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* `#error BYE_BYE_GOLDEN_SNAKE`

* Removing everything related to 2.7 from ci.yml

* Commenting-out Centos7

* Removing `PYTHON: 27` from .appveyor.yml

* "PY2" removal, mainly from tests. C++ code is not touched.

* Systematic removal of `u` prefix from `u"..."` and `u'...'` literals. Collateral cleanup of a couple minor other things.

* Cleaning up around case-insensitive hits for `[^a-z]py.*2` in tests/.

* Removing obsolete Python 2 mention in compiling.rst

* Proper `#error` for Python 2.

* Using PY_VERSION_HEX to guard `#error "PYTHON 2 IS NO LONGER SUPPORTED.`

* chore: bump pre-commit

* style: run pre-commit for pyupgrade 3+

* tests: use sys.version_info, not PY

* chore: more Python 2 removal

* Uncommenting Centos7 block (PR #3691 showed that it is working again).

* Update pre-commit hooks

* Fix pre-commit hook

* refactor: remove Python 2 from CMake

* refactor: remove Python 2 from setup code

* refactor: simplify, better static typing

* feat: fail with nice messages

* refactor: drop Python 2 C++ code

* docs: cleanup for Python 3

* revert: intree

revert: intree

* docs: minor touchup to py2 statement
Co-authored-by: Henry Schreiner <henryschreineriii@gmail.com>
Co-authored-by: Aaron Gokaslan <skylion.aaron@gmail.com>

Fix a typo in class.rst

* Add additional pygrep pre-commit hooks

* Remove useless noqas with hook

* Fix all single rst backticks

* Simplify mypy pre-commit hook with upstream fixes

* Add back missing comment

* Add one last pygrep hook

* Fix `pybind11::object::operator=` to be safe if `*this` is accessible from Python

* Add `custom_type_setup` attribute

This allows for custom modifications to the PyHeapTypeObject prior to
calling `PyType_Ready`.  This may be used, for example, to define
`tp_traverse` and `tp_clear` functions.

* Apply blacken-docs and fix language-hints

* Add blacken-docs pre-commit hook

* Add pycln pre-commit hook

* Enable a few builtin hooks

* Black no longer ignores pyi files

* py::pickle typo

* correct dots and parentheses

* fix: use OVERRIDE instead of OVERLOAD

* docs: more accurate statement

Resolve empty statement warning when using PYBIND11_OVERLOAD_PURE_NAME and PYBIND11_OVERLOAD_PURE (#2325)

* Wrap PYBIND11_OVERLOAD_NAME and PYBIND11_OVERLOAD_PURE_NAME in do { ... } while (false), and resolve trailing semicolon

* Deprecate PYBIND11_OVERLOAD_* and get_overload in favor of PYBIND11_OVERRIDE_* and get_override

* Correct erroneous usage of 'overload' instead of 'override' in the implementation and internals

* Fix tests to use non-deprecated PYBIND11_OVERRIDE_* macros

* Update docs to use override instead of overload where appropriate, and add warning about deprecated aliases

* Add semicolons to deprecated PYBIND11_OVERLOAD macros to match original behavior

* Remove deprecation of PYBIND11_OVERLOAD_* macros and get_overload

* Add note to changelog and upgrade guide

* feat: type<T>()

* refactor: using py::type as class

* refactor: py::object as base

* wip: tigher api

* refactor: fix conversion and limit API further

* docs: some added notes from @EricCousineau-TRI

* refactor: use py::type::of

Adapt code example in advanced/classes.rst to new handling of forgetting to call the superclass __init__ (#2429)

To deal with exceptions that hit destructors or other noexcept functions.

Includes fixes to support Python 2.7 and extends documentation on
error handling.

@virtuald and @YannickJadoul both contributed to this PR.

- Fixes #2103

* (docs) convert note to real note

* (docs) Add information about (deep)copy

- Not currently supported on PyPy

Commit 2b045757 ("Improve documentation related to inheritance. (#1676)") left off
a ':' from a hyperlink, which breaks the Travis CI build.

* Adds section to the reference.
* Adds section to advanced classes page describing how to use `get_overload`.

I think this particular method binding should not be done with `PYBIND11_OVERLOAD_PURE` but instead `PYBIND11_OVERLOAD`.

Fixed typos (erroneous `;`) in `classes.rst`.

* Add basic support for tag-based static polymorphism

Sometimes it is possible to look at a C++ object and know what its dynamic type is,
even if it doesn't use C++ polymorphism, because instances of the object and its
subclasses conform to some other mechanism for being self-describing; for example,
perhaps there's an enumerated "tag" or "kind" member in the base class that's always
set to an indication of the correct type. This might be done for performance reasons,
or to permit most-derived types to be trivially copyable. One of the most widely-known
examples is in LLVM: https://llvm.org/docs/HowToSetUpLLVMStyleRTTI.html

This PR permits pybind11 to be informed of such conventions via a new specializable
detail::polymorphic_type_hook<> template, which generalizes the previous logic for
determining the runtime type of an object based on C++ RTTI. Implementors provide
a way to map from a base class object to a const std::type_info* for the dynamic
type; pybind11 then uses this to ensure that casting a Base* to Python creates a
Python object that knows it's wrapping the appropriate sort of Derived.

There are a number of restrictions with this tag-based static polymorphism support
compared to pybind11's existing support for built-in C++ polymorphism:

- there is no support for this-pointer adjustment, so only single inheritance is permitted
- there is no way to make C++ code call new Python-provided subclasses
- when binding C++ classes that redefine a method in a subclass, the .def() must be
  repeated in the binding for Python to know about the update

But these are not much of an issue in practice in many cases, the impact on the
complexity of pybind11's innards is minimal and localized, and the support for
automatic downcasting improves usability a great deal.

[skip ci]

[skip ci]

This is analogous to `py::init()` vs `__init__` + placement-new.
`py::pickle()` reuses most of the implementation details of `py::init()`.

The main point of `py::module_local` is to make the C++ -> Python cast
unique so that returning/casting a C++ instance is well-defined.
Unfortunately it also makes loading unique, but this isn't particularly
desirable: when an instance contains `Type` instance there's no reason
it shouldn't be possible to pass that instance to a bound function
taking a `Type` parameter, even if that function is in another module.

This commit solves the issue by allowing foreign module (and global)
type loaders have a chance to load the value if the local module loader
fails.  The implementation here does this by storing a module-local
loading function in a capsule in the python type, which we can then call
if the local (and possibly global, if the local type is masking a global
type) version doesn't work.

This allows you to use:

    cls.def(py::init(&factory_function));

where `factory_function` returns a pointer, holder, or value of the
class type (or a derived type).  Various compile-time checks
(static_asserts) are performed to ensure the function is valid, and
various run-time type checks where necessary.

Some other details of this feature:
- The `py::init` name doesn't conflict with the templated no-argument
  `py::init<...>()`, but keeps the naming consistent: the existing
  templated, no-argument one wraps constructors, the no-template,
  function-argument one wraps factory functions.
- If returning a CppClass (whether by value or pointer) when an CppAlias
  is required (i.e. python-side inheritance and a declared alias), a
  dynamic_cast to the alias is attempted (for the pointer version); if
  it fails, or if returned by value, an Alias(Class &&) constructor
  is invoked.  If this constructor doesn't exist, a runtime error occurs.
- for holder returns when an alias is required, we try a dynamic_cast of
  the wrapped pointer to the alias to see if it is already an alias
  instance; if it isn't, we raise an error.
- `py::init(class_factory, alias_factory)` is also available that takes
  two factories: the first is called when an alias is not needed, the
  second when it is.
- Reimplement factory instance clearing.  The previous implementation
  failed under python-side multiple inheritance: *each* inherited
  type's factory init would clear the instance instead of only setting
  its own type value.  The new implementation here clears just the
  relevant value pointer.
- dealloc is updated to explicitly set the leftover value pointer to
  nullptr and the `holder_constructed` flag to false so that it can be
  used to clear preallocated value without needing to rebuild the
  instance internals data.
- Added various tests to test out new allocation/deallocation code.
- With preallocation now done lazily, init factory holders can
  completely avoid the extra overhead of needing an extra
  allocation/deallocation.
- Updated documentation to make factory constructors the default
  advanced constructor style.
- If an `__init__` is called a second time, we have two choices: we can
  throw away the first instance, replacing it with the second; or we can
  ignore the second call.  The latter is slightly easier, so do that.

* Ensure :ref: for virtual_and_inheritance is parsed.

* Add quick blurb about __init__ with inherited types.

[skip ci]

Attempting to mix py::module_local and non-module_local classes results
in some unexpected/undesirable behaviour:

- if a class is registered non-local by some other module, a later
  attempt to register it locally fails.  It doesn't need to: it is
  perfectly acceptable for the local registration to simply override
  the external global registration.
- going the other way (i.e. module `A` registers a type `T` locally,
  then `B` registers the same type `T` globally) causes a more serious
  issue: `A.T`'s constructors no longer work because the `self` argument
  gets converted to a `B.T`, which then fails to resolve.

Changing the cast precedence to prefer local over global fixes this and
makes it work more consistently, regardless of module load order.

This commit adds a `py::module_local` attribute that lets you confine a
registered type to the module (more technically, the shared object) in
which it is defined, by registering it with:

    py::class_<C>(m, "C", py::module_local())

This will allow the same C++ class `C` to be registered in different
modules with independent sets of class definitions.  On the Python side,
two such types will be completely distinct; on the C++ side, the C++
type resolves to a different Python type in each module.

This applies `py::module_local` automatically to `stl_bind.h` bindings
when the container value type looks like something global: i.e. when it
is a converting type (for example, when binding a `std::vector<int>`),
or when it is a registered type itself bound with `py::module_local`.
This should help resolve potential future conflicts (e.g. if two
completely unrelated modules both try to bind a `std::vector<int>`.
Users can override the automatic selection by adding a
`py::module_local()` or `py::module_local(false)`.

Note that this does mildly break backwards compatibility: bound stl
containers of basic types like `std::vector<int>` cannot be bound in one
module and returned in a different module.  (This can be re-enabled with
`py::module_local(false)` as described above, but with the potential for
eventual load conflicts).