It depends on the codebase. Open source projects are probably a bad example, as they tend to be well (or at least decently) written: code has cohesion and is o

It depends on the codebase. Open source projects are probably a bad example, as they tend to be well (or at least decently) written: code has cohesion and is organised in a rational way, so that you can usually find pretty quickly the piece you are looking for, and you can often just change the functionality you are interested in without examining every single file. Other codebases, like the ones that you may find in your typical enterprise environment, have ofter much less cohesion, so that they require shotgun surgery, and behaviour is also spread throughout the codebase randomly. If this is the case, you'll be forced to look at more code, figure out the relationship between pieces and maybe proceed by trial and error. This is not a rule, as of course there are lots and lots of private enterprise codebases well written, but I haven't seen a lot of them to date.