bpo-38870: Throw ValueError on invalid yield from usage (GH-17798)

Lib/ast.py

@@ -736,8 +736,8 @@ class _Unparser(NodeVisitor):
     def visit_YieldFrom(self, node):
         with self.delimit("(", ")"):
             self.write("yield from ")
-            if node.value:
-                self.write(" ")
+            if not node.value:
+                raise ValueError("Node can't be used without a value attribute.")
             self.traverse(node.value)
 
     def visit_Raise(self, node):

Lib/test/test_unparse.py

@@ -278,6 +278,8 @@ class UnparseTestCase(ASTTestCase):
     def test_invalid_set(self):
         self.check_invalid(ast.Set(elts=[]))
 
+    def test_invalid_yield_from(self):
+        self.check_invalid(ast.YieldFrom(value=None))
 
 class DirectoryTestCase(ASTTestCase):
     """Test roundtrip behaviour on all files in Lib and Lib/test."""