docs: def devices_by_subsystem(subsys, only=lambda x: True):
      """
      Iterator that yields devices that belong to specified subsystem. Returned
      values are ``pydev.Device`` instances.
  
      The ``only`` argument can be used to further filter devices. It should be
      a function that takes a ``pyudev.Device`` object, and returns ``True`` or
      ``False`` to indicate whether device should be returned.
  
      Example::
  
          >>> devices_by_subsystem('net')
          [Device('/sys/devices/pci0000:00/0000:00:1c.3/0000:02:00.0/net/wlp2s0'),
           Device('/sys/devices/virtual/net/lo')]
      """
      ctx = pyudev.Context()
      for d in ctx.list_devices():
          if d.subsystem != subsys:
              continue
          if not only(d):
              continue
          yield d