deprecate indexing of SoundFile

[bastibe]

Using __getitem__ or __setitem__ should raise a warning, and the deprecation should be marked in the docstrings of SoundFile, pysoundfile, and the README.

[mgeier]

OK, I guess the warning should work like this:

from warnings import warn
warn("blah", DeprecationWarning)

See https://docs.python.org/3.4/library/warnings.html#warnings.warn.

I think this is one of the few cases where the import should happen directly before using the function.

[bastibe]

This warning only shows up if you explicitly ask for it. Normal users will never see this. Is this really what we wanted?

[mgeier]

Hmmm ... I wasn't aware of that. Apparently, since Python 2.7, DeprecationWarning is silenced by default.

I don't think this is what we wanted.

We could just get a bit more aggressive by removing the implementation and just raising a NotImplementedError.
And for the release after the next one we can completely remove the methods.

Alternatively, we could change the warning to a plain Warning, then it should be shown by default.

[bastibe]

They make a point in the documentation, that the way we are doing it right now is the preferred way of doing it, since users don't typically need to be aware of deprecations, and developers typically should be running programs with all warnings enabled anyway.

I agree that we should elevate this to a plain Warning.

[mgeier]

Now the warning shows up, let's merge!