Your concern applies not just to rounded_rectangle(), but also to rectangle().

from PIL import Image, ImageDraw
xy = (10, 20, 190, 180)
im = Image.new("RGB", (200, 200))
draw = ImageDraw.Draw(im)
draw.rectangle(xy, fill="red", outline="green", width=5)

from PIL import Image, ImageDraw
xy = (190, 20, 10, 180)
im = Image.new("RGB", (200, 200))
draw = ImageDraw.Draw(im)
draw.rectangle(xy, fill="red", outline="green", width=5)

It also applies to pieslice(), and perhaps other methods.

But I'm unsure about automatically correcting user's mistakes like this. If we silently handle this bug, it might lead to other problems in the user's code when they don't realise that they have their co-ordinates the wrong way around.

@hugovk any thoughts?

I though it looked like rectangle already had C code to handle this, but I hadn't tested it. Apparently it doesn't completely work. The hline functions do reorder their x coordinates, but the line functions don't.

I hadn't noticed pieslice(); it says it's the same as arc() though (with extra lines between the start/end and middle), which does specify "where x1 >= x0 and y1 >= y0". chord() is also the same as arc(), but with a line between the start and end. ellipse() also takes two points and specifies "where x1 >= x0 and y1 >= y0". Neither rounded_rectangle() nor rectangle() specify that the points have to be in a specific order. A lot of the drawing functions take a single point, and some take a list of points, so this doesn't affect them.

Thinking about what might cause points to be out of order:

• user types in coordinates, got them in the wrong order. For this, it might be nice to fix it.

• coordinates are data passed from somewhere else. This might indicate there's another problem in their code, and better not hide it (as @radarhere suggested).

I expect it's had this behaviour for a long time.

We could update the docs, and/or raise a ValueError for invalid input.

Ok, I've created PR #6978, to update the docs for rectangle() and rounded_rectangle(), and to raise an error if the co-ordinates are the wrong way around.

#6978 has been merged instead.