gh-136616: Improve assert syntax error messages (#136653)

Author: sobolevn

Grammar/python.gram

@@ -212,7 +212,9 @@ del_stmt[stmt_ty]:
 
 yield_stmt[stmt_ty]: y=yield_expr { _PyAST_Expr(y, EXTRA) }
 
-assert_stmt[stmt_ty]: 'assert' a=expression b=[',' z=expression { z }] { _PyAST_Assert(a, b, EXTRA) }
+assert_stmt[stmt_ty]:
+    | invalid_assert_stmt
+    | 'assert' a=expression b=[',' z=expression { z }] { _PyAST_Assert(a, b, EXTRA) }
 
 import_stmt[stmt_ty]:
     | invalid_import
@@ -1302,6 +1304,17 @@ invalid_raise_stmt:
 invalid_del_stmt:
     | 'del' a=star_expressions {
         RAISE_SYNTAX_ERROR_INVALID_TARGET(DEL_TARGETS, a) }
+invalid_assert_stmt:
+    | 'assert' a=expression '=' b=expression {
+        RAISE_SYNTAX_ERROR_KNOWN_RANGE(
+            a, b,
+            "cannot assign to %s here. Maybe you meant '==' instead of '='?",
+            _PyPegen_get_expr_name(a)) }
+    | 'assert' expression ',' a=expression '=' b=expression {
+        RAISE_SYNTAX_ERROR_KNOWN_RANGE(
+            a, b,
+            "cannot assign to %s here. Maybe you meant '==' instead of '='?",
+            _PyPegen_get_expr_name(a)) }
 invalid_block:
     | NEWLINE !INDENT { RAISE_INDENTATION_ERROR("expected an indented block") }
 invalid_comprehension:

Lib/test/test_syntax.py

@@ -2686,6 +2686,71 @@ def f(x: *b)
     >>> f(x = 5, *:)
     Traceback (most recent call last):
     SyntaxError: Invalid star expression
+
+Asserts:
+
+    >>> assert (a := 1)  # ok
+    >>> # TODO(@sobolevn): improve this message in the next PR
+    >>> assert a := 1
+    Traceback (most recent call last):
+    SyntaxError: invalid syntax
+
+    >>> assert 1 = 2 = 3
+    Traceback (most recent call last):
+    SyntaxError: cannot assign to literal here. Maybe you meant '==' instead of '='?
+
+    >>> assert 1 = 2
+    Traceback (most recent call last):
+    SyntaxError: cannot assign to literal here. Maybe you meant '==' instead of '='?
+
+    >>> assert (1 = 2)
+    Traceback (most recent call last):
+    SyntaxError: cannot assign to literal here. Maybe you meant '==' instead of '='?
+
+    >>> assert 'a' = a
+    Traceback (most recent call last):
+    SyntaxError: cannot assign to literal here. Maybe you meant '==' instead of '='?
+
+    >>> assert x[0] = 1
+    Traceback (most recent call last):
+    SyntaxError: cannot assign to subscript here. Maybe you meant '==' instead of '='?
+
+    >>> assert (yield a) = 2
+    Traceback (most recent call last):
+    SyntaxError: cannot assign to yield expression here. Maybe you meant '==' instead of '='?
+
+    >>> assert a = 2
+    Traceback (most recent call last):
+    SyntaxError: cannot assign to name here. Maybe you meant '==' instead of '='?
+
+    >>> assert (a = 2)
+    Traceback (most recent call last):
+    SyntaxError: invalid syntax. Maybe you meant '==' or ':=' instead of '='?
+
+    >>> assert a = b
+    Traceback (most recent call last):
+    SyntaxError: cannot assign to name here. Maybe you meant '==' instead of '='?
+
+    >>> assert 1, 1 = b
+    Traceback (most recent call last):
+    SyntaxError: cannot assign to literal here. Maybe you meant '==' instead of '='?
+
+    >>> assert 1, (1 = b)
+    Traceback (most recent call last):
+    SyntaxError: cannot assign to literal here. Maybe you meant '==' instead of '='?
+
+    >>> assert 1, a = 1
+    Traceback (most recent call last):
+    SyntaxError: cannot assign to name here. Maybe you meant '==' instead of '='?
+
+    >>> assert 1, (a = 1)
+    Traceback (most recent call last):
+    SyntaxError: invalid syntax. Maybe you meant '==' or ':=' instead of '='?
+
+    >>> assert 1 = a, a = 1
+    Traceback (most recent call last):
+    SyntaxError: cannot assign to literal here. Maybe you meant '==' instead of '='?
+
 """
 
 import re

Misc/NEWS.d/next/Core_and_Builtins/2025-07-14-17-01-23.gh-issue-136616.FQjXE_.rst

@@ -0,0 +1,2 @@
+Improve :exc:`SyntaxError` error messages for invalid :keyword:`assert`
+usages.