The just-updated flake8 package hits a bunch of:
E741 ambiguous variable name 'l'
warnings. This commit renames them all from `l` to `lst` (they are all
list values) to avoid the error.
There are two separate additions:
1. `py::hash(obj)` is equivalent to the Python `hash(obj)`.
2. `.def(hash(py::self))` registers the hash function defined by
`std::hash<T>` as the Python hash function.
Currently types that are capable of conversion always call their convert
function when invoked with a `py::object` which is actually the correct
type. This means that code such as `py::cast<py::list>(obj)` and
`py::list l(obj.attr("list"))` make copies, which was an oversight
rather than an intentional feature.
While at first glance there might be something behind having
`py::list(obj)` make a copy (as it would in Python), this would be
inconsistent when you dig a little deeper because `py::list(l)`
*doesn't* make a copy for an existing `py::list l`, and having an
inconsistency within C++ would be worse than a C++ <-> Python
inconsistency.
It is possible to get around the copying using a
`reinterpret_borrow<list>(o)` (and this commit fixes one place, in
`embed.h`, that does so), but that seems a misuse of
`reinterpret_borrow`, which is really supposed to be just for dealing
with raw python-returned values, not `py::object`-derived wrappers which
are supposed to be higher level.
This changes the constructor of such converting types (i.e. anything
using PYBIND11_OBJECT_CVT -- `str`, `bool_`, `int_`, `float_`, `tuple`,
`dict`, `list`, `set`, `memoryview`) to reference rather than copy when
the check function passes.
It also adds an `object &&` constructor that is slightly more efficient
by avoiding an inc_ref when the check function passes.
