add as_tuple() method to PyList

[samuelcolvin]

From benchmarks, this is significantly faster than PyTuple::new(py, the_list):

test list_as_tuple_direct        ... bench:         299 ns/iter (+/- 51)
test list_as_tuple_iterate       ... bench:         521 ns/iter (+/- 41)

[davidhewitt]

Looks good other than the change on the .gitignore which I'm not convinced is necessary!

[davidhewitt]

Thanks!

bors r+

[bors]

Build succeeded:

• conclusion

[mejrs]

I would prefer a From<&PyList> for &PyTuple implementation.

[samuelcolvin]

I would prefer a From<&PyList> for &PyTuple implementation.

I see that but it would be less obvious what's going on, and why the reverse can't be done.

[davidhewitt]

I would prefer a From<&PyList> for &PyTuple implementation.

Actually this is a great point, I think the reverse could be implemented by tuple.as_sequence().list(). However I would like to ask we put this on the backlog for the moment just because with reboot of #1308 there would be ownership difference and then it's less clear if the implementation should be From<&'_ PyList<'py>> for PyTuple<'py> or From<PyList<'py>> for PyTuple<'py> (the difference being whether a reference to the PyList is borrowed or consumed).

However this also makes me notice another snag, which is naming convention. This is as_tuple() (and matches the C API name) but in Rust the as_ methods are usually zero-cost conversions, with to_ conversions being more costly e.g. to_string vs as_str. So I think there's a few considerations:

• Call this just .tuple() to match PySequence::tuple().
• Call this to_tuple() and rename sequence methods to PySequence::to_tuple() and PySequence::to_list().
  • I'm tempted to open a PR to rename the sequence methods anyway, I think the to_ prefix may be strictly better.
• Revert this and recommend just using the sequence API. It looks like PySequence_Tuple has a special case to call PyList_AsTuple so there would be negligible perf difference.

[mejrs]

The to_* renaming seems like a good idea

[samuelcolvin]

👍 for to_*.

[davidhewitt]

Pushed #3043 and I'll add separate PRs for other renaming and also PyTuple::to_list when I get a moment.