REF: make _concat methods @classmethods

Author: toobaz

pandas/core/indexes/base.py

@@ -1685,6 +1685,7 @@ def append(self, other):
         res.name = name
         return res
 
+    @classmethod
     def _concat(self, to_concat):
         """
         Concatenate to_concat to single Index
@@ -1695,7 +1696,8 @@ def _concat(self, to_concat):
         else:
             return _concat._concat_index_asobject(to_concat)
 
-    def _concat_same_dtype(self, to_concat):
+    @classmethod
+    def _concat_same_dtype(cls, to_concat):
         """
         Concatenate to_concat which has the same dtype
         """

pandas/core/indexes/category.py

@@ -622,14 +622,15 @@ def insert(self, loc, item):
         codes = np.concatenate((codes[:loc], code, codes[loc:]))
         return self._create_from_codes(codes)
 
-    def _concat_same_dtype(self, to_concat):
+    @classmethod
+    def _concat_same_dtype(cls, to_concat):
         """
         Concatenate to_concat which has the same class
         ValueError if other is not in the categories
         """
-        to_concat = [self._is_dtype_compat(c) for c in to_concat]
-        codes = np.concatenate([c.codes for c in to_concat])
-        return self._create_from_codes(codes)
+        compat = [to_concat[0]._is_dtype_compat(c) for c in to_concat]
+        codes = np.concatenate([c.codes for c in compat])
+        return to_concat[0]._create_from_codes(codes)
 
     def _codes_for_groupby(self, sort):
         """ Return a Categorical adjusted for groupby """

pandas/core/indexes/range.py

@@ -443,10 +443,11 @@ def join(self, other, how='left', level=None, return_indexers=False,
         return super(RangeIndex, self).join(other, how, level, return_indexers,
                                             sort)
 
-    def _concat_same_dtype(self, indexes):
+    @classmethod
+    def _concat_same_dtype(cls, indexes):
 
         if not all([isinstance(i, RangeIndex) for i in indexes]):
-            return super(RangeIndex, self)._concat_same_dtype(indexes)
+            return super(RangeIndex, cls)._concat_same_dtype(indexes)
 
         start = step = next = None
 
@@ -462,13 +463,13 @@ def _concat_same_dtype(self, indexes):
             elif step is None:
                 # First non-empty index had only one element
                 if obj._start == start:
-                    return super(RangeIndex, self)._concat_same_dtype(indexes)
+                    return super(RangeIndex, cls)._concat_same_dtype(indexes)
                 step = obj._start - start
 
             non_consecutive = ((step != obj._step and len(obj) > 1) or
                                (next is not None and obj._start != next))
             if non_consecutive:
-                return super(RangeIndex, self)._concat_same_dtype(indexes)
+                return super(RangeIndex, cls)._concat_same_dtype(indexes)
 
             if step is not None:
                 next = obj[-1] + step