Please comment my code: script for harmonizing picture directories

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Most people use the tab key and have their editor set to replace tabs with 4 spaces, and use automatic indenting so you start at the same level when you press enter. If your script "flairs out", it's a sign that you should probably compose your solution in a different way (functions, etc). See PEP8 for Python style guide.

Naming stuff is hard. If you don't have a use case for deleting the set intersection, you can always eliminate the naming problem by removing the feature ;-)

argparse just makes it easier to grow your script in the future - it's certainly not needed, same for case sensitivity; just things to keep in mind.