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Thinking/reasoning about code is my main approach to finding and preventing bugs, and I would argue that it is by far the most important way to do so, since it is one of few ways to understand the ideas that lie behind the code as written, and to build a theory/mental model of what happens. Still, there are cases where you will build a mental model that is wrong either due to minor bugs or larger design problems. As a former physicist, I think of this like theoretical vs experimental physics. First you build the theory (by reviewing the code), then you run experiments (by first running the code). If you encounter surprises, you run more detailed experiements to pinpoint where your theory is wrong, and for this you can use print/log statements and/or a debugger. In my experience, print/log statements are ok for relatively linear logic (immutable or functional patterns, for instance in a data pipeline), while the debugger may be more helpful for highly non-sequential patterns, where it is difficult to grasp what states the program may end up in (complex state machines driven by random/user input, for instance). Then there are cases that are complicated by asynchronous, often high frequency input or that involves third party services that you cannot control (ie trading systems, logistics systems and similar systems that use a lot of concurrent transaction management, and are often connected to external systems, or even generic software like OS's, database engines, etc). Those may be impossible to reproduce properly in a debugger, and may instead need some kind of statistical approach, by bombarding the software with either live or synthetic input, and use collected metrics to build a theory that can explain the problem. (Which will trigger further experiments to validate.) Depending on what software you are writing, using a debugger can be anything from a superpower to nearly useless. The ability to reason about code, on the other hand, is universally useful. Arguably even for commercial software where you do not have access to the code. (As a physicist, I could not "see" quarks directly, but I could still detect them statistically by properly designed experiments.) |