This reminds me so much about a tip in Effective Java .. 'Always provide an option for users to access every relevant part of your object. If not people will start to parse your toString output and you will have created an inofficial API that you have to support whether you want or not' (paraphrased from my faulty memory). That programmers make that mistake again and again is just sad. :(
With a sufficient number of users of an API, it does not matter what you promise in the contract: all observable behaviors of your system will be depended on by somebody.
I'm in a team where all the consumers are only other internal teams and yet this still happens. I've found that it doesn't matter if you explicitly state to not rely on a particular behavior in your documentation, clients still will. It's always your fault if you break it because "it was your change that broke this client" and "it was working fine yesterday".
For example in java the iteration order of hash maps or the behavior of sorts in presence of non-reflexive comparators changed and people did depend on that. Sun was able to change it because it was not part of the API contract.
Which has been the source of a number of bugs in various GPG clients in the past (a few even on the HN frontpage for a few moments) and probably will into the future until GPG is no longer used.
tbh, GPG should just provide an RPC option so applications can securely pass data back and forth and receive proper error messages and codes. But I bet this won't happen because GNU fears that sort of things considering they won't split up the GCC compiler.
But yes I mostly parse colon separated text.
That's unfounded, there are plenty of GNU projects that have an API.
I can see that the same reasoning here would be valid; someone could write a proprietary GPG frontend.
Furthermore the GCC decision is well documented in mailing list posts, have you ever seen anyone involved in GPG development claim that they won't allow a library/frontend split for fear of someone writing a proprietary frontend?
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.
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.
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).
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.
>>>