Question

So I’m experimenting in pygame and wrote some code for a rectangle that slowly changes color

a = 4
b = 3
c = 2

#some irrelevant code later

    if color[0]+a < 255:
        color[0] += a
    else:
        a *= -1
    if color[1]+b < 255:
        color[1] += b
    else:
        b *= -1
    if color[2]+c < 255:
        color[2] += c
    else:
        c *= -1

a, b, and c as the speed of change for red, green, and, blue.

The problem is that for some reason it will give me a TypeError: Invalid color argument after a few seconds of the program running, usually when the color is very blue. I don’t see any reason an invalid color argument would appear.

Answer 1

I would expect a ValueError instead of a TypeError, but it looks like what’s happening is that if we take a = 4 and work with color[0]

• color[0] == 250, so gets changed to 254
• color[0] == 254, so stays the same, a gets changed to -4
• color[0] == 254, so gets changed to 250
• … keeps substracting 4….

And I’m not sure -4 is a valid colour…

maybe look at using

>>> from itertools import izip, cycle
>>> a = range(0, 20, 4) + range(20, 0, -4)
>>> b = range(0, 20, 3) + range(20, 0, -3)
>>> c = range(0, 20, 2) + range(20, 0, -2)
>>> test = izip(cycle(a), cycle(b), cycle(c))
>>> for i in range(30):
    print next(test)

(0, 0, 0)
(4, 3, 2)
(8, 6, 4)
(12, 9, 6)
(16, 12, 8)
(20, 15, 10)
(16, 18, 12)
(12, 20, 14)
(8, 17, 16)
(4, 14, 18)
(0, 11, 20)
(4, 8, 18)
(8, 5, 16)
(12, 2, 14)
(16, 0, 12)
(20, 3, 10)
(16, 6, 8)
(12, 9, 6)
(8, 12, 4)
(4, 15, 2)
(0, 18, 0)
(4, 20, 2)
(8, 17, 4)
(12, 14, 6)
(16, 11, 8)
(20, 8, 10)
(16, 5, 12)
(12, 2, 14)
(8, 0, 16)
(4, 3, 18)