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.]

1 files changed, 2 insertions, 2 deletions

diff --git a/Python/ceval.c b/Python/ceval.c
index 6218397..1c51ccf 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -1320,7 +1320,7 @@ eval_code2(co, globals, locals,
 		case LOAD_FAST:
 			x = GETLOCAL(oparg);
 			if (x == NULL) {
-				PyErr_SetObject(PyExc_NameError,
+				PyErr_SetObject(PyExc_UnboundLocalError,
 					   PyTuple_GetItem(co->co_varnames,
 							oparg));
 				break;
@@ -1338,7 +1338,7 @@ eval_code2(co, globals, locals,
 		case DELETE_FAST:
 			x = GETLOCAL(oparg);
 			if (x == NULL) {
-				PyErr_SetObject(PyExc_NameError,
+				PyErr_SetObject(PyExc_UnboundLocalError,
 					   PyTuple_GetItem(co->co_varnames,
 							oparg));
 				break;