When I re-read the top of the file, I remembered that turtle.py has unique issues in relation to code ownership. (I was once told that Gregor had to approve any substantive code change.) I asked about that and about code cleanups on pydev (thread 'updating turtle.py').

Re your message: I was suggesting 'mapping' the code, not re-structuring it.

Testing: A complete 'unit' test would test each function in each layer. A minimal 'unit' test should at least test each top-level function and check response on the canvas. Assuming that one can get to tk root and canvas, some things should be possible. But I don't know what introspection functions a canvas has. An alternative would be to replace the canvas with a mock-canvas with extra introspection added. Another alternative would be a human-verified test, a turtle script that systematically called every function and said that it was doing for a person to verify. "Line width: 1, 2, 3, 5, 8, 12 17, 30" (with a slight pause for each width).

Versions: at most 2.7, 3.4, 3.5. The 3.4 and 3.5 turtle code should be close to identical.