input: Write a function to replace a given word in a sentence with another word. If the original word is not found in the sentence, return the original sentence unchanged.
  def replace_word(sentence, old_word, new_word):
      if old_word in sentence:
          return sentence.replace(old_word, new_word)
      else:
          return sentence
  
  assert replace_word("I love walking on the beach", "walking", "swimming") == "I love swimming on the beach"
  assert replace_word("I love reading books", "walking", "swimming") == "I love reading books"
  assert replace_word("", "walking", "swimming") == ""
  assert replace_word("I love walking on the beach", "beech", "ocean") == "I love walking on the beach"
  assert replace_word("walking is fun", "walking", "running") == "running is fun"
output: def replace_word(sentence, old_word, new_word):
      if old_word in sentence:
          return sentence.replace(old_word, new_word)
      else:
          return sentence