Factor-out common code for helper classes.

Lib/collections/__init__.py

@@ -633,7 +633,7 @@ class Counter(dict):
 
 
 ########################################################################
-###  ChainMap (helper for configparser)
+###  ChainMap (helper for configparser and string.Template)
 ########################################################################
 
 class _ChainMap(MutableMapping):

Lib/string.py

@@ -46,23 +46,7 @@ def capwords(s, sep=None):
 
 ####################################################################
 import re as _re
-
-class _multimap:
-    """Helper class for combining multiple mappings.
-
-    Used by .{safe_,}substitute() to combine the mapping and keyword
-    arguments.
-    """
-    def __init__(self, primary, secondary):
-        self._primary = primary
-        self._secondary = secondary
-
-    def __getitem__(self, key):
-        try:
-            return self._primary[key]
-        except KeyError:
-            return self._secondary[key]
-
+from collections import _ChainMap
 
 class _TemplateMetaclass(type):
     pattern = r"""
@@ -116,7 +100,7 @@ class Template(metaclass=_TemplateMetaclass):
         if not args:
             mapping = kws
         elif kws:
-            mapping = _multimap(kws, args[0])
+            mapping = _ChainMap(kws, args[0])
         else:
             mapping = args[0]
         # Helper function for .sub()
@@ -142,7 +126,7 @@ class Template(metaclass=_TemplateMetaclass):
         if not args:
             mapping = kws
         elif kws:
-            mapping = _multimap(kws, args[0])
+            mapping = _ChainMap(kws, args[0])
         else:
             mapping = args[0]
         # Helper function for .sub()