From d69cbeb99d5fd0d5464e937202cca6a2024d1bcf Mon Sep 17 00:00:00 2001
From: Pablo Galindo <Pablogsal@gmail.com>
Date: Mon, 23 Dec 2019 16:42:48 +0000
Subject: Revert "bpo-38870: Remove dependency on contextlib to avoid
 performance regression on import (GH-17376)" (GH-17687)

This reverts commit ded8888fbc33011dd39b7b1c86a5adfacc4943f3.
---
 Lib/ast.py | 19 ++++++-------------
 1 file changed, 6 insertions(+), 13 deletions(-)

diff --git a/Lib/ast.py b/Lib/ast.py
index 76e0cac..62f6e07 100644
--- a/Lib/ast.py
+++ b/Lib/ast.py
@@ -597,22 +597,15 @@ class _Unparser(NodeVisitor):
         self._buffer.clear()
         return value
 
-    class _Block:
+    @contextmanager
+    def block(self):
         """A context manager for preparing the source for blocks. It adds
         the character':', increases the indentation on enter and decreases
         the indentation on exit."""
-        def __init__(self, unparser):
-            self.unparser = unparser
-
-        def __enter__(self):
-            self.unparser.write(":")
-            self.unparser._indent += 1
-
-        def __exit__(self, exc_type, exc_value, traceback):
-            self.unparser._indent -= 1
-
-    def block(self):
-        return self._Block(self)
+        self.write(":")
+        self._indent += 1
+        yield
+        self._indent -= 1
 
     @contextmanager
     def delimit(self, start, end):
-- 
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