Wandering Thoughts archives

Sometimes print-based debugging is your only choice

Recently I had to investigate a mysterious issue in our Django based Python web application. This issue happened only when the application was actually running as part of the web server (using mod_wsgi, which effectively runs as an Apache process). The only particularly feasible way to dig into what was going on was everyone's stand-by, print based debugging (because I could print into Apache's error log; I could have used any form of logging that would surface the information). Even if I might have somehow been able to attach a debugger to things to debug a HTTP request in flight, using print based debugging was a lot easier and faster in practice.

I'm a long time fan of print based debugging. Sometimes this is because print based debugging is easier if you only dip into a language every so often, but that points to a deeper issue, which is that almost every environment can print or log. Print or log based 'debugging' is an almost universal way to extract information from a system, and sometimes you have no other practical way to do that.

(The low level programming people sometimes can't even print things out, but there are other very basic ways to communicate things.)

As in my example, one of the general cases where you have very little access other than logs is when your issue only shows up in some sort of isolated or encapsulated environment (a 'production' environment). We have a lot of ways of isolating things these days, things like daemon processes, containers, 'cattle' (virtual) servers, and so on, but they all share the common trait that they deliberately detach themselves away from you. There are good reasons for this (which often can be boiled down to wanting to run in a controlled and repeatable environment), but it has its downsides.

Should print based debugging be the first thing you reach for? Maybe not; some sorts of bugs cause me to reach for a debugger, and in general if you're a regular user of your chosen debugger you can probably get a lot of information with it quite easily, easier than sprinkling print statements all over. But I think that you probably should build up some print debugging capabilities, because sooner or later you'll probably need them.