input: Write a python function to generate a random hexadecimal color.
  assert len(generate_hex_color()) == 7
  assert generate_hex_color()[0] == '#'
  assert all(c in '0123456789ABCDEF' for c in generate_hex_color()[1:])
output: import random
  
  def generate_hex_color():
      return '#' + ''.join(random.choice('0123456789ABCDEF') for _ in range(6))