When trying to decide whether or not a given file needs to be
recompiled, the magic number and timestamp stored with the file should
be checked for equalty. This resmbles the way python internally
identifies a need for recompilation.

This helps to avoid problems when files get unpacked from some kind of
archive so that their mtimes are probably older than the mtimes of
compiled files already in place, even though the compiled files are
outdated. Downgrades are handled gracefully by this method as well.

References:
http://bugs.python.org/issue5128
http://bugs.gentoo.org/256953

2009-02-02 Martin von Gagern

Index: Lib/compileall.py
===================================================================
--- Lib/compileall.py	(revision 69204)
+++ Lib/compileall.py	(working copy)
@@ -15,6 +15,8 @@
 import os
 import sys
 import py_compile
+import struct
+import imp
 
 __all__ = ["compile_dir","compile_path"]
 
@@ -54,11 +56,18 @@
         if os.path.isfile(fullname):
             head, tail = name[:-3], name[-3:]
             if tail == '.py':
-                cfile = fullname + (__debug__ and 'c' or 'o')
-                ftime = os.stat(fullname).st_mtime
-                try: ctime = os.stat(cfile).st_mtime
-                except os.error: ctime = 0
-                if (ctime > ftime) and not force: continue
+                if not force:
+                    try:
+                        mtime = os.stat(fullname).st_mtime
+                        expect = struct.pack('<4sl', imp.get_magic(), mtime)
+                        cfile = fullname + (__debug__ and 'c' or 'o')
+                        chandle = open(cfile, 'rb')
+                        actual = chandle.read(8)
+                        chandle.close()
+                        if expect == actual:
+                            continue
+                    except IOError:
+                        pass
                 if not quiet:
                     print 'Compiling', fullname, '...'
                 try: