diff --git a/hgext/histedit.py b/hgext/histedit.py
--- a/hgext/histedit.py
+++ b/hgext/histedit.py
@@ -1315,7 +1315,7 @@
     our list of rules"""
     commands = []
     for rules in rules:
-        commands.append(b"{0} {1}\n".format(rules.action, rules.ctx))
+        commands.append(b'%s %s\n' % (rules.action, rules.ctx))
     return commands