Make turtle.pensize smarter

[adorilson]

Feature or enhancement

Proposal:

The turtle.pensize doc says:

turtle.pensize(width=None)
    Parameters:

        width – a positive number

However, if you put a non-positive number, the function will accept it without any error or warning.

Afterwards, a draw action (like forward) will raise an exception. See below.

>>> import turtle
>>> turtle.pensize(-1)
>>> turtle.forward(100)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 8, in forward
  File "/home/adorilson/anaconda3/lib/python3.12/turtle.py", line 1636, in forward
    self._go(distance)
  File "/home/adorilson/anaconda3/lib/python3.12/turtle.py", line 1597, in _go
    self._goto(ende)
  File "/home/adorilson/anaconda3/lib/python3.12/turtle.py", line 3248, in _goto
    screen._drawline(self.drawingLineItem,
  File "/home/adorilson/anaconda3/lib/python3.12/turtle.py", line 543, in _drawline
    self.cv.itemconfigure(lineitem, width=width)
  File "<string>", line 1, in itemconfigure
  File "/home/adorilson/anaconda3/lib/python3.12/tkinter/__init__.py", line 2988, in itemconfigure
    return self._configure(('itemconfigure', tagOrId), cnf, kw)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/adorilson/anaconda3/lib/python3.12/tkinter/__init__.py", line 1712, in _configure
    self.tk.call(_flatten((self._w, cmd)) + self._options(cnf))
_tkinter.TclError: bad screen distance "-1"
>>>

My suggestion is pensize (and probably others) raises a TurtleGraphicsError if the width argument is less than 1.

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

https://discuss.python.org/t/improving-the-turtle-library/61840/18

Linked PRs

• gh-133244: TPen.pensize raises TurtleGraphicsError if called with a negative number #135268

[StanFromIreland]

if the width argument is less than 1.

This is a backwards compatibility concern, since currently values less than one are accepted, even though they are equivalent to 1. This should be a separate feature issue.

[donbarbos]

This is a backwards compatibility concern, since currently values less than one are accepted, even though they are equivalent to 1. This should be a separate feature issue.

the thing is that anyone who uses negative values ​​still gets an error, just in a different place. it's unlikely that anyone calls pensize() without further action.

maybe wait to see what @willingc as a turtle expert says

I would also like to note that other methods expect positive numbers arguments: pensize (width alias), pen, shapesize (turtlesize alias), tracer (expecting nonnegative integer)

and shapesize has argument checking but only for zero:

cpython/Lib/turtle.py

Lines 2913 to 2914 in ad2f088

	if stretch_wid == 0 or stretch_len == 0:
	raise TurtleGraphicsError("stretch_wid/stretch_len must not be zero")

[adorilson]

This is a backwards compatibility concern, since currently values less than one are accepted, even though they are equivalent to 1. This should be a separate feature issue.

In fact, if it is 0, it works this way, but if it is a negative number, it doesn't. I took it into mind while opening the PR, and put in there an idea concerning 0.