handle lambda in f-string

Author: sunmy2019

Grammar/python.gram

@@ -1140,6 +1140,8 @@ invalid_expression:
         _PyPegen_check_legacy_stmt(p, a) ? NULL : p->tokens[p->mark-1]->level == 0 ? NULL :
         RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "invalid syntax. Perhaps you forgot a comma?") }
    | a=disjunction 'if' b=disjunction !('else'|':') { RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "expected 'else' after 'if' expression") }
+   | a='lambda' [lambda_params] b=':' &(FSTRING_MIDDLE | fstring_replacement_field)  {
+        RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "f-string: lambda expressions are not allowed without parentheses") }
 
 invalid_named_expression(memo):
     | a=expression ':=' expression {
@@ -1362,7 +1364,6 @@ invalid_replacement_field:
     | '{' 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_fstring.py

@@ -929,11 +929,23 @@ def test_lambda(self):
         self.assertEqual(f'{(lambda y:x*y)("8"):10}', "88888     ")
 
         # lambda doesn't work without parens, because the colon
-        #  makes the parser think it's a format_spec
+        # makes the parser think it's a format_spec
+        # emit warning if we can match a format_spec
         self.assertAllRaise(SyntaxError, 
-                            "f-string: lambda expression are not allowed "
+                            "f-string: lambda expressions are not allowed "
                             "without parentheses",
                             ["f'{lambda x:x}'",
+                             "f'{lambda :x}'",
+                             "f'{lambda *arg, :x}'",
+                             "f'{1, lambda:x}'",
+                             ])
+        
+        # but don't emit the paren warning in general cases
+        self.assertAllRaise(SyntaxError, 
+                            "f-string: expecting a valid expression after '{'",
+                            ["f'{lambda x:}'",
+                             "f'{lambda :}'",
+                             "f'{+ lambda:None}'",
                              ])
 
     def test_valid_prefixes(self):