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

Doc/library/turtle.rst

@@ -1079,6 +1079,8 @@
 
    :param width: a positive number
 
+   :raises TurtleGraphicsError: If *width* is less than zero.
+
    Set the line thickness to *width* or return it.  If resizemode is set to
    "auto" and turtleshape is a polygon, that polygon is drawn with the same line
    thickness.  If no argument is given, the current pensize is returned.
@@ -1090,6 +1092,9 @@
       1
       >>> turtle.pensize(10)   # from here on lines of width 10 are drawn
 
+   .. versionchanged:: next
+      Raise :exc:`TurtleGraphicsError` if *width* is less than zero.
+
 
 .. function:: pen(pen=None, **pendict)
 

Lib/test/test_turtle.py

@@ -480,6 +480,25 @@ def test_teleport(self):
             tpen.teleport(-100, -100, fill_gap=fill_gap_value)
             self.assertTrue(tpen.isdown())
 
+    def test_pensize_with_positive_numbers(self):
+        tpen = turtle.TPen()
+
+        tpen.pensize(42)
+        self.assertEqual(42, tpen.pensize())
+
+    def test_pensize_with_nonpositive_numbers(self):
+        tpen = turtle.TPen()
+
+        tpen.pensize(0)
+        self.assertEqual(0, tpen.pensize())
+
+        width = -1
+        msg = f"width argument must be a positive number, but got {width}."
+        with self.assertRaisesRegex(turtle.TurtleGraphicsError, re.escape(msg)):
+            tpen.pensize(width)
+
+        self.assertEqual(0, tpen.pensize())
+
 
 class TestTurtleScreen(unittest.TestCase):
     def test_save_raises_if_wrong_extension(self) -> None:

Lib/turtle.py

@@ -2141,7 +2141,11 @@ def pensize(self, width=None):
         """
         if width is None:
             return self._pensize
-        self.pen(pensize=width)
+        elif width < 0:
+            msg = f"width argument must be a positive number, but got {width}."
+            raise TurtleGraphicsError(msg)
+        else:
+            self.pen(pensize=width)
 
 
     def penup(self):

Misc/NEWS.d/next/Library/2025-06-09-08-11-47.gh-issue-133244.GINMl-.rst

@@ -0,0 +1 @@
+:meth:`!turtle.TPen.pensize` raises an exception if called with a negative *width* argument.