Cryptic error message if you draw in color on a grayscale image

[shirriff]

What did you do?

Draw a color rectangle on a grayscale image.

What did you expect to happen?

Should draw a rectangle, or at least give an error explaining that you can't draw in color on a grayscale image

What actually happened?

The error message:

TypeError: function takes exactly 1 argument (3 given)

This error message is confusing since the rect function isn't being given 3 arguments; the arguments to rect match the documentation.

I ran into this problem and put an explanation on stackoverflow. This has received dozens of upvotes, so a lot of other people are hitting this problem. (This is why I think it's worth filing an issue on this.)

What are your OS, Python and Pillow versions?

• OS: macOS Big Sur 11.2.3
• Python: 3.9.4
• Pillow: 8.2.0

Code to reproduce the problem:

from PIL import Image, ImageDraw
img = Image.new('L', (100, 100))
draw = ImageDraw.Draw(img)
draw.rectangle((0, 0, 10, 10), outline=(255, 0, 0))

[radarhere]

I've created PR #5504 to change the error message to "color must be int or single-element tuple".