Add nullcontext

Related to https://github.com/jazzband/contextlib2/issues/16

contextlib2.py

@@ -5,7 +5,8 @@ import warnings
 from collections import deque
 from functools import wraps
 
-__all__ = ["contextmanager", "closing", "ContextDecorator", "ExitStack",
+__all__ = ["contextmanager", "closing", "nullcontext",
+           "ContextDecorator", "ExitStack",
            "redirect_stdout", "redirect_stderr", "suppress"]
 
 # Backwards compatibility
@@ -439,3 +440,22 @@ class ContextStack(ExitStack):
 
     def preserve(self):
         return self.pop_all()
+
+
+class nullcontext(AbstractContextManager):
+    """Context manager that does no additional processing.
+    Used as a stand-in for a normal context manager, when a particular
+    block of code is only sometimes used with a normal context manager:
+    cm = optional_cm if condition else nullcontext()
+    with cm:
+        # Perform operation, using optional_cm if condition is True
+    """
+
+    def __init__(self, enter_result=None):
+        self.enter_result = enter_result
+
+    def __enter__(self):
+        return self.enter_result
+
+    def __exit__(self, *excinfo):
+        pass