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Conversions between Python's native (stdlib) enum types and C++ enums. #30005

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merged 25 commits into from
Mar 16, 2023

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@rwgk rwgk commented Mar 9, 2023

Description

Alternative to py::enum_, binding C++ enum types to Python's native (stdlib) enum types. For example:

enum class altitude : char {
    high = 'h',
    low = 'l'
};

PYBIND11_MODULE(native_enum_demo, m) {
    m += py::native_enum<altitude>("altitude")
             .value("high", altitude::high)
             .value("low", altitude::low);
}

Note that the py::native_enum design is different from the established py::enum_ design:

    py::enum_<altitude>(m, "altitude")
             .value("high", altitude::high)
             .value("low", altitude::low);

The py::native_enum design is a mostly a trick:

  • Python's native enums cannot be built incrementally, but all name/values need to be passed at once.
  • To achieve this, py::native_enum is merely a buffer for collecting the name/value pairs. The m += operation triggers harvesting those from the buffer to build the arguments for constructing a native Python enum type.

Sidesteps issues discovered under pybind/pybind11#2744.

This PR fixes pybind/pybind11#3643 (comment), more-or-less as a side-effect.

This PR was transferred from pybind/pybind11#4349, which in turn was transferred from pybind/pybind11#4329.

Suggested changelog entry:

Ralf W. Grosse-Kunstleve added 11 commits November 19, 2022 13:39
The patch .rej below are resolved, but THIS STATE DOES NOT BUILD:

```
clang++ -o pybind11/tests/test_constants_and_functions.os -c -std=c++17 -fPIC -fvisibility=hidden -O0 -g -Wall -Wextra -Wconversion -Wcast-qual -Wdeprecated -Wundef -Wnon-virtual-dtor -Wunused-result -Werror -isystem /usr/include/python3.10 -isystem /usr/include/eigen3 -DPYBIND11_STRICT_ASSERTS_CLASS_HOLDER_VS_TYPE_CASTER_MIX -DPYBIND11_ENABLE_TYPE_CASTER_ODR_GUARD_IF_AVAILABLE -DPYBIND11_TEST_BOOST -Ipybind11/include -I/usr/local/google/home/rwgk/forked/pybind11/include -I/usr/local/google/home/rwgk/clone/pybind11/include /usr/local/google/home/rwgk/forked/pybind11/tests/test_constants_and_functions.cpp
In file included from /usr/local/google/home/rwgk/forked/pybind11/tests/test_constants_and_functions.cpp:11:
In file included from /usr/local/google/home/rwgk/forked/pybind11/tests/pybind11_tests.h:3:
In file included from /usr/local/google/home/rwgk/forked/pybind11/include/pybind11/eval.h:14:
/usr/local/google/home/rwgk/forked/pybind11/include/pybind11/pybind11.h:1781:9: error: static_assert failed due to requirement '!holder_is_smart_holder == type_caster_type_is_type_caster_base_subtype' "py::class_ holder vs type_caster mismatch: missing PYBIND11_TYPE_CASTER_BASE_HOLDER(T, ...) or collision with custom py::detail::type_caster<T>?"
        static_assert(!holder_is_smart_holder == type_caster_type_is_type_caster_base_subtype,
        ^             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/google/home/rwgk/forked/pybind11/include/pybind11/pybind11.h:2460:11: note: in instantiation of function template specialization 'pybind11::class_<MyEnum>::class_<>' requested here
        : class_<Type>(scope, name, extra...), m_base(*this, scope) {
          ^
/usr/local/google/home/rwgk/forked/pybind11/tests/test_constants_and_functions.cpp:106:5: note: in instantiation of function template specialization 'pybind11::enum_<MyEnum>::enum_<>' requested here
    py::enum_<MyEnum>(m, "MyEnum")
    ^
1 error generated.
```

________

```
rwgk.c.googlers.com:~/forked/pybind11 $ patch -p 1 < ~/native_enum_git_diff_master_2022-11-19+131942.patch
patching file .github/workflows/python312.yml
patching file CMakeLists.txt
Hunk google#1 FAILED at 111.
Hunk google#2 succeeded at 138 (offset 5 lines).
1 out of 2 hunks FAILED -- saving rejects to file CMakeLists.txt.rej
patching file include/pybind11/cast.h
Hunk google#1 FAILED at 12.
Hunk google#2 succeeded at 78 (offset 30 lines).
Hunk google#3 succeeded at 1173 (offset 41 lines).
1 out of 3 hunks FAILED -- saving rejects to file include/pybind11/cast.h.rej
patching file include/pybind11/detail/abi_platform_id.h
patching file include/pybind11/detail/cross_extension_shared_state.h
patching file include/pybind11/detail/internals.h
Hunk google#1 FAILED at 16.
Hunk google#2 succeeded at 109 (offset 1 line).
Hunk google#3 FAILED at 203.
Hunk google#4 succeeded at 398 (offset 11 lines).
Hunk google#5 succeeded at 457 (offset 11 lines).
2 out of 5 hunks FAILED -- saving rejects to file include/pybind11/detail/internals.h.rej
patching file include/pybind11/detail/native_enum_data.h
patching file include/pybind11/detail/type_map.h
patching file include/pybind11/embed.h
patching file include/pybind11/native_enum.h
patching file include/pybind11/pybind11.h
Hunk google#1 FAILED at 12.
Hunk google#2 succeeded at 1269 (offset 1 line).
Hunk google#3 succeeded at 2457 (offset 255 lines).
1 out of 3 hunks FAILED -- saving rejects to file include/pybind11/pybind11.h.rej
patching file include/pybind11/pytypes.h
patching file tests/CMakeLists.txt
Hunk google#1 succeeded at 160 (offset 18 lines).
patching file tests/conftest.py
patching file tests/extra_python_package/test_files.py
Hunk google#2 FAILED at 47.
1 out of 2 hunks FAILED -- saving rejects to file tests/extra_python_package/test_files.py.rej
patching file tests/test_embed/test_interpreter.cpp
patching file tests/test_enum.cpp
patching file tests/test_enum.py
patching file tests/test_native_enum.cpp
patching file tests/test_native_enum.py
```
@rwgk rwgk added the gha_wf_python312dev_enable Enable python312dev.yml label Mar 9, 2023
@rwgk rwgk requested a review from wangxf123456 March 16, 2023 00:16
@rwgk rwgk marked this pull request as ready for review March 16, 2023 00:16
@rwgk
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rwgk commented Mar 16, 2023

For completeness: We reviewed this PR together in a meeting, spending a good 40 minutes looking at it.

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