gh-151303 : Improve SyntaxError suggestions for common operator typos and cross-language mistakes

Grammar/python.gram

@@ -801,8 +801,15 @@ compare_op_bitwise_or_pair[CmpopExprPair*]:
     | in_bitwise_or
     | isnot_bitwise_or
     | is_bitwise_or
+    | invalid_eqeqeq
 
 eq_bitwise_or[CmpopExprPair*]: '==' a=bitwise_or { _PyPegen_cmpop_expr_pair(p, Eq, a) }
+invalid_eqeqeq:
+    | a='==' b='=' {
+        _PyPegen_tokens_are_adjacent(a, b)
+        ? RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "invalid syntax.  Maybe you meant 'is' instead of '==='?")
+        : NULL
+    }
 noteq_bitwise_or[CmpopExprPair*]:
     | (tok='!=' { _PyPegen_check_barry_as_flufl(p, tok) ? NULL : tok}) a=bitwise_or {_PyPegen_cmpop_expr_pair(p, NotEq, a) }
 lte_bitwise_or[CmpopExprPair*]: '<=' a=bitwise_or { _PyPegen_cmpop_expr_pair(p, LtE, a) }
@@ -1313,6 +1320,21 @@ invalid_assignment:
             "'%s' is an illegal expression for augmented assignment",
             _PyPegen_get_expr_name(a)
         )}
+    | star_expressions a='=' b='<' {
+        _PyPegen_tokens_are_adjacent(a, b)
+        ? RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "invalid syntax.  Maybe you meant '<=' instead of '=<'?")
+        : NULL
+    }
+    | star_expressions a='=' b='>' {
+        _PyPegen_tokens_are_adjacent(a, b)
+        ? RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "invalid syntax.  Maybe you meant '>=' instead of '=>'?")
+        : NULL
+    }
+    | star_expressions a='=' b='!' {
+        _PyPegen_tokens_are_adjacent(a, b)
+        ? RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "invalid syntax.  Maybe you meant '!=' instead of '=!'?")
+        : NULL
+    }
 invalid_ann_assign_target[expr_ty]:
     | list
     | tuple

Lib/test/test_syntax.py

@@ -3505,6 +3505,79 @@ def test_ifexp_body_stmt_else_stmt(self):
         ]:
             self._check_error(f"x = {lhs_stmt} if 1 else {rhs_stmt}", msg)
 
+    def test_diamond_operator(self):
+        self._check_error(
+            "1<>2",
+            r'Are you trying to overthrow the SC\?  Use operator "!="!',
+            lineno=1,
+            end_lineno=1,
+            offset=2,
+            end_offset=4,
+        )
+        self._check_error(
+            "1 < > 2",
+            "invalid syntax",
+            lineno=1,
+            end_lineno=1,
+            offset=5,
+            end_offset=6,
+        )
+
+    def test_diamond_operator_barry_as_flufl(self):
+        # Under barry_as_FLUFL, '<>' is the valid "not equal" operator
+        compile(
+            "from __future__ import barry_as_FLUFL\n1<>2",
+            "<test>", "exec",
+        )
+
+    def test_triple_equal(self):
+        self._check_error(
+            "a === b",
+            r"Maybe you meant 'is' instead of '==='\?",
+            lineno=1,
+            end_lineno=1,
+            offset=3,
+            end_offset=6,
+        )
+        self._check_error(
+            "a == = b",
+            "invalid syntax",
+            lineno=1,
+            end_lineno=1,
+            offset=6,
+            end_offset=7,
+        )
+
+    def test_eq_lt_typo(self):
+        self._check_error(
+            "a =< b",
+            r"Maybe you meant '<=' instead of '=<'\?",
+            lineno=1,
+            end_lineno=1,
+            offset=3,
+            end_offset=5,
+        )
+
+    def test_eq_gt_typo(self):
+        self._check_error(
+            "a => b",
+            r"Maybe you meant '>=' instead of '=>'\?",
+            lineno=1,
+            end_lineno=1,
+            offset=3,
+            end_offset=5,
+        )
+
+    def test_eq_bang_typo(self):
+        self._check_error(
+            "a =! b",
+            r"Maybe you meant '!=' instead of '=!'\?",
+            lineno=1,
+            end_lineno=1,
+            offset=3,
+            end_offset=5,
+        )
+
 
 class LazyImportRestrictionTestCase(SyntaxErrorTestCase):
     """Test syntax restrictions for lazy imports."""

Misc/NEWS.d/next/Core_and_Builtins/2026-06-12-03-41-39.gh-issue-151303.mzlJxi.rst

@@ -0,0 +1 @@
+Improve :exc:`SyntaxError` messages for common operator typos coming from other languages: ``=<``, ``=>``, and ``=!`` now suggest ``<=``, ``>=``, and ``!=`` respectively, ``===`` suggests ``is``, and ``<>`` shows a tailored message suggesting ``!=``.

Parser/action_helpers.c

@@ -931,6 +931,10 @@ _PyPegen_check_barry_as_flufl(Parser *p, Token* t) {
         return -1;
     }
     if (!(p->flags & PyPARSE_BARRY_AS_BDFL)) {
+        if (strcmp(tok_str, "<>") == 0) {
+            RAISE_SYNTAX_ERROR("invalid syntax.  Are you trying to overthrow the SC?  Use operator \"!=\"!");
+            return -1;
+        }
         return strcmp(tok_str, "!=");
     }
     return 0;