input: Write a python function to split a dataset into training and testing sets.
  assert len(train_test_split([1, 2, 3, 4, 5], test_size=0.2)) == 4
  assert len(train_test_split([[1,2],[3,4],[5,6],[7,8]], test_size=0.5)) == 4
  assert len(train_test_split([10, 20, 30, 40], test_size=0.25)) == 4
output: def train_test_split(data, test_size=0.25):
      import random
      random.shuffle(data)
      split_index = int(len(data) * (1 - test_size))
      train_set = data[:split_index]
      test_set = data[split_index:]
      return train_set, test_set