From 4b03309bf75720e571d12fcddaf194dc5d0f4fce Mon Sep 17 00:00:00 2001
From: isidentical <batuhanosmantaskaya@gmail.com>
Date: Thu, 2 Jan 2020 20:43:57 +0300
Subject: [PATCH] bpo-38870: Throw ValueError on invalid yield from usage

---
 Lib/ast.py               | 8 ++++----
 Lib/test/test_unparse.py | 2 ++
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/Lib/ast.py b/Lib/ast.py
index 62f6e075a09fdf2..ece8b139e460eeb 100644
--- a/Lib/ast.py
+++ b/Lib/ast.py
@@ -735,10 +735,10 @@ def visit_Yield(self, node):
 
     def visit_YieldFrom(self, node):
         with self.delimit("(", ")"):
-            self.write("yield from")
-            if node.value:
-                self.write(" ")
-                self.traverse(node.value)
+            self.write("yield from ")
+            if not node.value:
+                raise ValueError("Node can't be used without a value attribute.")
+            self.traverse(node.value)
 
     def visit_Raise(self, node):
         self.fill("raise")
diff --git a/Lib/test/test_unparse.py b/Lib/test/test_unparse.py
index 49767dbac16d11f..e8b0d4b06f9e97b 100644
--- a/Lib/test/test_unparse.py
+++ b/Lib/test/test_unparse.py
@@ -278,6 +278,8 @@ def test_invalid_fstring_conversion(self):
     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."""