Pybind11

New features:

- Added `py::raise_from` to enable chaining exceptions. [3215](https://github.com/pybind/pybind11/pull/3215)
- Allow exception translators to be optionally registered local to a module instead of applying globally across all pybind11 modules. Use `register_local_exception_translator(ExceptionTranslator&& translator)` instead of `register_exception_translator(ExceptionTranslator&& translator)` to keep your exception remapping code local to the module. [2650](https://github.com/pybind/pybind11/pull/2650)
- Add `make_simple_namespace` function for instantiating Python `SimpleNamespace` objects. [2840](https://github.com/pybind/pybind11/pull/2840)
- `pybind11::scoped_interpreter` and `initialize_interpreter` have new arguments to allow `sys.argv` initialization. [2341](https://github.com/pybind/pybind11/pull/2341)
- Allow Python builtins to be used as callbacks in CPython. [1413](https://github.com/pybind/pybind11/pull/1413)
- Added `view` to view arrays with a different datatype. [987](https://github.com/pybind/pybind11/pull/987)
- Implemented `reshape` on arrays. [984](https://github.com/pybind/pybind11/pull/984)
- Enable defining custom `__new__` methods on classes by fixing bug preventing overriding methods if they have non-pybind11 siblings. [3265](https://github.com/pybind/pybind11/pull/3265)
- Add `make_value_iterator()`, and fix `make_key_iterator()` to return references instead of copies. [3293](https://github.com/pybind/pybind11/pull/3293)
- Improve the classes generated by `bind_map`: [3310](https://github.com/pybind/pybind11/pull/3310)
- Change `.items` from an iterator to a dictionary view.
- Add `.keys` and `.values` (both dictionary views).
- Allow `__contains__` to take any object.
- `pybind11::custom_type_setup` was added, for customizing the `PyHeapTypeObject` corresponding to a class, which may be useful for enabling garbage collection support, among other things. [3287](https://github.com/pybind/pybind11/pull/3287)

Changes:

- Set `__file__` constant when running `eval_file` in an embedded interpreter. [3233](https://github.com/pybind/pybind11/pull/3233)
- Python objects and (C++17) `std::optional` now accepted in `py::slice` constructor. [1101](https://github.com/pybind/pybind11/pull/1101)
- The pybind11 proxy types `str`, `bytes`, `bytearray`, `tuple`, `list` now consistently support passing `ssize_t` values for sizes and indexes. Previously, only `size_t` was accepted in several interfaces. [3219](https://github.com/pybind/pybind11/pull/3219)
- Avoid evaluating `PYBIND11_TLS_REPLACE_VALUE` arguments more than once. [3290](https://github.com/pybind/pybind11/pull/3290)

Fixes:

- Bug fix: enum value's `__int__` returning non-int when underlying type is bool or of char type [1334](https://github.com/pybind/pybind11/pull/1334)
- Fixes bug in setting error state in Capsule's pointer methods. [3261](https://github.com/pybind/pybind11/pull/3261)
- A long-standing memory leak in `py::cpp_function::initialize` was fixed. [3229](https://github.com/pybind/pybind11/pull/3229)
- Fixes thread safety for some `pybind11::type_caster` which require lifetime extension, such as for `std::string_view`. [3237](https://github.com/pybind/pybind11/pull/3237)
- Restore compatibility with gcc 4.8.4 as distributed by ubuntu-trusty, linuxmint-17. [3270](https://github.com/pybind/pybind11/pull/3270)

Build system improvements:

- Fix regression in CMake Python package config: improper use of absolute path. [3144](https://github.com/pybind/pybind11/pull/3144)
- Cached Python version information could become stale when CMake was re-run with a different Python version. The build system now detects this and updates this information. [3299](https://github.com/pybind/pybind11/pull/3299)
- Specified UTF8-encoding in setup.py calls of open(). [3137](https://github.com/pybind/pybind11/pull/3137)
- Fix a harmless warning from CMake 3.21 with the classic Python discovery. [3220](https://github.com/pybind/pybind11/pull/3220)
- Eigen repo and version can now be specified as cmake options. [3324](https://github.com/pybind/pybind11/pull/3324)

Backend and tidying up:

- Reduced thread-local storage required for keeping alive temporary data for type conversion to one key per ABI version, rather than one key per extension module. This makes the total thread-local storage required by pybind11 2 keys per ABI version. [3275](https://github.com/pybind/pybind11/pull/3275)
- Optimize NumPy array construction with additional moves. [3183](https://github.com/pybind/pybind11/pull/3183)
- Conversion to `std::string` and `std::string_view` now avoids making an extra copy of the data on Python >= 3.3. [3257](https://github.com/pybind/pybind11/pull/3257)
- Remove const modifier from certain C++ methods on Python collections (`list`, `set`, `dict`) such as (`clear()`, `append()`, `insert()`, etc...) and annotated them with `py-non-const`.
- Enable readability `clang-tidy-const-return` and remove useless consts. [3254](https://github.com/pybind/pybind11/pull/3254) [#3194](https://github.com/pybind/pybind11/pull/3194)
- The clang-tidy `google-explicit-constructor` option was enabled. [3250](https://github.com/pybind/pybind11/pull/3250)
- Mark a pytype move constructor as noexcept (perf). [3236](https://github.com/pybind/pybind11/pull/3236)
- Enable clang-tidy check to guard against inheritance slicing. [3210](https://github.com/pybind/pybind11/pull/3210)
- Legacy warning suppression pragma were removed from eigen.h. On Unix platforms, please use -isystem for Eigen include directories, to suppress compiler warnings originating from Eigen headers. Note that CMake does this by default. No adjustments are needed for Windows. [3198](https://github.com/pybind/pybind11/pull/3198)
- Format pybind11 with isort consistent ordering of imports [3195](https://github.com/pybind/pybind11/pull/3195)
- The warnings-suppression "pragma clamp" at the top/bottom of pybind11 was removed, clearing the path to refactoring and IWYU cleanup. [3186](https://github.com/pybind/pybind11/pull/3186)
- Enable most bugprone checks in clang-tidy and fix the found potential bugs and poor coding styles. [3166](https://github.com/pybind/pybind11/pull/3166)
- Add `clang-tidy-readability` rules to make boolean casts explicit improving code readability. Also enabled other misc and readability clang-tidy checks. [3148](https://github.com/pybind/pybind11/pull/3148)
- Move object in `.pop()` for list. [3116](https://github.com/pybind/pybind11/pull/3116)

Minor missing functionality added:

- Allow Python builtins to be used as callbacks in CPython. [1413](https://github.com/pybind/pybind11/pull/1413)

Bug fixes:

- Fix regression in CMake Python package config: improper use of absolute path. [3144](https://github.com/pybind/pybind11/pull/3144)
- Fix Mingw64 and add to the CI testing matrix. [3132](https://github.com/pybind/pybind11/pull/3132)
- Specified UTF8-encoding in setup.py calls of open(). [3137](https://github.com/pybind/pybind11/pull/3137)
- Add clang-tidy-readability rules to make boolean casts explicit improving code readability. Also enabled other misc and readability clang-tidy checks. [3148](https://github.com/pybind/pybind11/pull/3148)
- Move object in `.pop()` for list. [3116](https://github.com/pybind/pybind11/pull/3116)

Backend and tidying up:

- Removed and fixed warning suppressions. [3127](https://github.com/pybind/pybind11/pull/3127) [#3129](https://github.com/pybind/pybind11/pull/3129) [#3135](https://github.com/pybind/pybind11/pull/3135) [#3141](https://github.com/pybind/pybind11/pull/3141) [#3142](https://github.com/pybind/pybind11/pull/3142) [#3150](https://github.com/pybind/pybind11/pull/3150) [#3152](https://github.com/pybind/pybind11/pull/3152) [#3160](https://github.com/pybind/pybind11/pull/3160) [#3161](https://github.com/pybind/pybind11/pull/3161)

New features:

- Enable `py::implicitly_convertible<py::none, ...>` for `py::class_`-wrapped types. [3059](https://github.com/pybind/pybind11/pull/3059)
- Allow function pointer extraction from overloaded functions. [2944](https://github.com/pybind/pybind11/pull/2944)
- NumPy: added `.char_()` to type which gives the NumPy public `char` result, which also distinguishes types by bit length (unlike `.kind()`). [2864](https://github.com/pybind/pybind11/pull/2864)
- Add `pybind11::bytearray` to manipulate `bytearray` similar to `bytes`. [2799](https://github.com/pybind/pybind11/pull/2799)
- `pybind11/stl/filesystem.h` registers a type caster that, on C++17/Python 3.6+, converts `std::filesystem::path` to `pathlib.Path` and any `os.PathLike` to `std::filesystem::path`. [2730](https://github.com/pybind/pybind11/pull/2730)
- A `PYBIND11_VERSION_HEX` define was added, similar to `PY_VERSION_HEX`. [3120](https://github.com/pybind/pybind11/pull/3120)

Changes:

- `py::str` changed to exclusively hold `PyUnicodeObject`. Previously `py::str` could
also hold `bytes`, which is probably surprising, was never documented, and can mask bugs (e.g. accidental use of `py::str` instead of `py::bytes`). [2409](https://github.com/pybind/pybind11/pull/2409)
- Add a safety guard to ensure that the Python GIL is held when C++ calls back into Python via `object_api<>::operator()` (e.g. `py::function` `__call__`). (This feature is available for Python 3.6+ only.) [2919](https://github.com/pybind/pybind11/pull/2919)
- Catch a missing `self` argument in calls to `__init__()`. [2914](https://github.com/pybind/pybind11/pull/2914)
- Use `std::string_view` if available to avoid a copy when passing an object to a `std::ostream`. [3042](https://github.com/pybind/pybind11/pull/3042)
- An important warning about thread safety was added to the `iostream.h` documentation; attempts to make `py::scoped_ostream_redirect` thread safe have been removed, as it was only partially effective. [2995](https://github.com/pybind/pybind11/pull/2995)

Fixes:

- Performance: avoid unnecessary strlen calls. [3058](https://github.com/pybind/pybind11/pull/3058)
- Fix auto-generated documentation string when using `const T` in `pyarray_t`. [3020](https://github.com/pybind/pybind11/pull/3020)
- Unify error messages thrown by `simple_collector`/`unpacking_collector`. [3013](https://github.com/pybind/pybind11/pull/3013)
- `pybind11::builtin_exception` is now explicitly exported, which means the types included/defined in different modules are identical, and exceptions raised in different modules can be caught correctly. The documentation was updated to explain that custom exceptions that are used across module boundaries need to be explicitly exported as well. [2999](https://github.com/pybind/pybind11/pull/2999)
- Fixed exception when printing UTF-8 to a `scoped_ostream_redirect`. [2982](https://github.com/pybind/pybind11/pull/2982)
- Pickle support enhancement: `setstate` implementation will attempt to `setattr` `__dict__` only if the unpickled `dict` object is not empty, to not force use of `py::dynamic_attr()` unnecessarily. [2972](https://github.com/pybind/pybind11/pull/2972)
- Allow negative timedelta values to roundtrip. [2870](https://github.com/pybind/pybind11/pull/2870)
- Fix unchecked errors could potentially swallow signals/other exceptions. [2863](https://github.com/pybind/pybind11/pull/2863)
- Add null pointer check with `std::localtime`. [2846](https://github.com/pybind/pybind11/pull/2846)
- Fix the `weakref` constructor from `py::object` to create a new `weakref` on conversion. [2832](https://github.com/pybind/pybind11/pull/2832)
- Avoid relying on exceptions in C++17 when getting a `shared_ptr` holder from a `shared_from_this` class. [2819](https://github.com/pybind/pybind11/pull/2819)
- Allow the codec's exception to be raised instead of `RuntimeError` when casting from `py::str` to `std::string`. [2903](https://github.com/pybind/pybind11/pull/2903)

Build system improvements:

- In `setup_helpers.py`, test for platforms that have some multiprocessing features but lack semaphores, which `ParallelCompile` requires. [3043](https://github.com/pybind/pybind11/pull/3043)
- Fix `pybind11_INCLUDE_DIR` in case `CMAKE_INSTALL_INCLUDEDIR` is absolute. [3005](https://github.com/pybind/pybind11/pull/3005)
- Fix bug not respecting `WITH_SOABI` or `WITHOUT_SOABI` to CMake. [2938](https://github.com/pybind/pybind11/pull/2938)
- Fix the default `Pybind11Extension` compilation flags with a Mingw64 python. [2921](https://github.com/pybind/pybind11/pull/2921)
- Clang on Windows: do not pass `/MP` (ignored flag). [2824](https://github.com/pybind/pybind11/pull/2824)
- `pybind11.setup_helpers.intree_extensions` can be used to generate `Pybind11Extension` instances from cpp files placed in the Python
package source tree. [2831](https://github.com/pybind/pybind11/pull/2831)

Backend and tidying up:

- Enable clang-tidy performance, readability, and modernization checks throughout the codebase to enforce best coding practices. [3046](https://github.com/pybind/pybind11/pull/3046), [#3049](https://github.com/pybind/pybind11/pull/3049), [#3051](https://github.com/pybind/pybind11/pull/3051), [#3052](https://github.com/pybind/pybind11/pull/3052), [#3080](https://github.com/pybind/pybind11/pull/3080), and [#3094](https://github.com/pybind/pybind11/pull/3094)
- Checks for common misspellings were added to the pre-commit hooks. [3076](https://github.com/pybind/pybind11/pull/3076)
- Changed `Werror` to stricter `Werror-all` for Intel compiler and fixed minor issues. [2948](https://github.com/pybind/pybind11/pull/2948)
- Fixed compilation with GCC \< 5 when the user defines `_GLIBCXX_USE_CXX11_ABI`. [2956](https://github.com/pybind/pybind11/pull/2956)
- Added nox support for easier local testing and linting of contributions. [3101](https://github.com/pybind/pybind11/pull/3101) and [#3121](https://github.com/pybind/pybind11/pull/3121)
- Avoid RTD style issue with docutils 0.17+. [3119](https://github.com/pybind/pybind11/pull/3119)
- Support pipx run, such as `pipx run pybind11 --include` for a quick compile. [3117](https://github.com/pybind/pybind11/pull/3117)

Minor missing functionality added:

* enum: add missing Enum.value property. [2739](https://github.com/pybind/pybind11/pull/2739)
* Allow thread termination to be avoided during shutdown for CPython 3.7+ via `.disarm` for `gil_scoped_acquire`/`gil_scoped_release`. [2657](https://github.com/pybind/pybind11/pull/2657)

Fixed or improved behavior in a few special cases:

* Fix bug where the constructor of `object` subclasses would not throw on being passed a Python object of the wrong type. [2701](https://github.com/pybind/pybind11/pull/2701)
* The `type_caster` for integers does not convert Python objects with `__int__` anymore with `noconvert` or during the first round of trying overloads. [2698](https://github.com/pybind/pybind11/pull/2698)
* When casting to a C++ integer, `__index__` is always called and not considered as conversion, consistent with Python 3.8+. [2801](https://github.com/pybind/pybind11/pull/2801)

Build improvements:

* Setup helpers: `extra_compile_args` and `extra_link_args` automatically set by Pybind11Extension are now prepended, which allows them to be overridden by user-set `extra_compile_args` and `extra_link_args`. [2808](https://github.com/pybind/pybind11/pull/2808)
* Setup helpers: Don\'t trigger unused parameter warning. [2735](https://github.com/pybind/pybind11/pull/2735)
* CMake: Support running with `--warn-uninitialized` active. [2806](https://github.com/pybind/pybind11/pull/2806)
* CMake: Avoid error if included from two submodule directories. [2804](https://github.com/pybind/pybind11/pull/2804)
* CMake: Fix `STATIC` / `SHARED` being ignored in FindPython mode. [2796](https://github.com/pybind/pybind11/pull/2796)
* CMake: Respect the setting for `CMAKE_CXX_VISIBILITY_PRESET` if defined. [2793](https://github.com/pybind/pybind11/pull/2793)
* CMake: Fix issue with FindPython2/FindPython3 not working with `pybind11::embed`. [2662](https://github.com/pybind/pybind11/pull/2662)
* CMake: mixing local and installed pybind11\'s would prioritize the installed one over the local one (regression in 2.6.0). [2716](https://github.com/pybind/pybind11/pull/2716)

Bug fixes:

* Fixed segfault in multithreaded environments when using `scoped_ostream_redirect`. [2675](https://github.com/pybind/pybind11/pull/2675)
* Leave docstring unset when all docstring-related options are disabled, rather than set an empty string. [2745](https://github.com/pybind/pybind11/pull/2745)
* The module key in builtins that pybind11 uses to store its internals changed from std::string to a python str type (more natural on Python 2, no change on Python 3). [2814](https://github.com/pybind/pybind11/pull/2814)
* Fixed assertion error related to unhandled (later overwritten) exception in CPython 3.8 and 3.9 debug builds. [2685](https://github.com/pybind/pybind11/pull/2685)
* Fix `py::gil_scoped_acquire` assert with CPython 3.9 debug build. [2683](https://github.com/pybind/pybind11/pull/2683)
* Fix issue with a test failing on PyTest 6.2. [2741](https://github.com/pybind/pybind11/pull/2741)

Warning fixes:

* Fix warning modifying constructor parameter \'flag\' that shadows a field of \'set_flag\' `[-Wshadow-field-in-constructor-modified]`. [2780](https://github.com/pybind/pybind11/pull/2780)
* Suppressed some deprecation warnings about old-style `__init__`/`__setstate__` in the tests. [2759](https://github.com/pybind/pybind11/pull/2759)

Valgrind work:

* Fix invalid access when calling a pybind11 `__init__` on a non-pybind11 class instance. [2755](https://github.com/pybind/pybind11/pull/2755)
* Fixed various minor memory leaks in pybind11\'s test suite. [2758](https://github.com/pybind/pybind11/pull/2758)
* Resolved memory leak in cpp_function initialization when exceptions occurred. [2756](https://github.com/pybind/pybind11/pull/2756)
* Added a Valgrind build, checking for leaks and memory-related UB, to CI. [2746](https://github.com/pybind/pybind11/pull/2746)

Compiler support:

* Intel compiler was not activating C++14 support due to a broken define. [2679](https://github.com/pybind/pybind11/pull/2679)
* Support ICC and NVIDIA HPC SDK in C++17 mode. [2729](https://github.com/pybind/pybind11/pull/2729)
* Support Intel OneAPI compiler (ICC 20.2) and add to CI. [2573](https://github.com/pybind/pybind11/pull/2573)

- `py::exec`, `py::eval`, and `py::eval_file` now add the builtins module as `"__builtins__"` to their `globals` argument, better matching `exec` and `eval` in pure Python. [2616](https://github.com/pybind/pybind11/pull/2616)
- `setup_helpers` will no longer set a minimum macOS version higher than the current version. [2622](https://github.com/pybind/pybind11/pull/2622)
- Allow deleting static properties. [2629](https://github.com/pybind/pybind11/pull/2629)
- Seal a leak in `def_buffer`, cleaning up the `capture` object after the `class_` object goes out of scope.
[2634](https://github.com/pybind/pybind11/pull/2634)
- `pybind11_INCLUDE_DIRS` was incorrect, potentially causing a regression if it was expected to include `PYTHON_INCLUDE_DIRS` (please use targets instead). [2636](https://github.com/pybind/pybind11/pull/2636)
- Added parameter names to the `py::enum_` constructor and methods, avoiding `arg0` in the generated docstrings. [2637](https://github.com/pybind/pybind11/pull/2637)
- Added `needs_recompile` optional function to the `ParallelCompiler` helper, to allow a recompile to be skipped based on a user-defined function. [2643](https://github.com/pybind/pybind11/pull/2643)

New features:

- Keyword-only arguments supported in Python 2 or 3 with `py::kw_only()`. [2100](https://github.com/pybind/pybind11/pull/2100)
- Positional-only arguments supported in Python 2 or 3 with `py::pos_only()`. [2459](https://github.com/pybind/pybind11/pull/2459)
- `py::is_final()` class modifier to block subclassing (CPython only). [2151](https://github.com/pybind/pybind11/pull/2151)
- Added `py::prepend()`, allowing a function to be placed at the beginning of the overload chain. [1131](https://github.com/pybind/pybind11/pull/1131)
- Access to the type object now provided with `py::type::of<T>()` and `py::type::of(h)`. [2364](https://github.com/pybind/pybind11/pull/2364)
- Perfect forwarding support for methods. [2048](https://github.com/pybind/pybind11/pull/2048)
- Added `py::error_already_set::discard_as_unraisable()`. [2372](https://github.com/pybind/pybind11/pull/2372)
- `py::hash` is now public. [2217](https://github.com/pybind/pybind11/pull/2217)
- `py::class_<union_type>` is now supported. Note that writing to one data member of the union and reading another (type punning) is UB in C++. Thus pybind11-bound enums should never be used for such conversions. [2320](https://github.com/pybind/pybind11/pull/2320).
- Classes now check local scope when registering members, allowing a subclass to have a member with the same name as a parent (such as an enum). [2335](https://github.com/pybind/pybind11/pull/2335)

Code correctness features:

- Error now thrown when `__init__` is forgotten on subclasses. [2152](https://github.com/pybind/pybind11/pull/2152)
- Throw error if conversion to a pybind11 type if the Python object isn't a valid instance of that type, such as `py::bytes(o)` when `py::object o` isn't a bytes instance. [2349](https://github.com/pybind/pybind11/pull/2349)
- Throw if conversion to `str` fails. [2477](https://github.com/pybind/pybind11/pull/2477)

API changes:

- `py::module` was renamed `py::module_` to avoid issues with C++20 when used unqualified, but an alias `py::module` is provided for backward compatibility. [2489](https://github.com/pybind/pybind11/pull/2489)
- Public constructors for `py::module_` have been deprecated; please use `pybind11::module_::create_extension_module` if you were using the public constructor (fairly rare after `PYBIND11_MODULE` was introduced). [2552](https://github.com/pybind/pybind11/pull/2552)
- `PYBIND11_OVERLOAD*` macros and `get_overload` function replaced by correctly-named `PYBIND11_OVERRIDE*` and `get_override`, fixing inconsistencies in the presence of a closing `;` in these macros. `get_type_overload` is deprecated. [2325](https://github.com/pybind/pybind11/pull/2325)

Packaging / building improvements:

- The Python package was reworked to be more powerful and useful. [2433](https://github.com/pybind/pybind11/pull/2433)
- `build-setuptools` is easier thanks to a new `pybind11.setup_helpers` module, which provides utilities to use setuptools with pybind11. It can be used via PEP 518, `setup_requires`, or by directly importing or copying `setup_helpers.py` into your project.
- CMake configuration files are now included in the Python package. Use `pybind11.get_cmake_dir()` or `python -m pybind11 --cmakedir` to get the directory with the CMake configuration files, or include the site-packages location in your `CMAKE_MODULE_PATH`. Or you can use the new `pybind11[global]` extra when you install `pybind11`, which installs the CMake files and headers into your base environment in the standard location.
- `pybind11-config` is another way to write `python -m pybind11` if you have your PATH set up.
- Added external typing support to the helper module, code from `import pybind11` can now be type checked. [2588](https://github.com/pybind/pybind11/pull/2588)
- Minimum CMake required increased to 3.4. [2338](https://github.com/pybind/pybind11/pull/2338) and [#2370](https://github.com/pybind/pybind11/pull/2370)
- Full integration with CMake's C++ standard system and compile features replaces `PYBIND11_CPP_STANDARD`.
- Generated config file is now portable to different Python/compiler/CMake versions.
- Virtual environments prioritized if `PYTHON_EXECUTABLE` is not set (`venv`, `virtualenv`, and `conda`) (similar to the new FindPython mode).
- Other CMake features now natively supported, like `CMAKE_INTERPROCEDURAL_OPTIMIZATION`, `set(CMAKE_CXX_VISIBILITY_PRESET hidden)`.
- `CUDA` as a language is now supported.
- Helper functions `pybind11_strip`, `pybind11_extension`, `pybind11_find_import` added, see `cmake/index`.
- Optional `find-python-mode` and `nopython-mode` with CMake. [2370](https://github.com/pybind/pybind11/pull/2370)
- Uninstall target added. [2265](https://github.com/pybind/pybind11/pull/2265) and [#2346](https://github.com/pybind/pybind11/pull/2346)
- `pybind11_add_module()` now accepts an optional `OPT_SIZE` flag that switches the binding target to size-based optimization if the global build type can not always be fixed to `MinSizeRel` (except in debug mode, where optimizations remain disabled). `MinSizeRel` or this flag reduces binary size quite substantially (~25% on some platforms). [2463](https://github.com/pybind/pybind11/pull/2463)

Smaller or developer focused features and fixes:

- Moved `mkdoc.py` to a new repo, [pybind11-mkdoc](https://github.com/pybind/pybind11-mkdoc). There are no longer submodules in the main repo.
- `py::memoryview` segfault fix and update, with new `py::memoryview::from_memory` in Python 3, and documentation. [2223](https://github.com/pybind/pybind11/pull/2223)
- Fix for `buffer_info` on Python 2. [2503](https://github.com/pybind/pybind11/pull/2503)
- If `__eq__` defined but not `__hash__`, `__hash__` is now set to `None`. [2291](https://github.com/pybind/pybind11/pull/2291)
- `py::ellipsis` now also works on Python 2. [2360](https://github.com/pybind/pybind11/pull/2360)
- Pointer to `std::tuple` & `std::pair` supported in cast. [2334](https://github.com/pybind/pybind11/pull/2334)
- Small fixes in NumPy support. `py::array` now uses `py::ssize_t` as first argument type. [2293](https://github.com/pybind/pybind11/pull/2293)
- Added missing signature for `py::array`. [2363](https://github.com/pybind/pybind11/pull/2363)
- `unchecked_mutable_reference` has access to operator `()` and `[]` when const. [2514](https://github.com/pybind/pybind11/pull/2514)
- `py::vectorize` is now supported on functions that return void. [1969](https://github.com/pybind/pybind11/pull/1969)
- `py::capsule` supports `get_pointer` and `set_pointer`. [1131](https://github.com/pybind/pybind11/pull/1131)
- Fix crash when different instances share the same pointer of the same type. [2252](https://github.com/pybind/pybind11/pull/2252)
- Fix for `py::len` not clearing Python's error state when it fails and throws. [2575](https://github.com/pybind/pybind11/pull/2575)
- Bugfixes related to more extensive testing, new GitHub Actions CI. [2321](https://github.com/pybind/pybind11/pull/2321)
- Bug in timezone issue in Eastern hemisphere midnight fixed. [2438](https://github.com/pybind/pybind11/pull/2438)
- `std::chrono::time_point` now works when the resolution is not the same as the system. [2481](https://github.com/pybind/pybind11/pull/2481)
- Bug fixed where `py::array_t` could accept arrays that did not match the requested ordering. [2484](https://github.com/pybind/pybind11/pull/2484)
- Avoid a segfault on some compilers when types are removed in Python. [2564](https://github.com/pybind/pybind11/pull/2564)
- `py::arg::none()` is now also respected when passing keyword arguments. [2611](https://github.com/pybind/pybind11/pull/2611)
- PyPy fixes, PyPy 7.3.x now supported, including PyPy3. (Known issue with PyPy2 and Windows [2596](https://github.com/pybind/pybind11/issues/2596)). [#2146](https://github.com/pybind/pybind11/pull/2146)
- CPython 3.9.0 workaround for undefined behavior (macOS segfault). [2576](https://github.com/pybind/pybind11/pull/2576)
- CPython 3.9 warning fixes. [2253](https://github.com/pybind/pybind11/pull/2253)
- Improved C++20 support, now tested in CI. [2489](https://github.com/pybind/pybind11/pull/2489) [#2599](https://github.com/pybind/pybind11/pull/2599)
- Improved but still incomplete debug Python interpreter support. [2025](https://github.com/pybind/pybind11/pull/2025)
- NVCC (CUDA 11) now supported and tested in CI. [2461](https://github.com/pybind/pybind11/pull/2461)
- NVIDIA PGI compilers now supported and tested in CI. [2475](https://github.com/pybind/pybind11/pull/2475)
- At least Intel 18 now explicitly required when compiling with Intel. [2577](https://github.com/pybind/pybind11/pull/2577)
- Extensive style checking in CI, with [pre-commit](https://pre-commit.com) support. Code modernization, checked by clang-tidy.
- Expanded docs, including new main page, new installing section, and CMake helpers page, along with over a dozen new sections on existing pages.
- In GitHub, new docs for contributing and new issue templates.