bpo-32888: enhance ast.literal_eval error messagess with context information

Lib/ast.py

@@ -62,20 +62,23 @@ def literal_eval(node_or_string):
         node_or_string = parse(node_or_string, mode='eval')
     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}')
-    def _convert_num(node):
+    def _raise_malformed_node(node, context = None):
+        message = "malformed node or string"
+        if context is not None and isinstance(node, AST):
+            message += f" in {context} context"
+        raise ValueError(f'{message}: {node!r}')
+    def _convert_num(node, context):
         if not isinstance(node, Constant) or type(node.value) not in (int, float, complex):
-            _raise_malformed_node(node)
+            _raise_malformed_node(node, context)
         return node.value
-    def _convert_signed_num(node):
+    def _convert_signed_num(node, context):
         if isinstance(node, UnaryOp) and isinstance(node.op, (UAdd, USub)):
-            operand = _convert_num(node.operand)
+            operand = _convert_num(node.operand, "unary operation")
             if isinstance(node.op, UAdd):
                 return + operand
             else:
                 return - operand
-        return _convert_num(node)
+        return _convert_num(node, context)
     def _convert(node):
         if isinstance(node, Constant):
             return node.value
@@ -90,18 +93,18 @@ def _convert(node):
             return set()
         elif isinstance(node, Dict):
             if len(node.keys) != len(node.values):
-                _raise_malformed_node(node)
+                _raise_malformed_node(node, "dictionary")
             return dict(zip(map(_convert, node.keys),
                             map(_convert, node.values)))
         elif isinstance(node, BinOp) and isinstance(node.op, (Add, Sub)):
-            left = _convert_signed_num(node.left)
-            right = _convert_num(node.right)
+            left = _convert_signed_num(node.left, "binary operation")
+            right = _convert_num(node.right, "binary operation")
             if isinstance(left, (int, float)) and isinstance(right, complex):
                 if isinstance(node.op, Add):
                     return left + right
                 else:
                     return left - right
-        return _convert_signed_num(node)
+        return _convert_signed_num(node, "literal")
     return _convert(node_or_string)
 
 

Lib/test/test_ast.py

@@ -971,6 +971,22 @@ def test_literal_eval_malformed_dict_nodes(self):
         malformed = ast.Dict(keys=[ast.Constant(1)], values=[ast.Constant(2), ast.Constant(3)])
         self.assertRaises(ValueError, ast.literal_eval, malformed)
 
+    def test_literal_eval_message(self):
+        tests = {
+            "2 * 5": "in literal context",
+            "[] + []": "in binary operation context",
+            "+''": "in unary operation context",
+            ast.Dict(
+                keys=[ast.Constant(1)], values=[]
+            ): "in dictionary context",
+            ast.BinOp(
+                ast.Constant(1), ast.Add(), "oops"
+            ): "malformed node or string: 'oops'",
+        }
+        for test, message in tests.items():
+            with self.subTest(test=test, expected_message=message):
+                self.assertRaisesRegex(ValueError, message, ast.literal_eval, test)
+
     def test_bad_integer(self):
         # issue13436: Bad error message with invalid numeric values
         body = [ast.ImportFrom(module='time',

Lib/test/test_fstring.py

@@ -338,7 +338,10 @@ def g():
         self.assertIsNone(g.__doc__)
 
     def test_literal_eval(self):
-        with self.assertRaisesRegex(ValueError, 'malformed node or string'):
+        with self.assertRaisesRegex(
+            ValueError,
+            'malformed node or string in literal context'
+        ):
             ast.literal_eval("f'x'")
 
     def test_ast_compile_time_concat(self):

Misc/NEWS.d/next/Library/2019-12-19-18-20-08.bpo-32888.J5XUjV.rst

@@ -0,0 +1,2 @@
+Enhance :func:`ast.literal_eval` error messages with context information. Original
+patch is written by Chris Angelico, improved and maintained by Batuhan Taskaya.