bpo-43797: Improve syntax error for invalid comparisons (#25317)

* bpo-43797: Improve syntax error for invalid comparisons

* Update Lib/test/test_fstring.py

Co-authored-by: Guido van Rossum <[email protected]>

* Apply review comments

* can't -> cannot

Co-authored-by: Guido van Rossum <[email protected]>

Author: pablogsal

Grammar/python.gram

@@ -163,17 +163,20 @@ dotted_name[expr_ty]:
     | NAME
 
 if_stmt[stmt_ty]:
-    | 'if' a=named_expression &&':' b=block c=elif_stmt {
+    | 'if' a=named_expression ':' b=block c=elif_stmt {
         _PyAST_If(a, b, CHECK(asdl_stmt_seq*, _PyPegen_singleton_seq(p, c)), EXTRA) }
-    | 'if' a=named_expression &&':' b=block c=[else_block] { _PyAST_If(a, b, c, EXTRA) }
+    | 'if' a=named_expression ':' b=block c=[else_block] { _PyAST_If(a, b, c, EXTRA) }
+    | invalid_if_stmt
 elif_stmt[stmt_ty]:
-    | 'elif' a=named_expression &&':' b=block c=elif_stmt {
+    | 'elif' a=named_expression ':' b=block c=elif_stmt {
         _PyAST_If(a, b, CHECK(asdl_stmt_seq*, _PyPegen_singleton_seq(p, c)), EXTRA) }
-    | 'elif' a=named_expression &&':' b=block c=[else_block] { _PyAST_If(a, b, c, EXTRA) }
+    | 'elif' a=named_expression ':' b=block c=[else_block] { _PyAST_If(a, b, c, EXTRA) }
+    | invalid_elif_stmt
 else_block[asdl_stmt_seq*]: 'else' &&':' b=block { b }
 
 while_stmt[stmt_ty]:
-    | 'while' a=named_expression &&':' b=block c=[else_block] { _PyAST_While(a, b, c, EXTRA) }
+    | 'while' a=named_expression ':' b=block c=[else_block] { _PyAST_While(a, b, c, EXTRA) }
+    | invalid_while_stmt
 
 for_stmt[stmt_ty]:
     | 'for' t=star_targets 'in' ~ ex=star_expressions &&':' tc=[TYPE_COMMENT] b=block el=[else_block] {
@@ -438,10 +441,11 @@ star_named_expressions[asdl_expr_seq*]: a[asdl_expr_seq*]=','.star_named_express
 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) }
-    | expression !':='
     | invalid_named_expression
+    | expression !':='
 
 annotated_rhs[expr_ty]: yield_expr | star_expressions
 
@@ -772,6 +776,12 @@ 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_LOCATION(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(b, "cannot assign to %s here. Maybe you meant '==' instead of '='?",
+                                          _PyPegen_get_expr_name(a)) }
+
 invalid_assignment:
     | a=invalid_ann_assign_target ':' expression {
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(
@@ -841,9 +851,9 @@ invalid_for_target:
 
 invalid_group:
     | '(' a=starred_expression ')' {
-        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "can't use starred expression here") }
+        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "cannot use starred expression here") }
     | '(' a='**' expression ')' {
-        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "can't use double starred expression here") }
+        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "cannot use double starred expression here") }
 invalid_import_from_targets:
     | import_from_as_names ',' {
         RAISE_SYNTAX_ERROR("trailing comma not allowed without surrounding parentheses") }
@@ -860,6 +870,11 @@ invalid_except_block:
 
 invalid_match_stmt:
     | "match" subject_expr !':' { CHECK_VERSION(void*, 10, "Pattern matching is", RAISE_SYNTAX_ERROR("expected ':'") ) }
-
 invalid_case_block:
     | "case" patterns guard? !':' { RAISE_SYNTAX_ERROR("expected ':'") }
+invalid_if_stmt:
+    | 'if' named_expression NEWLINE { RAISE_SYNTAX_ERROR("expected ':'") }
+invalid_elif_stmt:
+    | 'elif' named_expression NEWLINE { RAISE_SYNTAX_ERROR("expected ':'") }
+invalid_while_stmt:
+    | 'while' named_expression NEWLINE { RAISE_SYNTAX_ERROR("expected ':'") }

Lib/test/test_cmd_line_script.py

@@ -600,8 +600,8 @@ def test_syntaxerror_unindented_caret_position(self):
             script_name = _make_test_script(script_dir, 'script', script)
             exitcode, stdout, stderr = assert_python_failure(script_name)
             text = io.TextIOWrapper(io.BytesIO(stderr), 'ascii').read()
-            # Confirm that the caret is located under the first 1 character
-            self.assertIn("\n    1 + 1 = 2\n    ^", text)
+            # Confirm that the caret is located under the '=' sign
+            self.assertIn("\n    1 + 1 = 2\n          ^\n", text)
 
     def test_syntaxerror_indented_caret_position(self):
         script = textwrap.dedent("""\
@@ -613,7 +613,7 @@ def test_syntaxerror_indented_caret_position(self):
             exitcode, stdout, stderr = assert_python_failure(script_name)
             text = io.TextIOWrapper(io.BytesIO(stderr), 'ascii').read()
             # Confirm that the caret is located under the first 1 character
-            self.assertIn("\n    1 + 1 = 2\n    ^", text)
+            self.assertIn("\n    1 + 1 = 2\n          ^\n", text)
 
             # Try the same with a form feed at the start of the indented line
             script = (
@@ -624,7 +624,7 @@ def test_syntaxerror_indented_caret_position(self):
             exitcode, stdout, stderr = assert_python_failure(script_name)
             text = io.TextIOWrapper(io.BytesIO(stderr), "ascii").read()
             self.assertNotIn("\f", text)
-            self.assertIn("\n    1 + 1 = 2\n    ^", text)
+            self.assertIn("\n    1 + 1 = 2\n          ^\n", text)
 
     def test_syntaxerror_multi_line_fstring(self):
         script = 'foo = f"""{}\nfoo"""\n'

Lib/test/test_codeop.py

@@ -275,7 +275,6 @@ def test_invalid(self):
         ai("a = 'a\\\n")
 
         ai("a = 1","eval")
-        ai("a = (","eval")
         ai("]","eval")
         ai("())","eval")
         ai("[}","eval")

Lib/test/test_exceptions.py

@@ -260,7 +260,7 @@ def baz():
         check('[*x for x in xs]', 1, 2)
         check('foo(x for x in range(10), 100)', 1, 5)
         check('for 1 in []: pass', 1, 5)
-        check('(yield i) = 2', 1, 2)
+        check('(yield i) = 2', 1, 11)
         check('def f(*):\n  pass', 1, 8)
 
     @cpython_only

Lib/test/test_fstring.py

@@ -990,7 +990,7 @@ def test_conversions(self):
                              ])
 
     def test_assignment(self):
-        self.assertAllRaise(SyntaxError, 'invalid syntax',
+        self.assertAllRaise(SyntaxError, r'invalid syntax',
                             ["f'' = 3",
                              "f'{0}' = x",
                              "f'{x}' = x",
@@ -1276,11 +1276,11 @@ def test_with_an_underscore_and_a_comma_in_format_specifier(self):
             f'{1:_,}'
 
     def test_syntax_error_for_starred_expressions(self):
-        error_msg = re.escape("can't use starred expression here")
+        error_msg = re.escape("cannot use starred expression here")
         with self.assertRaisesRegex(SyntaxError, error_msg):
             compile("f'{*a}'", "?", "exec")
 
-        error_msg = re.escape("can't use double starred expression here")
+        error_msg = re.escape("cannot use double starred expression here")
         with self.assertRaisesRegex(SyntaxError, error_msg):
             compile("f'{**a}'", "?", "exec")
 

Lib/test/test_generators.py

@@ -2013,7 +2013,7 @@ def printsolution(self, x):
 >>> def f(): (yield bar) = y
 Traceback (most recent call last):
   ...
-SyntaxError: cannot assign to yield expression
+SyntaxError: cannot assign to yield expression here. Maybe you meant '==' instead of '='?
 
 >>> def f(): (yield bar) += y
 Traceback (most recent call last):

Lib/test/test_genexps.py

@@ -103,7 +103,7 @@
     >>> dict(a = i for i in range(10))
     Traceback (most recent call last):
        ...
-    SyntaxError: invalid syntax
+    SyntaxError: invalid syntax. Maybe you meant '==' or ':=' instead of '='?
 
 Verify that parenthesis are required when used as a keyword argument value