So far…

• Have introduced the basic tools of programming

• But how do we know a program gives the right answer?

• We need to
  • write programs that check their own operation
  • write tests to catch the mistakes those self-checks miss

Confess!

Why don’t you write tests?

• “I don’t write buggy code”
• “It’s too hard”
• “It’s not interesting”
• “It takes too much time and I’ve research to do”

Ariane 5

• Ariane 5 used Ariane 4 code
• Ariane 5’s faster engines caused buffer overflow
• Buffer overflow caused Ariane 5 to explode!
• Unfortunately, code wasn’t properly tested

Millions of pounds down the drain, some very red faces

Consider Geoffrey Chang…

• Dept. of Molecular Biology, Scripps Institute
• 5th Annual Presidential Early Career Awards, 2000
• Beckerman Foundation Young Investigator Award, 2001

Three pharma publications in Science, 2001-2005

Letters – Retraction, Science 22 December 2006

And he’s not alone…

• “A Test of Corrections for Extraneous Signals in Gridded Surface Temperature Data”, R. McKitrick et al, Climate Research, 2004
  • “McKitrick screws up yet again”, T. Lambert’s blog
  • “McKitrick mucks it up”, J. Quiggin’s blog

“ERRATUM”, Climate Research, 2004

• “formula for computing cosine of absolute latitude … takes the angle in radians, but our data were entered in degrees”

What testing gives you

• Confidence that your code does what it is supposed to
  • That your research is built on a solid foundation
• Ability to detect, and fix, bugs more quickly
• Confidence to refactor or fix bugs without creating new bugs
• Examples of how to use your code
• “if it’s not tested, it’s broken”
  • bittermanandy, 10/09/2010

Learning Objectives

• how to write code defensively to guard against making errors;
• how to use a unit testing framework;
• when it’s useful to write tests before writing code.
• how Python reports and handles errors;