--- a/mercurial/formatter.py	Tue Jun 13 17:43:33 2017 -0400
+++ b/mercurial/formatter.py	Sat May 06 14:40:22 2017 +0900
@@ -399,7 +399,8 @@
         # is it a mapfile for a style?
         if os.path.basename(tmpl).startswith("map-"):
             return None, os.path.realpath(tmpl)
-        tmpl = open(tmpl).read()
+        with open(tmpl) as f:
+            tmpl = f.read()
         return tmpl, None
 
     # constant string?