F12.6 The digital divide and accessibility

Using the provided luminance, write contrast(a, b) returning the contrast ratio of two colours. For each (name, foreground, background) in pairs, print <name>: <ratio> pass if the ratio is 4.5 or more, or <name>: <ratio> FAIL if not, with the ratio rounded to 1 decimal place. At the end print failed: <n>, and set the LED to the foreground colour of the first pair that passes.

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()

def luminance(rgb):
    """How bright a colour looks, from 0 (black) to 1 (white)."""
    out = []
    for value in rgb:
        v = value / 255
        out.append(v / 12.92 if v <= 0.03928 else ((v + 0.055) / 1.055) ** 2.4)
    return 0.2126 * out[0] + 0.7152 * out[1] + 0.0722 * out[2]

# name, foreground, background
pairs = [
    ("grey on white", (150, 150, 150), (255, 255, 255)),
    ("navy on white", (0, 40, 120), (255, 255, 255)),
    ("yellow on white", (255, 220, 0), (255, 255, 255)),
    ("white on navy", (255, 255, 255), (0, 40, 120)),
]

The hint students can ask for: The ratio is the lighter luminance plus a small offset, divided by the darker one plus the same offset. Compare it with the threshold to decide pass or fail, count the fails, and remember the first foreground that passed for the LED.

A solution

from bugbot import *
connect()
def luminance(rgb):
    """How bright a colour looks, from 0 (black) to 1 (white)."""
    out = []
    for value in rgb:
        v = value / 255
        out.append(v / 12.92 if v <= 0.03928 else ((v + 0.055) / 1.055) ** 2.4)
    return 0.2126 * out[0] + 0.7152 * out[1] + 0.0722 * out[2]

pairs = [
    ("grey on white", (150, 150, 150), (255, 255, 255)),
    ("navy on white", (0, 40, 120), (255, 255, 255)),
    ("yellow on white", (255, 220, 0), (255, 255, 255)),
    ("white on navy", (255, 255, 255), (0, 40, 120)),
]

def contrast(a, b):
    la, lb = luminance(a), luminance(b)
    lighter, darker = max(la, lb), min(la, lb)
    return (lighter + 0.05) / (darker + 0.05)

failed = 0
best = None
for name, fg, bg in pairs:
    ratio = contrast(fg, bg)
    if ratio >= 4.5:
        print(f"{name}: {ratio:.1f} pass")
        if best is None:
            best = fg
    else:
        failed = failed + 1
        print(f"{name}: {ratio:.1f} FAIL")
print("failed:", failed)
if best:
    led(*best)

Any program that meets the task's checks is marked correct in the simulator; this is one way, not the only way.