IMO there's a difference between "use at your own risk" and "this is actually not meant to be called from the outside and if it is it will violate some invariant in the code". Those are two different concepts and it makes sense to distinguish them. Of course in high level highly managed languages like python it makes sense that the difference between the two can be rather fuzzy at times but in low level code there's often a clear difference.

Take for instance a socket class in C++, you might have a private method that deals with the low level details of the libc's socket calls. It's called at construction time and never later, and calling it would cause the current socket to be replaced by the new one (leaking the fd in the process) because it's only meant to be used at init time. Clearly it's not "use at your own risk", it's "code that calls this from the outside is fundamentally broken". Having the compiler enforce this invariant is a useful feature.

Meanwhile you can also have "use at your own risk" methods, for instance "get_raw_fd" if you want to be able to access the underlying socket. It makes it easy to break things but has legitimate use cases.

Of course you could say that you could just tag these private in a certain way and let coder discipline do the rest, but then again you could say that of pretty much all static validation (which, I suppose, makes sense if you like very dynamic languages like Python).

> IMO there's a difference between "use at your own risk" and "this is actually not meant to be called from the outside and if it is it will violate some invariant in the code". Those are two different concepts and it makes sense to distinguish them.

That's what double underscores are for:

  $ python3
  Python 3.5.2 (default, Nov 23 2017, 16:37:01) 
  [GCC 5.4.0 20160609] on linux
  Type "help", "copyright", "credits" or "license" for more information.
  >>> class Foo:
  ...     def foo(self):
  ...         print("I'm a public method.")
  ...     def _foo(self):
  ...         print("I'm a private method.")
  ...     def __foo(self):
  ...         print("I'm so private that you have to really know what you're doing to even call me.")
  ... 
  >>> foo = Foo()
  >>> foo.foo()
  I'm a public method.
  >>> foo._foo()
  I'm a private method.
  >>> foo.__foo()
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  AttributeError: 'Foo' object has no attribute '__foo'
  >>> foo._Foo__foo()
  I'm so private that you have to really know what you're doing to even call me.
  >>> 

Now in theory it's not quite what you're talking about because it does rely on coder discipline to some extent, but in practice I've never seen it become an issue (although arguably that could be because not many people know that you can access double underscore variable from outside a class).

Where did you get that idea from? Double underscores have to do with name mangling, nothing else https://stackoverflow.com/questions/1301346/what-is-the-mean...

It's just how I've always seen it used.

IME if you allow someone to use your private API you can be sure they will do at some point. And then they will be unhappy when you change it. Sure, you can go with "I said it was private, your problem", but that is the road to zero users.

So much this. I've seen it happen time and time again.

At least in Java, the reflection API serves as a "break glass" barrier to make sure folks understand they're doing something they shouldn't. Someone will still do it, and they'll still blame you when their app breaks later... but I like to believe it at least scares some folks away.

Honestly, I don't see people getting unhappy at breaking private API functionality. Most grab a library when they need and never upgrade it. They only do so when there's a necessary security risk or some wiz-bang new feature they need.

The common example: programmer grabs a library to solve a problem; a few weeks later they realize it doesn't really solve the problem and requires modifications to the library to do some custom job; they make the modifications. That's normally the end of the story for several years. No point in upgrading the library unless it really solves a business problem.

That constitutes the vast majority of developers.

Of course there are other organizations that always want the latest and greatest version and are constantly upgrading. To them, I'd say sure, stick to the public methods, otherwise you're creating a big headache down the road. But I wouldn't require it of them.

Closed source dependencies treat programmers as infants. With open source dependencies, private methods are just an organization tool, not an actual limitation on the programmer.

If a programmer cannot access some variable or function because another developer thought that they "shouldn't", how is that not treating them like infants?