下面的程序允许我用鼠标和键在一个小盒子里创建形状。我在理解下面两条代码时遇到了困难。在

从下面的代码中，当在第一个if语句(与下一个elif语句for-)中按+键时，python如何更改大小？此外，当连续多次按下+按钮时，大小如何增加。嵌套在下面语句中的if语句不应允许由于最后一个\u键而改变大小！=“+”：错误，因为按“+键一次后，该值将存储为最后一个_键。我已经看了一会儿了，但似乎不太明白。在def main():

size = INITIAL_SIZE # current diameter of circle or length of square

set_fill_color(1, 0, 0) # shapes are initially red

last_key = "" # no key pressed yet

drawing_circle = False # are we drawing circles?

drawing_square = False # are we drawing squares?

mouse_was_down = False # was the mouse button most recently down?

while not window_closed():

if is_key_pressed("+"): # increase size?

if last_key != "+":

size = min(size + SIZE_CHANGE, MAX_SIZE)

last_key = "+"

elif is_key_pressed("-"): # decrease size?

if last_key != '-':

size = max(size - SIZE_CHANGE, MIN_SIZE)

last_key = '-'

elif is_key_pressed("r"): # draw in red?

set_fill_color(1, 0, 0)

last_key = "r"

elif is_key_pressed("g"): # draw in green?

set_fill_color(0, 1, 0)

last_key = "g"

elif is_key_pressed("b"): # draw in blue?

set_fill_color(0, 0, 1)

last_key = "b"

elif is_key_pressed("c"): # draw circles?

drawing_circle = True

drawing_square = False

last_key = "c"

elif is_key_pressed("s"): # draw squares?

drawing_circle = False

drawing_square = True

last_key = "s"

else:

last_key = "" # no key pressed

if mouse_down():

if not mouse_was_down: # beginning of a mouse click?

if drawing_circle:

draw_circle(mouse_x(), mouse_y(), size/2)

elif drawing_square:

draw_rectangle(mouse_x() - size/2, mouse_y() - size/2, size, size)

mouse_was_down = True

else:

mouse_was_down = False

request_redraw()

sleep(0.05)

启动图形(主)