class_docstr: Can be used as no-function dummy object in place of ``threading.lock``.
  
  The ``_NullLock`` is an object which can be used in place of a
  ``threading.Lock`` object, but doesn't actually do anything.
  
  It is used by the ``read_segments`` function in the event that a
  ``Lock`` is not provided by the caller.
code: def __enter__(self):
      pass
  
imports: import operator
  from functools import reduce
  from mmap import mmap
  from numbers import Integral
  import numpy
label_desc: Use image node_id 1 for calling the _NullLock obj's underlying member method code with example usage: obj.__enter__() without return types