How can you be sure it’s one line of code? What if there are several codepaths, and venvs are activated in different places? And in any case, even if there is only one conditional needed, that is still one branch more than necessary to test.
Your symlink example does not make sense. There is someting that is changing. In fact, it may even be the opposite: if you need to use file A in s container, and file B otherwise, it may make perfect sense to symlink the correct file to C, so thst your code does not need to care about it.
Sure If that happens. But it may also not. Which is actually usually the case. Sure, it’s not 100% safe, but it is safer.