diff --git a/build/pymake/pymake/data.py b/build/pymake/pymake/data.py
index cb916a35c5f2..935ddcd0d25e 100644
--- a/build/pymake/pymake/data.py
+++ b/build/pymake/pymake/data.py
@@ -573,7 +573,8 @@ class Pattern(object):
     def __init__(self, s):
         r = []
         i = 0
-        while i < len(s):
+        slen = len(s)
+        while i < slen:
             c = s[i]
             if c == '\\':
                 nc = s[i + 1]