Python bindings: accessing deprecated code should emit DeprecationWarning

[AllSeeingEyeTolledEweSew]

Warnings in python are soft errors, which can be configured at runtime to either be ignored, raise an exception, or print a message to stderr.

There are a few built-in classes of warnings. DeprecationWarning is ignored by default, but are printed when encountered by unit test runners, which makes it a good choice for complaining to developers about using deprecated functions, while not annoying users.

I think all deprecated code in the python bindings should emit DeprecationWarning. Adding warnings isn't expected to break any usage, so it should be reasonable to emit new warnings in all branches, even in a point release.

Ideally, even getting a deprecated attribute would emit the warning. That is, it should be emitted by both of these:

my_func = libtorrent.get_http_category
my_value = libtorrent.get_http_category()

The functions to emit warnings from python C API are PyErr_WarnEx() and friends.

[latot]

Yes please!, this would be great :)

[arvidn]

I think this ought to cover some of it at least. I haven't come up with a good way of reporting this for properties yet.
would anyone mind giving it a try?

#6123

[AllSeeingEyeTolledEweSew]

I found #6123 broke some things:

fprint = lt.fingerprint("AB", 1, 2, 3, 4)
self.assertEqual(fprint.major_version, 1)
"""
TypeError: No Python class registered for C++ class deprecate_visitor<int libtorrent::fingerprint::*>
"""

[AllSeeingEyeTolledEweSew]

There's also a quirk: with deprecated functions enabled, parse_magnet_uri(...) always returns an empty "url" item, session.add_torrent(parse_magnet_uri(...)) will always raise DeprecationWarning due to this. I think this is fine in view of #5992, but maybe it wasn't what you intended

[AllSeeingEyeTolledEweSew]

With #6335, I think the only outstanding work is to raise DeprecationWarning stuff that gets bound with .attr(...), class_(...), enum_(...) or enum_(...).value(...).

My C++ isn't strong enough to figure out a nice way to get these things to raise DeprecationWarning though, I'll need some help.