Revert "bpo-38870: Remove dependency on contextlib to avoid performance regression on import (GH-17376)" (GH-17687)

pablogsal · web-flow · commit d69cbeb99d5f · 2019-12-23T16:42:48.000Z
This reverts commit ded8888.
diff --git a/Lib/ast.py b/Lib/ast.py
@@ -597,22 +597,15 @@ def buffer(self):
         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):
