code: def test_fallback_train_one():
      algo = basic.Fallback(Bias())
      algo.fit(lktu.ml_test.ratings)
      assert len(algo.algorithms) == 1
      assert isinstance(algo.algorithms[0], Bias)
      assert algo.algorithms[0].mean_ == approx(
          lktu.ml_test.ratings.rating.mean()
      )
  
file_path: lkpy/tests/test_fallback.py
imports: from lenskit.algorithms import basic
  from lenskit.algorithms.bias import Bias
  from lenskit import util
  import pandas
  import numpy
  import binpickle
  import lenskit.util.test
  from pytest import approx
label_desc: Use image node_id 1 for calling a global function with example usage: test_fallback_train_one() without return types