bpo-43797: Improve syntax error for invalid comparisons

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) a=bitwise_or b='=' bitwise_or !('='|':='|',') {
+        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(b, "cannot assign to %s. Maybe you meant '==' instead of '='?",
+                                          _PyPegen_get_expr_name(a)) }
+
 invalid_assignment:
     | a=invalid_ann_assign_target ':' expression {
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(
@@ -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

@@ -601,7 +601,7 @@ def test_syntaxerror_unindented_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)
 
     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, 'invalid syntax.',
                             ["f'' = 3",
                              "f'{0}' = x",
                              "f'{x}' = x",

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. 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
 
@@ -153,7 +153,7 @@
     >>> (y for y in (1,2)) = 10
     Traceback (most recent call last):
        ...
-    SyntaxError: cannot assign to generator expression
+    SyntaxError: cannot assign to generator expression. Maybe you meant '==' instead of '='?
 
     >>> (y for y in (1,2)) += 10
     Traceback (most recent call last):

Lib/test/test_syntax.py

@@ -33,15 +33,15 @@
 
 >>> None = 1
 Traceback (most recent call last):
-SyntaxError: cannot assign to None
+SyntaxError: cannot assign to None. Maybe you meant '==' instead of '='?
 
 >>> obj.True = 1
 Traceback (most recent call last):
 SyntaxError: invalid syntax
 
 >>> True = 1
 Traceback (most recent call last):
-SyntaxError: cannot assign to True
+SyntaxError: cannot assign to True. Maybe you meant '==' instead of '='?
 
 >>> (True := 1)
 Traceback (most recent call last):
@@ -61,7 +61,7 @@
 
 >>> f() = 1
 Traceback (most recent call last):
-SyntaxError: cannot assign to function call
+SyntaxError: cannot assign to function call. Maybe you meant '==' instead of '='?
 
 >>> yield = 1
 Traceback (most recent call last):
@@ -73,27 +73,27 @@
 
 >>> a + 1 = 2
 Traceback (most recent call last):
-SyntaxError: cannot assign to operator
+SyntaxError: cannot assign to expression. Maybe you meant '==' instead of '='?
 
 >>> (x for x in x) = 1
 Traceback (most recent call last):
-SyntaxError: cannot assign to generator expression
+SyntaxError: cannot assign to generator expression. Maybe you meant '==' instead of '='?
 
 >>> 1 = 1
 Traceback (most recent call last):
-SyntaxError: cannot assign to literal
+SyntaxError: cannot assign to literal. Maybe you meant '==' instead of '='?
 
 >>> "abc" = 1
 Traceback (most recent call last):
-SyntaxError: cannot assign to literal
+SyntaxError: cannot assign to literal. Maybe you meant '==' instead of '='?
 
 >>> b"" = 1
 Traceback (most recent call last):
-SyntaxError: cannot assign to literal
+SyntaxError: cannot assign to literal. Maybe you meant '==' instead of '='?
 
 >>> ... = 1
 Traceback (most recent call last):
-SyntaxError: cannot assign to Ellipsis
+SyntaxError: cannot assign to Ellipsis. Maybe you meant '==' instead of '='?
 
 >>> `1` = 1
 Traceback (most recent call last):
@@ -126,15 +126,15 @@
 
 >>> [a, b, c + 1] = [1, 2, 3]
 Traceback (most recent call last):
-SyntaxError: cannot assign to operator
+SyntaxError: cannot assign to expression
 
 >>> [a, b[1], c + 1] = [1, 2, 3]
 Traceback (most recent call last):
-SyntaxError: cannot assign to operator
+SyntaxError: cannot assign to expression
 
 >>> [a, b.c.d, c + 1] = [1, 2, 3]
 Traceback (most recent call last):
-SyntaxError: cannot assign to operator
+SyntaxError: cannot assign to expression
 
 >>> a if 1 else b = 1
 Traceback (most recent call last):
@@ -181,7 +181,7 @@
 
 >>> for (*a, b, c+1) in b: pass
 Traceback (most recent call last):
-SyntaxError: cannot assign to operator
+SyntaxError: cannot assign to expression
 
 >>> for (x, *(y, z.d())) in b: pass
 Traceback (most recent call last):
@@ -193,7 +193,7 @@
 
 >>> for a, b, (c + 1, d()): pass
 Traceback (most recent call last):
-SyntaxError: cannot assign to operator
+SyntaxError: cannot assign to expression
 
 >>> for i < (): pass
 Traceback (most recent call last):
@@ -217,7 +217,7 @@
 
 >>> with a as (*b, c, d+1): pass
 Traceback (most recent call last):
-SyntaxError: cannot assign to operator
+SyntaxError: cannot assign to expression
 
 >>> with a as (x, *(y, z.d())): pass
 Traceback (most recent call last):
@@ -465,7 +465,7 @@
 # SyntaxError: expression cannot contain assignment, perhaps you meant "=="?
 # >>> f(True=2)
 # Traceback (most recent call last):
-# SyntaxError: cannot assign to True
+# SyntaxError: cannot assign to True. Maybe you meant '==' instead of '='?
 >>> f(__debug__=1)
 Traceback (most recent call last):
 SyntaxError: cannot assign to __debug__
@@ -684,15 +684,15 @@
    ...   pass
    Traceback (most recent call last):
      ...
-   SyntaxError: cannot assign to function call
+   SyntaxError: cannot assign to function call. Maybe you meant '==' instead of '='?
 
    >>> if 1:
    ...   pass
    ... elif 1:
    ...   x() = 1
    Traceback (most recent call last):
      ...
-   SyntaxError: cannot assign to function call
+   SyntaxError: cannot assign to function call. Maybe you meant '==' instead of '='?
 
    >>> if 1:
    ...   x() = 1
@@ -702,7 +702,7 @@
    ...   pass
    Traceback (most recent call last):
      ...
-   SyntaxError: cannot assign to function call
+   SyntaxError: cannot assign to function call. Maybe you meant '==' instead of '='?
 
    >>> if 1:
    ...   pass
@@ -712,7 +712,7 @@
    ...   pass
    Traceback (most recent call last):
      ...
-   SyntaxError: cannot assign to function call
+   SyntaxError: cannot assign to function call. Maybe you meant '==' instead of '='?
 
    >>> if 1:
    ...   pass
@@ -722,7 +722,7 @@
    ...   x() = 1
    Traceback (most recent call last):
      ...
-   SyntaxError: cannot assign to function call
+   SyntaxError: cannot assign to function call. Maybe you meant '==' instead of '='?
 
     Missing ':' before suites:
 
@@ -894,19 +894,19 @@
 
 >>> {1, 2, 3} = 42
 Traceback (most recent call last):
-SyntaxError: cannot assign to set display
+SyntaxError: cannot assign to set display. Maybe you meant '==' instead of '='?
 
 >>> {1: 2, 3: 4} = 42
 Traceback (most recent call last):
-SyntaxError: cannot assign to dict display
+SyntaxError: cannot assign to dict display. Maybe you meant '==' instead of '='?
 
 >>> f'{x}' = 42
 Traceback (most recent call last):
-SyntaxError: cannot assign to f-string expression
+SyntaxError: cannot assign to f-string expression. Maybe you meant '==' instead of '='?
 
 >>> f'{x}-{y}' = 42
 Traceback (most recent call last):
-SyntaxError: cannot assign to f-string expression
+SyntaxError: cannot assign to f-string expression. Maybe you meant '==' instead of '='?
 
 >>> from t import x,
 Traceback (most recent call last):
@@ -988,7 +988,7 @@ def _check_error(self, code, errtext,
     def test_expression_with_assignment(self):
         self._check_error(
             "print(end1 + end2 = ' ')",
-            'expression cannot contain assignment, perhaps you meant "=="?',
+            "cannot assign to expression. Maybe you meant '==' instead of '='?",
             offset=19
         )
 
@@ -1015,15 +1015,15 @@ def test_assign_del(self):
         self._check_error("del f(), x", "delete function call")
         self._check_error("del [a, b, ((c), (d,), e.f())]", "delete function call")
         self._check_error("del (a if True else b)", "delete conditional")
-        self._check_error("del +a", "delete operator")
-        self._check_error("del a, +b", "delete operator")
-        self._check_error("del a + b", "delete operator")
-        self._check_error("del (a + b, c)", "delete operator")
-        self._check_error("del (c[0], a + b)", "delete operator")
-        self._check_error("del a.b.c + 2", "delete operator")
-        self._check_error("del a.b.c[0] + 2", "delete operator")
-        self._check_error("del (a, b, (c, d.e.f + 2))", "delete operator")
-        self._check_error("del [a, b, (c, d.e.f[0] + 2)]", "delete operator")
+        self._check_error("del +a", "delete expression")
+        self._check_error("del a, +b", "delete expression")
+        self._check_error("del a + b", "delete expression")
+        self._check_error("del (a + b, c)", "delete expression")
+        self._check_error("del (c[0], a + b)", "delete expression")
+        self._check_error("del a.b.c + 2", "delete expression")
+        self._check_error("del a.b.c[0] + 2", "delete expression")
+        self._check_error("del (a, b, (c, d.e.f + 2))", "delete expression")
+        self._check_error("del [a, b, (c, d.e.f[0] + 2)]", "delete expression")
         self._check_error("del (a := 5)", "delete named expression")
         # We don't have a special message for this, but make sure we don't
         # report "cannot delete name"

Misc/NEWS.d/next/Core and Builtins/2021-04-09-19-12-48.bpo-43797.HfRqNP.rst

@@ -0,0 +1,2 @@
+Improve ``SyntaxError`` error messages for invalid comparisons. Patch by
+Pablo Galindo.