diff -r 4e3c76cb0e8a Lib/os.py
--- a/Lib/os.py	Tue Mar 25 18:19:49 2014 +0100
+++ b/Lib/os.py	Fri Mar 28 10:10:07 2014 +0100
@@ -208,7 +208,10 @@ SEEK_END = 2
 
 
 def _get_masked_mode(mode):
-    mask = umask(0)
+    # Setting the umask changes it process-wide and so affect other threads,
+    # but there is no other way to get the current umask. Use 022 because this
+    # mask is the usual default.
+    mask = umask(0o022)
     umask(mask)
     return mode & ~mask