import pygame
import pygame.locals
import random

pygame.init()
screen=pygame.display.set_mode((240,200))
pygame.display.set_caption('Large Letter')

def newletter(let):
  global text
  global button_rect

  # clear screen
  screen.fill((255,255,255))
  # add the letter
  font = pygame.font.Font(None,200)
  # (10,10,10) is the colour
  text = font.render(let, 1, (10,10,10))
  screen.blit(text, letter_offset)

  # 'New' button
  font2 = pygame.font.Font(None,30)
  font2.set_underline(True)
  button_text = font2.render('New', 1, (10,10,10))
  # margin is the distance of the text from the edge of the screen
  margin = 6
  # put the button in the top left hand corner
  # and keep the return value (a Rect) in a global variable
  # so that we can detect mouse clicks on it
  button_rect = screen.blit(button_text, (margin, margin))
  pygame.display.update()

# we use letter_offset later to adjust the mouse coordinates
letter_offset = (70, 10)
newletter('m')

oldpos = (0, 0)
running = True

while running:
  # get one event off the event queue
  # waiting until there is one
  event = pygame.event.wait()
  if event.type == pygame.locals.QUIT:
    running = False
  if event.type == pygame.locals.MOUSEMOTION:
    newpos = event.pos
    # is the left button down?
    if event.buttons[0]:
      # see if the mouse is over the letter
      # the letter background is the same colour as the letter
      # but with zero alpha, ie transparent
      # ".a" fetches the alpha (transparency) value
      # adjust the mouse coordinates to be relative
      # to the corner of the letter surface
      offpos = (newpos[0]-letter_offset[0], newpos[1]-letter_offset[1])
      try:
        if text.get_at(offpos).a > 50:
          # on the text
          # https://www.pygame.org/docs/ref/color_list.html
          colour = 'yellow'
        else:
          # on the background
          colour = 'blue'
      except IndexError:
        # off the edge of the letter's surface (variable 'text')
        colour = 'cornflowerblue'
      # draw a line from oldpos to newpos
      # the last parameter is the line width
      pygame.draw.line(screen, pygame.Color(colour), oldpos, newpos, 3)
    # remember the previous mouse position even if the button is up
    oldpos = newpos
    pygame.display.update()
  if event.type == pygame.locals.MOUSEBUTTONDOWN:
    if button_rect.collidepoint(event.pos):
      # clicking the button clears the screen
      # and generates a new letter
      newletter(chr(random.randint(ord('a'), ord('z'))))
  # ignore all other events