References

Python Official Docs - easy to read, many examples

  https://docs.python.org/3/library/unittest.html

Real Python - good explanation & how to run unit tests in IDE

  https://realpython.com/python-testing/

Python Hitchhiker's Guide to Testing

  https://docs.python-guide.org/writing/tests/

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Examples of  common testing tools

Python Cookbook, Chapter 14

     How to test many common situations, including I/O