bpo-28964: add line number of node (if available) to ast.literal_eval error messages (GH-23677)

Lib/ast.py

@@ -63,7 +63,10 @@ def literal_eval(node_or_string):
     if isinstance(node_or_string, Expression):
         node_or_string = node_or_string.body
     def _raise_malformed_node(node):
-        raise ValueError(f'malformed node or string: {node!r}')
+        msg = "malformed node or string"
+        if lno := getattr(node, 'lineno', None):
+            msg += f' on line {lno}'
+        raise ValueError(msg + f': {node!r}')
     def _convert_num(node):
         if not isinstance(node, Constant) or type(node.value) not in (int, float, complex):
             _raise_malformed_node(node)

Lib/test/test_ast.py

@@ -1011,6 +1011,18 @@ Module(
         self.assertEqual(ast.literal_eval(" \t -1"), -1)
         self.assertRaises(IndentationError, ast.literal_eval, "\n -1")
 
+    def test_literal_eval_malformed_lineno(self):
+        msg = r'malformed node or string on line 3:'
+        with self.assertRaisesRegex(ValueError, msg):
+            ast.literal_eval("{'a': 1,\n'b':2,\n'c':++3,\n'd':4}")
+
+        node = ast.UnaryOp(
+            ast.UAdd(), ast.UnaryOp(ast.UAdd(), ast.Constant(6)))
+        self.assertIsNone(getattr(node, 'lineno', None))
+        msg = r'malformed node or string:'
+        with self.assertRaisesRegex(ValueError, msg):
+            ast.literal_eval(node)
+
     def test_bad_integer(self):
         # issue13436: Bad error message with invalid numeric values
         body = [ast.ImportFrom(module='time',

Misc/NEWS.d/next/Library/2020-12-07-13-21-00.bpo-28964.UTQikc.rst

@@ -0,0 +1 @@
+:func:`ast.literal_eval` adds line number information (if available) in error message for malformed nodes.