input: Write a function to create a dictionary that contains the data structure in the form of [x:x^2], where x is the key and x^2 is the value, given a list of numbers. If the list is empty, return an empty dictionary.
  def square_dict(numbers):
      return {num: num**2 for num in numbers}
  
  assert square_dict([1, 2, 3, 4, 5])=={1: 1, 2: 4, 3: 9, 4: 16, 5: 25}
  assert square_dict([])=={}
output: def square_dict(numbers):
      return {num: num**2 for num in numbers}