On first look I see nothing obvious, but I am not Python specialist.
Use the debugger to see what your code is doing. It allow you to execute lines 1 by 1 and to inspect variables as it execute.
Debugger - Wikipedia, the free encyclopedia[
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Mastering Debugging in Visual Studio 2010 - A Beginner's Guide[
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The debugger is here to show you what your code is doing and your task is to compare with what it should do.
There is no magic in the debugger, it don't find bugs, it just help you to. When the code don't do what is expected, you are close to a bug.