diff -r df8292735bf4 Lib/site.py
--- a/Lib/site.py	Wed Apr 22 21:42:19 2009 -0700
+++ b/Lib/site.py	Thu Apr 23 00:12:23 2009 -0700
@@ -161,11 +161,9 @@
 def addsitedir(sitedir, known_paths=None):
     """Add 'sitedir' argument to sys.path if missing and handle .pth files in
     'sitedir'"""
-    if known_paths is None:
+    reset = known_paths is None
+    if reset:
         known_paths = _init_pathinfo()
-        reset = 1
-    else:
-        reset = 0
     sitedir, sitedircase = makepath(sitedir)
     if not sitedircase in known_paths:
         sys.path.append(sitedir)        # Add path component
@@ -245,15 +243,30 @@
     return known_paths
 
 
+def add_prefixes():
+    global ENABLE_USER_SITE, PREFIXES
+    prefix_path = os.environ.get("PYTHONPREFIXES", None)
+    if not (ENABLE_USER_SITE and prefix_path):
+        return
+    prefixes = []
+    for prefix in prefix_path.split(os.pathsep):
+        prefix = prefix.rstrip(os.sep + (os.altsep or ""))
+        if os.path.isdir(prefix):
+            prefixes.append(prefix)
+    if prefixes:
+        PREFIXES = prefixes + PREFIXES
+
+
+
 def addsitepackages(known_paths):
     """Add site-packages (and possibly site-python) to sys.path"""
     sitedirs = []
-    seen = []
+    seen = set()
 
     for prefix in PREFIXES:
         if not prefix or prefix in seen:
             continue
-        seen.append(prefix)
+        seen.add(prefix)
 
         if sys.platform in ('os2emx', 'riscos'):
             sitedirs.append(os.path.join(prefix, "Lib", "site-packages"))
@@ -500,6 +513,7 @@
     if ENABLE_USER_SITE is None:
         ENABLE_USER_SITE = check_enableusersite()
     known_paths = addusersitepackages(known_paths)
+    add_prefixes()
     known_paths = addsitepackages(known_paths)
     if sys.platform == 'os2emx':
         setBEGINLIBPATH()