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.]

Include/pyerrors.h

@@ -78,6 +78,7 @@ extern DL_IMPORT(PyObject *) PyExc_SyntaxError;
 extern DL_IMPORT(PyObject *) PyExc_SystemError;
 extern DL_IMPORT(PyObject *) PyExc_SystemExit;
 extern DL_IMPORT(PyObject *) PyExc_TypeError;
+extern DL_IMPORT(PyObject *) PyExc_UnboundLocalError;
 extern DL_IMPORT(PyObject *) PyExc_ValueError;
 extern DL_IMPORT(PyObject *) PyExc_ZeroDivisionError;
 

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):

Python/bltinmodule.c

@@ -2232,6 +2232,7 @@ PyObject *PyExc_NotImplementedError;
 PyObject *PyExc_SyntaxError;
 PyObject *PyExc_SystemError;
 PyObject *PyExc_SystemExit;
+PyObject *PyExc_UnboundLocalError;
 PyObject *PyExc_TypeError;
 PyObject *PyExc_ValueError;
 PyObject *PyExc_ZeroDivisionError;
@@ -2261,6 +2262,11 @@ bltin_exc[] = {
 	{"KeyError",           &PyExc_KeyError,           1},
 	{"KeyboardInterrupt",  &PyExc_KeyboardInterrupt,  1},
 	{"MemoryError",        &PyExc_MemoryError,        1},
+	/* Note: NameError is not a leaf in exceptions.py, but unlike
+	   the other non-leafs NameError is meant to be raised directly
+	   at times -- the leaf_exc member really seems to mean something
+	   like "this is an abstract base class" when false.
+	*/
 	{"NameError",          &PyExc_NameError,          1},
 	{"OverflowError",      &PyExc_OverflowError,      1},
 	{"RuntimeError",       &PyExc_RuntimeError,       1},
@@ -2268,6 +2274,7 @@ bltin_exc[] = {
 	{"SyntaxError",        &PyExc_SyntaxError,        1},
 	{"SystemError",        &PyExc_SystemError,        1},
 	{"SystemExit",         &PyExc_SystemExit,         1},
+	{"UnboundLocalError",  &PyExc_UnboundLocalError,  1},
 	{"TypeError",          &PyExc_TypeError,          1},
 	{"ValueError",         &PyExc_ValueError,         1},
 	{"ZeroDivisionError",  &PyExc_ZeroDivisionError,  1},
@@ -2420,6 +2427,14 @@ initerrors(dict)
 	PyTuple_SET_ITEM(PyExc_EnvironmentError, 1, PyExc_OSError);
 	PyDict_SetItemString(dict, "EnvironmentError", PyExc_EnvironmentError);
 
+	/* Make UnboundLocalError an alias for NameError */
+	Py_INCREF(PyExc_NameError);
+	Py_DECREF(PyExc_UnboundLocalError);
+	PyExc_UnboundLocalError = PyExc_NameError;
+	if (PyDict_SetItemString(dict, "UnboundLocalError",
+				 PyExc_NameError) != 0)
+		Py_FatalError("Cannot create string-based exceptions");
+
 	/* missing from the StandardError tuple: Exception, StandardError,
 	 * and SystemExit
 	 */

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;