input: Write a python function to swap keys and values in a dictionary.
  assert swap_dict({'fruit': 'apple', 'animal': 'cat', 'city': 'London'}) == {'apple': 'fruit', 'cat': 'animal', 'London': 'city'}
  assert swap_dict({'a': 1, 'b': 2, 'c': 3}) == {1: 'a', 2: 'b', 3: 'c'}
  assert swap_dict({'x': 10, 'y': 20, 'z': 30}) == {10: 'x', 20: 'y', 30: 'z'}
output: def swap_dict(d):
      return {v: k for k, v in d.items()}