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Much of the material in this document is taken from Appendix F in the book A Primer on Scientific Programming with Python, 4th edition, by the same author, published by Springer, 2014.

How to debug

Most programmers will claim that writing code consumes a small portion of the time it takes to develop a program: the major portion of the work concerns testing the program and finding errors.

Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it. Brian W. Kernighan, computer scientist, 1942-.
Newcomers to programming often panic when their program runs for the first time and aborts with a seemingly cryptic error message. How do you approach the art of debugging? This document summarizes some important working habits in this respect. Some of the tips are useful for problem solving in general, not only when writing and testing Python programs.