input: Write a function to parse a JSON data structure and extract the values of the keys 'name', 'age', and 'occupation'. If the keys do not exist, return None for each missing key.
  
  def parse_json(data):
   data = json.loads(data)
   name = data.get('name', None)
   age = data.get('age', None)
   occupation = data.get('occupation', None)
   return name, age, occupation
  
  data = json.dumps({
   "name": "John",
   "age": 18,
   "occupation": "Software Engineer"
  })
  
  name, age, occupation = parse_json(data)
  assert (name, age, occupation) == ("John", 18, "Software Engineer")
  
  data = json.dumps({
   "name": "Jane",
   "age": 25
  })
  
  name, age, occupation = parse_json(data)
  assert (name, age, occupation) == ("Jane", 25, None)
  
  data = json.dumps({
   "name": "Alice",
   "occupation": "Data Scientist"
  })
  
  name, age, occupation = parse_json(data)
  assert (name, age, occupation) == ("Alice", None, "Data Scientist")
  
  data = json.dumps({
   "age": 30
  })
  
  name, age, occupation = parse_json(data)
  assert (name, age, occupation) == (None, 30, None)
output: def parse_json(data):
      data = json.loads(data)
      name = data.get('name', None)
      age = data.get('age', None)
      occupation = data.get('occupation', None)
      return name, age, occupation