bpo-44180: Fix edge cases in invalid assigment rules in the parser (GH-26283)

The invalid assignment rules are very delicate since the parser can
easily raise an invalid assignment when a keyword argument is provided.
As they are very deep into the grammar tree, is very difficult to
specify in which contexts these rules can be used and in which don't.
For that, we need to use a different version of the rule that doesn't do
error checking in those situations where we don't want the rule to raise
(keyword arguments and generator expressions).

We also need to check if we are in left-recursive rule, as those can try
to eagerly advance the parser even if the parse will fail at the end of
the expression. Failing to do this allows the parser to start parsing a
call as a tuple and incorrectly identify a keyword argument as an
invalid assignment, before it realizes that it was not a tuple after all.

Author: pablogsal

Grammar/python.gram

@@ -509,13 +509,13 @@ star_named_expression[expr_ty]:
     | '*' a=bitwise_or { _PyAST_Starred(a, Load, EXTRA) }
     | named_expression
 
-named_expression[expr_ty]:
-    | a=NAME ':=' ~ b=expression { _PyAST_NamedExpr(CHECK(expr_ty, _PyPegen_set_expr_context(p, a, Store)), b, EXTRA) }
-    | invalid_named_expression
-    | expression !':='
 
-direct_named_expression[expr_ty]:
+assigment_expression[expr_ty]:
     | a=NAME ':=' ~ b=expression { _PyAST_NamedExpr(CHECK(expr_ty, _PyPegen_set_expr_context(p, a, Store)), b, EXTRA) }
+
+named_expression[expr_ty]:
+    | assigment_expression
+    | invalid_named_expression
     | expression !':='
 
 annotated_rhs[expr_ty]: yield_expr | star_expressions
@@ -706,7 +706,7 @@ group[expr_ty]:
     | '(' a=(yield_expr | named_expression) ')' { a }
     | invalid_group
 genexp[expr_ty]:
-    | '(' a=direct_named_expression b=for_if_clauses ')' { _PyAST_GeneratorExp(a, b, EXTRA) }
+    | '(' a=( assigment_expression | expression !':=') b=for_if_clauses ')' { _PyAST_GeneratorExp(a, b, EXTRA) }
     | invalid_comprehension
 set[expr_ty]: '{' a=star_named_expressions '}' { _PyAST_Set(a, EXTRA) }
 setcomp[expr_ty]:
@@ -745,27 +745,29 @@ arguments[expr_ty] (memo):
     | a=args [','] &')' { a }
     | invalid_arguments
 args[expr_ty]:
-    | a[asdl_expr_seq*]=','.(starred_expression | direct_named_expression !'=')+ b=[',' k=kwargs {k}] { _PyPegen_collect_call_seqs(p, a, b, EXTRA) }
+    | a[asdl_expr_seq*]=','.(starred_expression | ( assigment_expression | expression !':=') !'=')+ b=[',' k=kwargs {k}] {
+        _PyPegen_collect_call_seqs(p, a, b, EXTRA) }
     | a=kwargs { _PyAST_Call(_PyPegen_dummy_name(p),
                           CHECK_NULL_ALLOWED(asdl_expr_seq*, _PyPegen_seq_extract_starred_exprs(p, a)),
                           CHECK_NULL_ALLOWED(asdl_keyword_seq*, _PyPegen_seq_delete_starred_exprs(p, a)),
                           EXTRA) }
+
 kwargs[asdl_seq*]:
     | a=','.kwarg_or_starred+ ',' b=','.kwarg_or_double_starred+ { _PyPegen_join_sequences(p, a, b) }
     | ','.kwarg_or_starred+
     | ','.kwarg_or_double_starred+
 starred_expression[expr_ty]:
     | '*' a=expression { _PyAST_Starred(a, Load, EXTRA) }
 kwarg_or_starred[KeywordOrStarred*]:
+    | invalid_kwarg
     | a=NAME '=' b=expression {
         _PyPegen_keyword_or_starred(p, CHECK(keyword_ty, _PyAST_keyword(a->v.Name.id, b, EXTRA)), 1) }
     | a=starred_expression { _PyPegen_keyword_or_starred(p, a, 0) }
-    | invalid_kwarg
 kwarg_or_double_starred[KeywordOrStarred*]:
+    | invalid_kwarg
     | a=NAME '=' b=expression {
         _PyPegen_keyword_or_starred(p, CHECK(keyword_ty, _PyAST_keyword(a->v.Name.id, b, EXTRA)), 1) }
     | '**' a=expression { _PyPegen_keyword_or_starred(p, CHECK(keyword_ty, _PyAST_keyword(NULL, a, EXTRA)), 1) }
-    | invalid_kwarg
 
 # NOTE: star_targets may contain *bitwise_or, targets may not.
 star_targets[expr_ty]:
@@ -838,29 +840,37 @@ invalid_arguments:
     | a=args ',' '*' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "iterable argument unpacking follows keyword argument unpacking") }
     | a=expression b=for_if_clauses ',' [args | expression for_if_clauses] {
         RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, PyPegen_last_item(b, comprehension_ty)->target, "Generator expression must be parenthesized") }
+    | a=NAME b='=' expression for_if_clauses {
+        RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "invalid syntax. Maybe you meant '==' or ':=' instead of '='?")}
     | a=args for_if_clauses { _PyPegen_nonparen_genexp_in_call(p, a) }
     | args ',' a=expression b=for_if_clauses {
         RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, asdl_seq_GET(b, b->size-1)->target, "Generator expression must be parenthesized") }
     | a=args ',' args { _PyPegen_arguments_parsing_error(p, a) }
 invalid_kwarg:
+    | a=NAME b='=' expression for_if_clauses {
+        RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "invalid syntax. Maybe you meant '==' or ':=' instead of '='?")}
     | !(NAME '=') a=expression b='=' {
         RAISE_SYNTAX_ERROR_KNOWN_RANGE(
             a, b, "expression cannot contain assignment, perhaps you meant \"==\"?") }
 
+expression_without_invalid[expr_ty]:
+    | a=disjunction 'if' b=disjunction 'else' c=expression { _PyAST_IfExp(b, a, c, EXTRA) }
+    | disjunction
+    | lambdef
 invalid_expression:
     # !(NAME STRING) is not matched so we don't show this error with some invalid string prefixes like: kf"dsfsdf"
     # Soft keywords need to also be ignored because they can be parsed as NAME NAME
-    | !(NAME STRING | SOFT_KEYWORD) a=disjunction b=expression {
+    | !(NAME STRING | SOFT_KEYWORD) a=disjunction b=expression_without_invalid {
         RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "invalid syntax. Perhaps you forgot a comma?") }
 
 invalid_named_expression:
     | a=expression ':=' expression {
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(
             a, "cannot use assignment expressions with %s", _PyPegen_get_expr_name(a)) }
-    | a=NAME '=' b=bitwise_or !('='|':='|',') {
-        RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "invalid syntax. Maybe you meant '==' or ':=' instead of '='?") }
-    | !(list|tuple|genexp|'True'|'None'|'False') a=bitwise_or b='=' bitwise_or !('='|':='|',') {
-        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "cannot assign to %s here. Maybe you meant '==' instead of '='?",
+    | a=NAME '=' b=bitwise_or !('='|':=') {
+        p->in_raw_rule ? NULL : RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "invalid syntax. Maybe you meant '==' or ':=' instead of '='?") }
+    | !(list|tuple|genexp|'True'|'None'|'False') a=bitwise_or b='=' bitwise_or !('='|':=') {
+        p->in_raw_rule ? NULL : RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "cannot assign to %s here. Maybe you meant '==' instead of '='?",
                                           _PyPegen_get_expr_name(a)) }
 
 invalid_assignment:

Lib/test/test_syntax.py

@@ -1128,6 +1128,26 @@
 Traceback (most recent call last):
 SyntaxError: cannot assign to f-string expression here. Maybe you meant '==' instead of '='?
 
+>>> (x, y, z=3, d, e)
+Traceback (most recent call last):
+SyntaxError: invalid syntax. Maybe you meant '==' or ':=' instead of '='?
+
+>>> [x, y, z=3, d, e]
+Traceback (most recent call last):
+SyntaxError: invalid syntax. Maybe you meant '==' or ':=' instead of '='?
+
+>>> [z=3]
+Traceback (most recent call last):
+SyntaxError: invalid syntax. Maybe you meant '==' or ':=' instead of '='?
+
+>>> {x, y, z=3, d, e}
+Traceback (most recent call last):
+SyntaxError: invalid syntax. Maybe you meant '==' or ':=' instead of '='?
+
+>>> {z=3}
+Traceback (most recent call last):
+SyntaxError: invalid syntax. Maybe you meant '==' or ':=' instead of '='?
+
 >>> from t import x,
 Traceback (most recent call last):
 SyntaxError: trailing comma not allowed without surrounding parentheses