Surprising Assertions

Recently during code review (for a Python application). I asked somebody if they intentionally used the assert keyword rather than an if check outside of test, and if they were aware that calling the Python interpreter with the -O option would disable their assertions. This came as a surprise to the author (and rightfully so IMO, not many teams use the interpreter options and just take the default) who assumed they could safely take the pattern they used in test and apply it to some validation functions in the application.

A similar thing happens in C if NDEBUG is defined catching some engineers by surprise when their validation functions are no longer behaving as expected once their application moves from debug to release builds (or other optimized environments).

It's interesting (surprising?) behavior, and it makes me wonder what the history or context of the decision was around assert and how it can be removed from execution in both languages. Surprises like this are part of what makes engineering hard. If you expectations/mental model don't actually line up with the execution context and model your application can enter into surprising states that become harder to debug because you have to figure out that your mental model is disconnected from the actual state of the program.