other])
 
+    def concat_same_type(self, others):
+        """
+        Concatenate list of single blocks of the same type.
+        """
+        values = np.concatenate([self.values] + [o.values for o in others])
+        return self.make_block_same_class(
+            values, placement=slice(0, len(values), 1))
+
     def reindex_axis(self, indexer, method=None, axis=1, fill_value=None,
                      limit=None, mask_info=None):
         """
@@ -2684,6 +2692,21 @@ def shift(self, periods, axis=0, mgr=None):
         return [self.make_block_same_class(new_values,
                                            placement=self.mgr_locs)]
 
+    def concat_same_type(self, others):
+        """
+        Concatenate list of single blocks of the same type.
+        """
+        # can maybe replace
+        # from pandas.core.dtypes.concat._concat_datetimetz ?
+        to_concat = [self.values] + [o.values for o in others]
+
+        if len(set([str(x.dtype) for x in to_concat])) != 1:
+            raise ValueError('to_concat must have the same tz')
+
+        values = to_concat[0]._concat_same_dtype(to_concat, None)
+        return self.make_block_same_class(
+            values, placement=slice(0, len(values), 1))
+
 
 class SparseBlock(NonConsolidatableMixIn, Block):
     """ implement as a list of sparse arrays of the same dtype """

pandas/core/reshape/concat.py

@@ -362,6 +362,16 @@ def get_result(self):
 
             # stack blocks
             if self.axis == 0:
+                name = com._consensus_name_attr(self.objs)
+
+                # check if all series are of the same block type:
+                blocks = [obj._data.blocks[0] for obj in self.objs]
+                if all([type(b) == type(blocks[0]) for b in blocks[1:]]):
+                    new_block = blocks[0].concat_same_type(blocks[1:])
+                    return (Series(new_block, index=self.new_axes[0],
+                                   name=name, fastpath=True)
+                            .__finalize__(self, method='concat'))
+
                 # concat Series with length to keep dtype as much
                 non_empties = [x for x in self.objs if len(x) > 0]
                 if len(non_empties) > 0:
@@ -370,7 +380,6 @@ def get_result(self):
                     values = [x._values for x in self.objs]
                 new_data = _concat._concat_compat(values)
 
-                name = com._consensus_name_attr(self.objs)
                 cons = _concat._get_series_result_type(new_data)
 
                 return (cons(new_data, index=self.new_axes[0],