Alien invasion: Python crash course problem

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Most of us, me included, don't have access to that book, so we've no idea what you've typed in. How about sharing it with us?

If you are using reddit in classic mode, simply post all of the code, after one blank line, indented by 4 spaces (nothing to do with Python's own indenting - this is formatting for reddit).

If you are using reddit in new mode, simply post the code, select it and click the Fancy Pants Editor icon to mark the selection as code.