code: def test_flatten_dict():
      assert func.flatten_dict({"a": 1, "b": 2}) == {"a": 1, "b": 2}
      assert func.flatten_dict({"a": 1}) == {"a": 1}
      assert func.flatten_dict({}) == {}
      assert func.flatten_dict({"a": 1, "b": {"c": {"d": 2}}}) == {
          "a": 1,
          "b.c.d": 2,
      }
      assert func.flatten_dict(
          {"a": 1, "b": {"c": {"d": 2}}}, separator="/"
      ) == {"a": 1, "b/c/d": 2}
  
file_path: catboost/library/python/func/ut/test_func.py
imports: import pytest
  import multiprocessing
  import random
  import threading
  import time
  import library.python.func
label_desc: Use image node_id 6 for calling a global function with example usage: test_flatten_dict() without return types