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Check the traceback carefully, it shows exactly where the problem is, though it's not immediately obvious until it's bitten you a time or two. You are running /Users/a5/Desktop/z.py which imports turtle, but you have a turtle.py in /Users/a5/Desktop which is found before the standard library turtle. Your command line test works because it is run from a different directory. Just remove or rename /Users/a5/Desktop/turtle.py and it will work.
There have been other reports about this lately, and there are on-going discussions about what can be done to try to prevent this situation.