The Python Standard Library - this library reference manual describes the standard library (built-in functions, constants, types, etc) that is distributed with Python.
Real Python Tutorials - “at Real Python you can learn all things Python from the ground up. Everything from the absolute basics of Python, to web development and web scraping, to data visualization, and beyond.”
Python 3 Module of the Week - “PyMOTW-3 is a series of articles written by Doug Hellmann to demonstrate how to use the modules of the Python 3 standard library.”
A Whirlwind Tour of Python - a free tutorial for Python - “a brief yet comprehensive introduction to Python for engineers, researchers, and data scientists who are already familiar with another programming language.” Also available in a free PDF file