code: def test_bitmask_from_length(self):
      length = 4
      mask = BitMask.from_length(length)
      self.assertEqual(mask.to_bin(), "0b1111")
  
imports: import unittest
  from copy import deepcopy
  from random import randint, random, seed
  from lightly.api.bitmask import BitMask
label_desc: Use image node_id 4 for calling the TestBitMask obj's underlying member method code with example usage: obj.test_bitmask_from_length() without return types