如何在 Python 的 for 循环中设置条件语句？

Question

I am using turtle in python to create simple shapes that have a change of gradient starting at white and ending at black, black is 0,0,0. The way I have it set up currently is that it will continue to subtract beyond zero within my nested loop. I want my code to print the color black once it equals or is less than 0,0,0. I would appreciate it if someone could give me pointers and hints without straight up giving me the solution. Thanks!

a = int(input("Enter int angle:"))
q = int(input("Enter int length:"))
print("Click turtle screen to exit...")
r=1
g=1
b=1
import turtle
wn = turtle.Screen()
wn.bgcolor("white")
alex = turtle.Turtle()
alex.speed(500)
alex.pencolor("white")
for i in range (q):      
   alex.forward(i)
   alex.left(a)
   for c in range (q):
      alex.pencolor(r,g,b)
      r=(r-.0001)
      g=(g-.0001)
      b=(b-.0001)

    
wn.exitonclick()

Answer 1

In your final for loop, you should have

for c in range (q):
      alex.pencolor(r,g,b)
      if condition (color is black or less than black):
          result (print black)
      else:
          r=(r-.0001)
          g=(g-.0001)
          b=(b-.0001)

Answer 2

If we make the color decrement 1 over the number of iterations, then the color should reach black on our last iteration:

from turtle import Screen, Turtle

screen = Screen()

angle = screen.numinput("Angle", "Enter angle: ", default=110, minval=1, maxval=359)
length = int(screen.numinput("Length", "Enter integer length: ", default=75, minval=10, maxval=200))

gray = 1.0

turtle = Turtle()
turtle.speed('fastest')

for _ in range(length):
    turtle.pencolor(gray, gray, gray)
    turtle.forward(length)
    turtle.left(angle)

    gray = gray - 1.0 / length

screen.exitonclick()

But, if that still gives you problems, you can wrap it in a max() :

    gray = max(0, gray - 1.0 / length)