bpo-43823: Improve syntax errors for invalid dictionary literals (GH-25378)

Grammar/python.gram

@@ -650,6 +650,8 @@ dict[expr_ty]:
             CHECK(asdl_expr_seq*, _PyPegen_get_keys(p, a)),
             CHECK(asdl_expr_seq*, _PyPegen_get_values(p, a)),
             EXTRA) }
+    | '{' invalid_double_starred_kvpairs '}'
+
 dictcomp[expr_ty]:
     | '{' a=kvpair b=for_if_clauses '}' { _PyAST_DictComp(a->key, a->value, b, EXTRA) }
     | invalid_dict_comprehension
@@ -882,3 +884,12 @@ invalid_elif_stmt:
     | 'elif' named_expression NEWLINE { RAISE_SYNTAX_ERROR("expected ':'") }
 invalid_while_stmt:
     | 'while' named_expression NEWLINE { RAISE_SYNTAX_ERROR("expected ':'") }
+
+invalid_double_starred_kvpairs:
+    | ','.double_starred_kvpair+ ',' invalid_kvpair
+    | expression ':' a='*' bitwise_or { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "cannot use a starred expression in a dictionary value") }
+    | expression a=':' &('}'|',') { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "expression expected after dictionary key and ':'") }
+invalid_kvpair:
+    | a=expression !(':') { RAISE_SYNTAX_ERROR("':' expected after dictionary key") }
+    | expression ':' a='*' bitwise_or { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "cannot use a starred expression in a dictionary value") }
+    | expression a=':' {RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "expression expected after dictionary key and ':'") }

Lib/test/test_syntax.py

@@ -880,6 +880,38 @@ Ensure that early = are not matched by the parser as invalid comparisons
    Traceback (most recent call last):
    SyntaxError: invalid syntax
 
+Incomplete dictionary literals
+
+   >>> {1:2, 3:4, 5}
+   Traceback (most recent call last):
+   SyntaxError: ':' expected after dictionary key
+
+   >>> {1:2, 3:4, 5:}
+   Traceback (most recent call last):
+   SyntaxError: expression expected after dictionary key and ':'
+
+   >>> {1: *12+1, 23: 1}
+   Traceback (most recent call last):
+   SyntaxError: cannot use a starred expression in a dictionary value
+
+   >>> {1: *12+1}
+   Traceback (most recent call last):
+   SyntaxError: cannot use a starred expression in a dictionary value
+
+   >>> {1: 23, 1: *12+1}
+   Traceback (most recent call last):
+   SyntaxError: cannot use a starred expression in a dictionary value
+
+   >>> {1:}
+   Traceback (most recent call last):
+   SyntaxError: expression expected after dictionary key and ':'
+
+   # Ensure that the error is not raise for syntax errors that happen after sets
+
+   >>> {1} $
+   Traceback (most recent call last):
+   SyntaxError: invalid syntax
+
 Make sure that the old "raise X, Y[, Z]" form is gone:
    >>> raise X, Y
    Traceback (most recent call last):

Misc/NEWS.d/next/Core and Builtins/2021-04-13-03-06-09.bpo-43823.xpuHBi.rst

@@ -0,0 +1,2 @@
+Improve syntax errors for invalid dictionary literals. Patch by Pablo
+Galindo.