code: def test_add_var(ddof):
      A = stats.Var(ddof)
      B = stats.Var(ddof)
      C = stats.Var(ddof)
  
      X = [random.random() for _ in range(30)]
      Y = [random.random() for _ in range(30)]
      W = [1 for _ in range(30)]
  
      for i, (x, y, w) in enumerate(zip(X, Y, W)):
          A.update(x, w)
          B.update(y, w)
          C.update(x, w)
          C.update(y, w)
  
          D = A + B
          assert math.isclose(C.get(), D.get())
  
          E = A.clone(include_attributes=True)
          E += B
          assert math.isclose(C.get(), E.get())
  
          if i >= ddof:
              assert math.isclose(
                  C.get(),
                  _weighted_var(
                      X[: i + 1] + Y[: i + 1],
                      W[: i + 1] + W[: i + 1],
                      ddof=ddof,
                  ),
              )
  
file_path: river/river/stats/test_parallel.py
imports: from __future__ import annotations
  import copy
  import math
  import random
  import numpy
  import pytest
  from river import stats
label_desc: Use image node_id 3 for calling a global function with example usage: test_add_var(ddof) without return types