Ask HN: Why do you make class members private?

> Ouch, that's a pretty careless reason to start with

I think it's fine. This is probably the reason that I used to have for using private until I started thinking about it.

> there should never be a reason for consumers to handle that stuff directly

Are we assuming here that the organization is structured so that only one team works on a class, so that that's the team that knows how the bookkeeping is done and no other team should touch it?

> Making variables and methods private means making them inaccessible outside the class and I gave the typical reasons/best practices for that.

The point they are making is in python there's no private method, everything is public. There's only a convention of "avoid using anything starting with "_"

1. It's private, only internal routines can use it. You aren't telling anyone to avoid using it, you are constraining where it can be used. You can use it directly in every internal routine if you want, that's not a problem. But no one outside the class/module can directly access it so if you change it (in whatever fashion) then those outside users will not be impacted because they are only dependent on the public interface. Now if you change the public interface, then they are impacted but that's often rare after the initial development effort, in my experience.

2. Forget about people, it's about place. I don't care how many people alter a particular class or module. What I care about is how many places some information has to exist within the code and has to be maintained and synchronized as a result.

As a mostly-useless-after-CS101 example, consider a bounded stack. An implementation might have a number (perhaps variable) which describes its limit, an array backing it, and another number indicating where the current "top" of the stack is (in languages where arrays carry their size you don't necessarily need that limit number as a separate thing).

If you leave everything public then every user of this bounded stack could directly alter the backing array and change the "top" of the stack, artificially indicating that something had been popped off or incorrectly incrementing beyond the limit. The limit itself could be altered without actually changing the backing array. The backing array could be made smaller or larger without correspondingly changing the limit. All of that would make this data structure useless, because it would be in an arbitrary, likely invalid, state.

In order to preserve the invariants of the system (limit == array size, 0 correctly use it they have to preserve all these invariants everywhere they use the data structure. The code is now scattered and contains many duplications. Again, this isn't about people, it's about places and the number of them.

If you want to change the internal structure of this bounded stack, you have to change every place that currently accesses the public fields. Or you can be a sane developer, use private fields and public routines that manipulate the state so that it's always in a valid state. Now when you change the internal structure you only have to change those public routines and the private fields, no other place has to be altered. Every use of this bounded stack will look exactly the same as before, just push and pop and some error handling for when the stack is full or empty.

> Are we assuming here that the organization is structured so that only one team works on a class, so that that's the team that knows how the bookkeeping is done and no other team should touch it?

No. If you're in such an organization, anybody on any team should be able to read the code and discover how the bookkeeping is done and locate bugs therein. A good API will obscure the necessity of bookkeeping from consumers in order to reduce bug-ridden boilerplate. Said in more highfalutin' language, privacy should be used to prevent leaky abstractions.

> The only variables and functions that should be public are those that cannot put an object into an invalid state.

That's an interesting one. So a reason to use private is to tell programmers that calling this function or changing this variable can leave the object in an invalid state? I guess invalid state then means that you can't call a public function on that object.

It might not be clear what an invalid state is though. I mean, every state has its place. Example, after I decrease the health of the player to 0 but before the death animation is played, is that a valid state? I should not be able to play in that state but it's a state that the player might be in for a frame and it will be visible on the screen.