improve several corner cases related with argument names in parenthesis

- Fix #7362: give a good error message for parenthesized arguments with
  defaults.

- Add a py3k warning for any parenthesized arguments since those are not allowed
  in Py3.  This warning is not given in tuple unpacking, since that incurs the
  tuple unpacking warning.

Lib/test/test_py3kwarn.py

@@ -30,6 +30,18 @@ class TestPy3KWarnings(unittest.TestCase):
             exec "`2`" in {}
         self.assertWarning(None, w, expected)
 
+    def test_paren_arg_names(self):
+        expected = 'parenthesized argument names are invalid in 3.x'
+        def check(s):
+            exec s in {}
+            self.assertWarning(None, w, expected)
+        with check_warnings() as w:
+            check("def f((x)): pass")
+            check("def f((((x))), (y)): pass")
+            check("def f((x), (((y))), m=32): pass")
+            # Something like def f((a, (b))): pass will raise the tuple
+            # unpacking warning.
+
     def test_forbidden_names(self):
         # So we don't screw up our globals
         def safe_exec(expr):

Lib/test/test_syntax.py

@@ -493,10 +493,14 @@ class SyntaxTestCase(unittest.TestCase):
                 self.fail("SyntaxError is not a %s" % subclass.__name__)
             mo = re.search(errtext, str(err))
             if mo is None:
-                self.fail("SyntaxError did not contain '%r'" % (errtext,))
+                self.fail("%s did not contain '%r'" % (err, errtext,))
         else:
             self.fail("compile() did not raise SyntaxError")
 
+    def test_paren_arg_with_default(self):
+        self._check_error("def f((x)=23): pass",
+                          "parenthesized arg with default")
+
     def test_assign_call(self):
         self._check_error("f() = 1", "assign")
 

Python/ast.c

@@ -692,7 +692,8 @@ ast_for_arguments(struct compiling *c, const node *n)
     while (i < NCH(n)) {
         ch = CHILD(n, i);
         switch (TYPE(ch)) {
-            case fpdef:
+            case fpdef: {
+                int complex_args = 0, parenthesized = 0;
             handle_fpdef:
                 /* XXX Need to worry about checking if TYPE(CHILD(n, i+1)) is
                    anything other than EQUAL or a comma? */
@@ -707,6 +708,12 @@ ast_for_arguments(struct compiling *c, const node *n)
                     found_default = 1;
                 }
                 else if (found_default) {
+                    /* def f((x)=4): pass should raise an error.
+                       def f((x, (y))): pass will just incur the tuple unpacking warning. */
+                    if (parenthesized && !complex_args) {
+                        ast_error(n, "parenthesized arg with default");
+                        goto error;
+                    }
                     ast_error(n, 
                              "non-default argument follows default argument");
                     goto error;
@@ -719,6 +726,7 @@ ast_for_arguments(struct compiling *c, const node *n)
                         if (Py_Py3kWarningFlag && !ast_warn(c, ch,
                             "tuple parameter unpacking has been removed in 3.x"))
                             goto error;
+                        complex_args = 1;
                         asdl_seq_SET(args, k++, compiler_complex_args(c, ch));
                         if (!asdl_seq_GET(args, k-1))
                                 goto error;
@@ -726,6 +734,7 @@ ast_for_arguments(struct compiling *c, const node *n)
                         /* def foo((x)): setup for checking NAME below. */
                         /* Loop because there can be many parens and tuple
                            unpacking mixed in. */
+                        parenthesized = 1;
                         ch = CHILD(ch, 0);
                         assert(TYPE(ch) == fpdef);
                         goto handle_fpdef;
@@ -747,7 +756,13 @@ ast_for_arguments(struct compiling *c, const node *n)
                                          
                 }
                 i += 2; /* the name and the comma */
+                if (parenthesized && Py_Py3kWarningFlag &&
+                    !ast_warn(c, ch, "parenthesized argument names "
+                              "are invalid in 3.x"))
+                    goto error;
+
                 break;
+            }
             case STAR:
                 if (!forbidden_check(c, CHILD(n, i+1), STR(CHILD(n, i+1))))
                     goto error;