specialize error message for lambda

Author: sunmy2019

Grammar/python.gram

@@ -1358,10 +1358,11 @@ invalid_kvpair:
 invalid_starred_expression:
     | a='*' expression '=' b=expression { RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "cannot assign to iterable argument unpacking") }
 invalid_replacement_field:
-    | '{' a='=' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "f-string: expression required before '='") }
-    | '{' a='!' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "f-string: expression required before '!'") }
-    | '{' a=':' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "f-string: expression required before ':'") }
-    | '{' a='}' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "f-string: expression required before '}'") }
+    | '{' a='=' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "f-string: valid expression required before '='") }
+    | '{' a='!' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "f-string: valid expression required before '!'") }
+    | '{' a=':' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "f-string: valid expression required before ':'") }
+    | '{' a='}' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "f-string: valid expression required before '}'") }
+    | '{' a='lambda' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "f-string: lambda expression are not allowed without parentheses") }
     | '{' !(yield_expr | star_expressions) { RAISE_SYNTAX_ERROR_ON_NEXT_TOKEN("f-string: expecting a valid expression after '{'")}
     | '{' (yield_expr | star_expressions) !('=' | '!' | ':' | '}') {
         PyErr_Occurred() ? NULL : RAISE_SYNTAX_ERROR_ON_NEXT_TOKEN("f-string: expecting '=', or '!', or ':', or '}'") }

Lib/test/test_cmd_line_script.py

@@ -638,7 +638,7 @@ def test_syntaxerror_multi_line_fstring(self):
                 [
                     b'    foo = f"""{}',
                     b'               ^',
-                    b'SyntaxError: f-string: expression required before \'}\'',
+                    b'SyntaxError: f-string: valid expression required before \'}\'',
                 ],
             )
 

Lib/test/test_fstring.py

@@ -722,7 +722,8 @@ def __format__(self, spec):
         self.assertEqual(f'{x} {x}', '1 2')
 
     def test_missing_expression(self):
-        self.assertAllRaise(SyntaxError, "f-string: expression required before '}'",
+        self.assertAllRaise(SyntaxError,
+                            "f-string: valid expression required before '}'",
                             ["f'{}'",
                              "f'{ }'"
                              "f' {} '",
@@ -734,7 +735,8 @@ def test_missing_expression(self):
                              "f'''{\t\f\r\n}'''",
                              ])
 
-        self.assertAllRaise(SyntaxError, "f-string: expression required before '!'",
+        self.assertAllRaise(SyntaxError,
+                            "f-string: valid expression required before '!'",
                             ["f'{!r}'",
                              "f'{ !r}'",
                              "f'{!}'",
@@ -755,15 +757,17 @@ def test_missing_expression(self):
                              "f'{ !xr:a}'",
                              ])
 
-        self.assertAllRaise(SyntaxError, "f-string: expression required before ':'",
+        self.assertAllRaise(SyntaxError,
+                            "f-string: valid expression required before ':'",
                             ["f'{:}'",
                              "f'{ :!}'",
                              "f'{:2}'",
                              "f'''{\t\f\r\n:a}'''",
                              "f'{:'",
                              ])
 
-        self.assertAllRaise(SyntaxError, "f-string: expression required before '='",
+        self.assertAllRaise(SyntaxError,
+                            "f-string: valid expression required before '='",
                             ["f'{=}'",
                              "f'{ =}'",
                              "f'{ =:}'",
@@ -879,7 +883,8 @@ def test_backslashes_in_expression_part(self):
         self.assertEqual(f'{"\N{LEFT CURLY BRACKET}"}', '{')
         self.assertEqual(rf'{"\N{LEFT CURLY BRACKET}"}', '{')
 
-        self.assertAllRaise(SyntaxError, "f-string: expression required before '}'",
+        self.assertAllRaise(SyntaxError,
+                            "f-string: valid expression required before '}'",
                             ["f'{\n}'",
                              ])
 
@@ -926,7 +931,8 @@ def test_lambda(self):
         # lambda doesn't work without parens, because the colon
         #  makes the parser think it's a format_spec
         self.assertAllRaise(SyntaxError, 
-                            "f-string: expecting a valid expression after '{'",
+                            "f-string: lambda expression are not allowed "
+                            "without parentheses",
                             ["f'{lambda x:x}'",
                              ])