With interactive debuggers like python's pdb you can actively manipulate data, print it out, basically do whatever you need to do in a REPL. It is way more effective than adding print statements, rerunning your script, etc. That being said I typically use a combination of both: I use a print to get me to where I think the problem begins, then an interactive debugger (pdb.set_trace / breakpoint / etc) to drill down into the details.