check for assignment to __debug__ during AST generation

Also, give assignment to None a better error message

Lib/test/test_syntax.py

@@ -27,15 +27,13 @@ In ast.c, syntax errors are raised by calling ast_error().
 
 Errors from set_context():
 
-TODO(jhylton): "assignment to None" is inconsistent with other messages
-
 >>> obj.None = 1
 Traceback (most recent call last):
-SyntaxError: assignment to None (<doctest test.test_syntax[1]>, line 1)
+SyntaxError: cannot assign to None (<doctest test.test_syntax[1]>, line 1)
 
 >>> None = 1
 Traceback (most recent call last):
-SyntaxError: assignment to None (<doctest test.test_syntax[2]>, line 1)
+SyntaxError: cannot assign to None (<doctest test.test_syntax[2]>, line 1)
 
 It's a syntax error to assign to the empty tuple.  Why isn't it an
 error to assign to the empty list?  It will always raise some error at
@@ -95,7 +93,7 @@ From compiler_complex_args():
 >>> def f(None=1):
 ...     pass
 Traceback (most recent call last):
-SyntaxError: assignment to None (<doctest test.test_syntax[14]>, line 1)
+SyntaxError: cannot assign to None (<doctest test.test_syntax[14]>, line 1)
 
 
 From ast_for_arguments():
@@ -108,17 +106,17 @@ SyntaxError: non-default argument follows default argument (<doctest test.test_s
 >>> def f(x, None):
 ...     pass
 Traceback (most recent call last):
-SyntaxError: assignment to None (<doctest test.test_syntax[16]>, line 1)
+SyntaxError: cannot assign to None (<doctest test.test_syntax[16]>, line 1)
 
 >>> def f(*None):
 ...     pass
 Traceback (most recent call last):
-SyntaxError: assignment to None (<doctest test.test_syntax[17]>, line 1)
+SyntaxError: cannot assign to None (<doctest test.test_syntax[17]>, line 1)
 
 >>> def f(**None):
 ...     pass
 Traceback (most recent call last):
-SyntaxError: assignment to None (<doctest test.test_syntax[18]>, line 1)
+SyntaxError: cannot assign to None (<doctest test.test_syntax[18]>, line 1)
 
 
 From ast_for_funcdef():
@@ -126,7 +124,7 @@ From ast_for_funcdef():
 >>> def None(x):
 ...     pass
 Traceback (most recent call last):
-SyntaxError: assignment to None (<doctest test.test_syntax[19]>, line 1)
+SyntaxError: cannot assign to None (<doctest test.test_syntax[19]>, line 1)
 
 
 From ast_for_call():
@@ -231,7 +229,7 @@ Traceback (most recent call last):
 SyntaxError: augmented assignment to generator expression not possible (<doctest test.test_syntax[31]>, line 1)
 >>> None += 1
 Traceback (most recent call last):
-SyntaxError: assignment to None (<doctest test.test_syntax[32]>, line 1)
+SyntaxError: cannot assign to None (<doctest test.test_syntax[32]>, line 1)
 >>> f() += 1
 Traceback (most recent call last):
 SyntaxError: illegal expression for augmented assignment (<doctest test.test_syntax[33]>, line 1)

Python/ast.c

@@ -130,7 +130,9 @@ static int
 forbidden_check(struct compiling *c, const node *n, const char *x)
 {
     if (!strcmp(x, "None"))
-        return ast_error(n, "assignment to None");
+        return ast_error(n, "cannot assign to None");
+    if (!strcmp(x, "__debug__"))
+        return ast_error(n, "cannot assign to __debug__");
     if (Py_Py3kWarningFlag) {
         if (!(strcmp(x, "True") && strcmp(x, "False")) &&
             !ast_warn(c, n, "assignment to True or False is forbidden in 3.x"))

Python/compile.c

@@ -2344,12 +2344,6 @@ compiler_nameop(struct compiler *c, identifier name, expr_context_ty ctx)
 	PyObject *mangled;
 	/* XXX AugStore isn't used anywhere! */
 
-	/* First check for assignment to __debug__. Param? */
-	if ((ctx == Store || ctx == AugStore || ctx == Del)
-	    && !strcmp(PyString_AS_STRING(name), "__debug__")) {
-		return compiler_error(c, "can not assign to __debug__");
-	}
-
 	mangled = _Py_Mangle(c->u->u_private, name);
 	if (!mangled)
 		return 0;