Revision on assignment errors (#97)

* Revision on assignment expression errors

 * added rule for __debug__ (should be a keyword)
 * reviewed error messages
 * added new failing samples

* Adjustment upon Dave's review

 * rewind several changes in assignment errors
 * patched is_definition: command not found for assignment expressions
 * patched Python 2 inconsistent error messages in test_python_errors.py: command not found

parso/python/errors.py

@@ -914,6 +914,14 @@ class _CheckAssignmentRule(SyntaxRule):
                     if second.type == 'yield_expr':
                         error = 'yield expression'
                     elif second.type == 'testlist_comp':
+                        # ([a, b] := [1, 2])
+                        # ((a, b) := [1, 2])
+                        if is_namedexpr:
+                            if first == '(':
+                                error = 'tuple'
+                            elif first == '[':
+                                error = 'list'
+
                         # This is not a comprehension, they were handled
                         # further above.
                         for child in second.children[::2]:
@@ -963,6 +971,8 @@ class _CheckAssignmentRule(SyntaxRule):
 
         if error is not None:
             if is_namedexpr:
+                # c.f. CPython bpo-39176, should be changed in next release
+                # message = 'cannot use assignment expressions with %s' % error
                 message = 'cannot use named assignment with %s' % error
             else:
                 cannot = "can't" if self._normalizer.version < (3, 8) else "cannot"

parso/python/tree.py

@@ -233,6 +233,8 @@ class Name(_LeafWithoutNewlines):
         while node is not None:
             if node.type == 'suite':
                 return None
+            if node.type == 'namedexpr_test':
+                return node.children[0]
             if node.type in _GET_DEFINITION_TYPES:
                 if self in node.get_defined_names(include_setitem):
                     return node

test/failing_examples.py

@@ -347,4 +347,14 @@ if sys.version_info[:2] >= (3, 8):
         # Not in that issue
         '(await a := x)',
         '((await a) := x)',
+        # new discoveries
+        '((a, b) := (1, 2))',
+        '([a, b] := [1, 2])',
+        '({a, b} := {1, 2})',
+        '({a: b} := {1: 2})',
+        '(a + b := 1)',
+        '(True := 1)',
+        '(False := 1)',
+        '(None := 1)',
+        '(__debug__ := 1)',
     ]

test/test_python_errors.py

@@ -321,4 +321,3 @@ def test_invalid_fstrings(code, message):
 def test_trailing_comma(code):
     errors = _get_error_list(code)
     assert not errors
-