Patch by Tim Peters:

Introduce a new builtin exception, UnboundLocalError, raised when ceval.c
tries to retrieve or delete a local name that isn't bound to a value.
Currently raises NameError, which makes this behavior a FAQ since the same
error is raised for "missing" global names too:  when the user has a global
of the same name as the unbound local, NameError makes no sense to them.
Even in the absence of shadowing, knowing whether a bogus name is local or
global is a real aid to quick understanding.

Example:

D:\src\PCbuild>type local.py
x = 42

def f():
    print x
    x = 13
    return x

f()

D:\src\PCbuild>python local.py
Traceback (innermost last):
  File "local.py", line 8, in ?
    f()
  File "local.py", line 4, in f
    print x
UnboundLocalError: x

D:\src\PCbuild>

Note that UnboundLocalError is a subclass of NameError, for compatibility
with existing class-exception code that may be trying to catch this as a
NameError.  Unfortunately, I see no way to make this wholly compatible
with -X (see comments in bltinmodule.c):  under -X, [UnboundLocalError
is an alias for NameError --GvR].

[The ceval.c patch differs slightly from the second version that Tim
submitted; I decided not to raise UnboundLocalError for DELETE_NAME,
only for DELETE_LOCAL.  DELETE_NAME is only generated at the module
level, and since at that level a NameError is raised for referencing
an undefined name, it should also be raised for deleting one.]

Lib/exceptions.py

@@ -40,6 +40,8 @@ Exception(*)
       |    +-- NotImplementedError(*)
       |
       +-- NameError
+      |    |
+      |    +-- UnboundLocalError(*)
       +-- AttributeError
       +-- SyntaxError
       +-- TypeError
@@ -208,7 +210,11 @@ class AttributeError(StandardError):
     pass
 
 class NameError(StandardError):
-    """Name not found locally or globally."""
+    """Name not found globally."""
+    pass
+
+class UnboundLocalError(NameError):
+    """Local name referenced but not bound to a value."""
     pass
 
 class MemoryError(StandardError):