Mike Pirnat - How to Make Mistakes in Python

• 2015-09-25: First Release

To err is human; to really foul things up requires a computer.

Bill Vaughan

I started programming with Pythonin 2000,at the very tail end of The Bubble.In that time,Ivedone things.Things Im not proud of.Some of them simple,some of them profound,all with good intentions.Mistakes, as they say, have been made.Some have been costly,many of them embarrassing.By talking about them,by investigating them,by peeling them back layer by layer,I hope tosave you some of thetoe-stubbing and face-palmingthat Ive caused myself.

As Ive reflected onthe kinds of errors Ive madeas a Python programmer,Ive observed that theyfall more or less intothe categories thatare presented here:

How an incautiously prepared environment has hampered me.

The trivial mistakes that waste a disproportionate amount of my energy.

Poor stylistic decisions that impede readability.

Assembling code in ways that make change more difficult.

Those sudden shocking mysteries that only time can turn from OMG to LOL.

There are a couple of quick thingsthat should be addressedbefore we get started.

First, this work does not aimto be an exhaustive referenceon potential programming pitfallsitwould have to be much, much longer,and would probably never be completebutstrives insteadto be a meaningful tourof the greatest hitsof my sins.

My experiences are largely basedon working with real-world butclosed-source code;though authentic examplesare used where possible,code samples that appear heremay be abstracted and hyperbolizedfor effect,with variable names changedto protect the innocent.They may also refer toundefined variables or functions.Code samples make liberal useof the ellipsis ()to gloss over reams of codethat would otherwise obscurethe point of the discussion.Examples from real-world codemay contain more flawsthan those under direct examination.

Due to formatting constraints,some sample code thatsdescribed as one linemay appear on more than one line;I humbly ask the useof your imagination in such cases.

Code examples in this bookare written for Python 2,though the concepts under considerationare relevant to Python 3and likely far beyond.

Thanks are due toHeather Scherer,who coordinated this project;to Leonardo Alemeida,Allen Downey,and Stuart Williams,who provided valuable feedback; to Kristen Brown and Sonia Saruba, who helped tidy everything up; and especially to editorMeghan Blanchette,who picked my weird ideaover all of the safe onesand encouraged me to run with it.

Finally,though the material discussed hereis rooted in my professional life,it should not be construedas representing the current stateof the applications I work with.Rather, its drawn from over 15 years(an eternity on the web!)and much has changedin that time.Im deeply grateful to my workplacefor the opportunityto make mistakes,to grow as a programmer,and to share what Ive learnedalong the way.

With any luck,after reading thisyou will be in a positionto make a more interestingcaliber of mistake:with an awarenessof what can go wrong,and how to avoid it,you will be freedto make the exciting,messy, significantsorts of mistakesthat push the artof programming,or the domain of your work,forward.

Im eager to seewhat kind of troubleyoull get up to.