docs: def environment_variable(key, value):
      """ Temporarily overrides an environment variable. """
  
      if key not in os.environ:
          previous = None
      else:
          previous = os.environ[key]
  
      os.environ[key] = value
  
      yield
  
      if previous is None:
          del os.environ[key]
      else:
          os.environ[key] = previous