The case-insensitive _Environ class was lacking a case-insensitive has_key().

Lib/os.py

@@ -245,6 +245,8 @@ else:
                 self.data[key] = item
             def __getitem__(self, key):
                 return self.data[string.upper(key)]
+            def has_key(self, key):
+                return self.data.has_key(string.upper(key))
 
     else:  # Where Env Var Names Can Be Mixed Case
         class _Environ(UserDict.UserDict):