This is the second part of the python bioinformatics course that I have taught biologists. This module is about control flow and how to handle input and output. Control flow is needed in mainly two different situations, either that a decision based on data has to be made, or that a piece of code should be repeated. In Python, decisions are made using an IF statement, while iterations (repeating code) are done with either a FOR loop, or a WHILE loop. How to handle input and output from files is also described – in most cases that is where the data in question is to be found, and it is easier to keep track of results if the program prints the results to a file.