It depends on what you want to do, but I'd say just getting familiar with the standard library's modules would be a great start. It's so extensive and helpful that Guido (Python's creator) is reputed to have a time machine that lets him create modules the day before people request them. :)
Doug Hellman's Python Module of the Week PyMOTW blog does a great job of providing examples of Python's standard library. It mostly covers Python 2, but they still apply to Python 3:
http://pymotw.com/2/
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https://automatetheboringstuff.com
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