write(): Special case the common situation of a stream that's only

          being used to dump output (no seeks), so we can avoid a lot
          of extra checks being made.

Lib/StringIO.py

@@ -152,6 +152,10 @@ class StringIO:
         # Force s to be a string or unicode
         if not isinstance(s, basestring):
             s = str(s)
+        if self.pos == self.len:
+            self.buflist.append(s)
+            self.len = self.pos = self.pos + len(s)
+            return
         if self.pos > self.len:
             self.buflist.append('\0'*(self.pos - self.len))
             self.len = self.pos