bpo-32023: Disallow genexprs without parenthesis in class definitions.

Doc/whatsnew/3.7.rst

@@ -638,10 +638,14 @@ Changes in Python behavior
 
       f(1 for x in [1],)
 
+      class C(1 for x in [1]):
+          pass
+
   Python 3.7 now correctly raises a :exc:`SyntaxError`, as a generator
   expression always needs to be directly inside a set of parentheses
-  and cannot have a comma on either side.
-  (Contributed by Serhiy Storchaka in :issue:`32012`.)
+  and cannot have a comma on either side, and the duplication of the
+  parentheses can be omitted only on calls.
+  (Contributed by Serhiy Storchaka in :issue:`32012` and :issue:`32023`.)
 
 
 Changes in the Python API

Lib/test/test_syntax.py

@@ -153,6 +153,10 @@
 SyntaxError: Generator expression must be parenthesized
 >>> f((x for x in L), 1)
 [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+>>> class C(x for x in L):
+...     pass
+Traceback (most recent call last):
+SyntaxError: invalid syntax
 
 >>> def g(*args, **kwargs):
 ...     print(args, sorted(kwargs.items()))

Misc/NEWS.d/next/Core and Builtins/2017-11-15-10-49-35.bpo-32023.XnCGT5.rst

@@ -0,0 +1,3 @@
+SyntaxError is now correctly raised when a generator expression without
+parenthesis is used instead of an inheritance list in a class definition.
+The duplication of the parentheses can be omitted only on calls.

Python/ast.c

@@ -11,6 +11,7 @@
 #include "pythonrun.h"
 
 #include <assert.h>
+#include <stdbool.h>
 
 static int validate_stmts(asdl_seq *);
 static int validate_exprs(asdl_seq *, expr_context_ty, int);
@@ -611,7 +612,7 @@ static stmt_ty ast_for_with_stmt(struct compiling *, const node *, int);
 static stmt_ty ast_for_for_stmt(struct compiling *, const node *, int);
 
 /* Note different signature for ast_for_call */
-static expr_ty ast_for_call(struct compiling *, const node *, expr_ty);
+static expr_ty ast_for_call(struct compiling *, const node *, expr_ty, bool);
 
 static PyObject *parsenumber(struct compiling *, const char *);
 static expr_ty parsestrplus(struct compiling *, const node *n);
@@ -1545,7 +1546,7 @@ ast_for_decorator(struct compiling *c, const node *n)
         name_expr = NULL;
     }
     else {
-        d = ast_for_call(c, CHILD(n, 3), name_expr);
+        d = ast_for_call(c, CHILD(n, 3), name_expr, true);
         if (!d)
             return NULL;
         name_expr = NULL;
@@ -2368,7 +2369,7 @@ ast_for_trailer(struct compiling *c, const node *n, expr_ty left_expr)
             return Call(left_expr, NULL, NULL, LINENO(n),
                         n->n_col_offset, c->c_arena);
         else
-            return ast_for_call(c, CHILD(n, 1), left_expr);
+            return ast_for_call(c, CHILD(n, 1), left_expr, true);
     }
     else if (TYPE(CHILD(n, 0)) == DOT) {
         PyObject *attr_id = NEW_IDENTIFIER(CHILD(n, 1));
@@ -2705,7 +2706,7 @@ ast_for_expr(struct compiling *c, const node *n)
 }
 
 static expr_ty
-ast_for_call(struct compiling *c, const node *n, expr_ty func)
+ast_for_call(struct compiling *c, const node *n, expr_ty func, bool allowgen)
 {
     /*
       arglist: argument (',' argument)*  [',']
@@ -2728,6 +2729,10 @@ ast_for_call(struct compiling *c, const node *n, expr_ty func)
                 nargs++;
             else if (TYPE(CHILD(ch, 1)) == comp_for) {
                 nargs++;
+                if (!allowgen) {
+                    ast_error(c, ch, "invalid syntax");
+                    return NULL;
+                }
                 if (NCH(n) > 1) {
                     ast_error(c, ch, "Generator expression must be parenthesized");
                     return NULL;
@@ -3973,7 +3978,7 @@ ast_for_classdef(struct compiling *c, const node *n, asdl_seq *decorator_seq)
         if (!dummy_name)
             return NULL;
         dummy = Name(dummy_name, Load, LINENO(n), n->n_col_offset, c->c_arena);
-        call = ast_for_call(c, CHILD(n, 3), dummy);
+        call = ast_for_call(c, CHILD(n, 3), dummy, false);
         if (!call)
             return NULL;
     }