[zaphar]

I think there is an implied context here that the author didn't do a good job of communicating. Usually I will also assume the function does what it says when I use it in code I'm writing.

But what happens when I'm debugging a problem in code I may or may not have written myself? Or when I'm debugging code that I wrote so long ago that I no longer remember all the details. That's when I need to be able to follow the code in the function I'm consuming. You are right that when you are consuming code it's good when you can treat that code as a black box. But that's not actually where I spend the most time as a developer. I spend far more time reading code I don't have full state about and trying to figure why something is broken.

Pi was probably a bad example here since we are conditioned to think of it as a constant. But there are similar uses in code where a function is doing bad things and it's not clear why because you didn't realize the function had an undeclared input from global mutable state. I think that is what the author was trying to say in his post.