--- contrib/python/tornado/tornado-4/tornado/httputil.py	(index)
+++ contrib/python/tornado/tornado-4/tornado/httputil.py	(working tree)
@@ -31,6 +31,11 @@ import numbers
 import re
 import time
 
+try:
+    from collections.abc import MutableMapping
+except ImportError:
+    from collections import MutableMapping
+
 from tornado.escape import native_str, parse_qs_bytes, utf8
 from tornado.log import gen_log
 from tornado.util import ObjectDict, PY3
@@ -103,7 +108,7 @@ class _NormalizedHeaderCache(dict):
 _normalized_headers = _NormalizedHeaderCache(1000)
 
 
-class HTTPHeaders(collections.MutableMapping):
+class HTTPHeaders(MutableMapping):
     """A dictionary that maintains ``Http-Header-Case`` for all keys.
 
     Supports multiple values per key via a pair of new methods,
 END()