pydantic-bind 1.0.4

C++/pybind generation from Pydantic classes

pydantic-bind

Table of Contents

1. Overview
2. Why Not Protobufs ?
3. No Copy
4. Supported Types
5. Inheritance
6. Msgpack
7. Generated Code

Overview

This project has helpers for automatically generating C++ structs and corresponding pybind marshalling code for dataclasses and pydantic-based classes.

This is achieved via a cmake rule: pydantic_bind_add_module(<path to module>)

Add a module this way and it will be scanned for:

• dataclasses
• classes derived from pydantic's BaseModel
• enums

For any of these which are encountered, a definition will be added to a .h file, with re˚lative path matching the module and pybind11 code for binding objects added to a corresponding .cpp file.

The intended use of this package is for defining behaviour-less data classes, to be shared between python and C++. E.g., a common object model for financial modelling. Furthr, we want idiomatic classes for each language, not mutants like Protobuf-generated python classes.

Note that the typcal python developer experience is now somewhat changed, in that it's necessary to build/install the project. I personally use JetBrains CLion, in place of PyCharm for such projects.

You can create an instance of the pybind class from your original using get_pybind_instance(), e.g.,

my_class.py:

from dataclasses import dataclass

@dataclass
clas MyClass:
    my_int: int
    my_string: str | None

CMakeLists.txt:

cmake_minimum_required(VERSION 3.9)
project(my_project)

set(CMAKE_CXX_STANDARD 20)

find_package(python3 REQUIRED COMPONENTS Interpreter Development)
find_package(pydantic_bind REQUIRED COMPONENTS HINTS "${python3_SITELIB}")

pydantic_bind_add_module(my_class.py)

my_util.py

from pydantic_bind import get_pybind_value
from my_class imnport MyClass

orig = MyClass(my_int=123, my_string="hello")
generated = get_pybind_value(orig)

Why Not Protobufs?

A very good question. Protobufs are frankly a PITA to use: they have poor to no variant support, the generated code is ugly and idiosyncratic, they're large and painful to copy around etc.

AVRO is more friendly but generates python classes dynamically, which confuses IDEs like Pycharm. I do think a good solution is something like pydantic_avro where one can define the classes using pydantic, generate the AVRO schema and then the generateed C++ etc. I might well try and converge this project with that approach.

I was inspired to some degree by this blog.

No Copy

One annoyance of multi-language representations of data objects is that you often end up copying data around where you'd prefer to share a single copy. This is the raison d'etre for Protobufs and its ilk. In this project I've created implementations of BaseModel and dataclass which allow python to use the underlying C++ data representation, rather than holding its own copy.

Deriving from this BaseModel will give you equivalent functionality of as pydantic's BaseModel. The annotations are re-written using computed_field, with property getters and setters operating on the generated pybind class, which is instantiated behind the scenes in init. Note that this will make some operations (especially those that access dict) less efficient. I've also plumbed the computed fields into the JSON schema, so these objects can be used with FastAPI.

dataclass works similarly, adding properties to the dataclass, so that the exisitng get and set functionality works seamless in accessing the generated pybind class (also set via a shimmed init).

Using regular dataclass or BaseModel as members of classes defined with the pydantic_bind versions is very inefficient and not recommended.

Supported Types

The following python -> C++ mappings are supported (there are likely others I should consider):

• bool --> bool
• float --> double
• int --> int
• str --> std::string
• datetime.date --> std::chrono::system_clock::time_point
• datetime.datetime --> std::chrono::system_clock::time_point
• datetime.time --> std::chrono::system_clock::time_point
• datetime.timedelta --> std::chrono::duration
• pydantic.BaseModel --> struct
• pydantic_bind.BaseModel --> struct
• dataclass --> struct
• pydantic_bind.dataclass --> struct
• Enum --> enum

Inheritance

I have tested single inheritance (see Generated Code). Multiple inheritance may work ... or it may not. I'd generally advise against using it for data classes.

Msgpack

A rather rudimentary msgpack implementation is added to the generated C++ structs, using a slightly modified version of cpppack. It wasn't clear to me whether this package is maintained or accepting submissions, so I copied and slightly modified msgpack.h (also, I couldn't work out how to add to my project with my rather rudimentary cmake skillz!) Changes include:

• Fixing includes
• Support for std::optional
• Support for std::variant
• Support for enums

A likely future enhancement will be to use cereal and add a mgspack adaptor. However, I haven't quite worked out how to do that yet.

Generated Code

Code is generated into a directory structure underneath <top level>/generated.

Headers are installed to <top level>/include.

Compiled pybind modules are installed into <original module path>/__pybind__.

For C++ usage, you need only the headers, the compiled code is for pybind/python usage only.

For the example below, common_object_model/common_object_model/v1/common/__pybind__/foo.cpython-311-darwin.so will be installed (obviously with corresponding qualifiers for Linux/Windows). get_pybind_value() searches this directory.

Imports/includes should work seamlessly (the python import scheme will be copied). I have tested this but not completely rigorously.

common_object_model/common_object_model/v1/common/foo.py:

from dataclasses import dataclass
import datetime as dt
from enum import Enum, auto
from typing import Union

from pydantic_bind import BaseModel


class Weekday(Enum):
    MONDAY = auto()
    TUESDAY = auto()
    WEDNESDAY = auto()
    THURSDAY = auto()
    FRIDAY = auto()
    SATURDAY = auto()
    SUNDAY = auto()


@dataclass
class DCFoo:
    my_int: int
    my_string: str | None


class Foo(BaseModel):
    my_bool: bool = True
    my_day: Weekday = Weekday.SUNDAY


class Bar(Foo):
    my_int: int = 123
    my_string: str
    my_optional_string: str | None = None


class Baz(BaseModel):
    my_variant: Union[str, float] = 123.
    my_date: dt.date
    my_foo: Foo
    my_dc_foo: DCFoo

will generate the following files:

common_object_model/generated/common_object_model/v1/common/foo.h:

#ifndef COMMON_OBJECT_MODEL_FOO_H
#define COMMON_OBJECT_MODEL_FOO_H

#include <string>
#include <optional>
#include <variant>
#include <msgpack/msgpack.h>
#include <chrono>

namespace common_object_model
{
    enum Weekday { MONDAY = 1, TUESDAY = 2, WEDNESDAY = 3, THURSDAY = 4, FRIDAY = 5, SATURDAY = 6, SUNDAY = 7
    };

    struct DCFoo
    {
        DCFoo() :
            my_string(), my_int()
        {
        }
    
        DCFoo(std::optional<std::string> my_string, int my_int) :
            my_string(my_string), my_int(my_int)
        {
        }

        std::optional<std::string> my_string;
        int my_int;
    
        MSGPACK_DEFINE(my_string, my_int);
    };

    struct Foo
    {
        Foo(bool my_bool=true, Weekday my_day=SUNDAY) :
            my_bool(my_bool), my_day(my_day)
        {
        }

        bool my_bool;
        Weekday my_day;
    
        MSGPACK_DEFINE(my_bool, my_day);
    };

    struct Bar : public Foo
    {
        Bar() :
            Foo(),
            my_string(), my_int(123), my_optional_string(std::nullopt)
        {
        }
    
        Bar(std::string my_string, bool my_bool=true, Weekday my_day=SUNDAY, int my_int=123, std::optional<std::string>
            my_optional_string=std::nullopt) :
            Foo(my_bool, my_day),
            my_string(std::move(my_string)), my_int(my_int), my_optional_string(my_optional_string)
        {
        }

        std::string my_string;
        int my_int;
        std::optional<std::string> my_optional_string;
    
        MSGPACK_DEFINE(my_string, my_bool, my_day, my_int, my_optional_string);
    };

    struct Baz
    {
        Baz() :
            my_dc_foo(), my_foo(), my_date(), my_variant(123.0)
        {
        }
    
        Baz(DCFoo my_dc_foo, Foo my_foo, std::chrono::system_clock::time_point my_date, std::variant<std::string, double>
            my_variant=123.0) :
            my_dc_foo(std::move(my_dc_foo)), my_foo(std::move(my_foo)), my_date(my_date),
            my_variant(my_variant)
        {
        }

        DCFoo my_dc_foo;
        Foo my_foo;
        std::chrono::system_clock::time_point my_date;
        std::variant<std::string, double> my_variant;
    
        MSGPACK_DEFINE(my_dc_foo, my_foo, my_date, my_variant);
    };
} // common_object_model

#endif // COMMON_OBJECT_MODEL_FOO_H

common_object_model/generated/common_object_model/v1/common/foo.cpp:

#include <pybind11/pybind11.h>
#include <pybind11/stl.h>
#include <pybind11/chrono.h>

#include "foo.h"

namespace py = pybind11;
using namespace common_object_model;


PYBIND11_MODULE(foo, m)
{
    py::enum_<Weekday>(m, "Weekday").value("MONDAY", Weekday::MONDAY)
        .value("TUESDAY", Weekday::TUESDAY)
        .value("WEDNESDAY", Weekday::WEDNESDAY)
        .value("THURSDAY", Weekday::THURSDAY)
        .value("FRIDAY", Weekday::FRIDAY)
        .value("SATURDAY", Weekday::SATURDAY)
        .value("SUNDAY", Weekday::SUNDAY);

    py::class_<DCFoo>(m, "DCFoo")
        .def(py::init<>())
        .def(py::init<std::optional<std::string>, int>(), py::arg("my_string"), py::arg("my_int"))
        .def("to_msg_pack", &DCFoo::to_msg_pack)
        .def_static("from_msg_pack", &DCFoo::from_msg_pack<Baz>)
        .def_readwrite("my_string", &DCFoo::my_string)
        .def_readwrite("my_int", &DCFoo::my_int);

    py::class_<Foo>(m, "Foo")
        .def(py::init<bool, Weekday>(), py::arg("my_bool")=true, py::arg("my_day")=SUNDAY)
        .def("to_msg_pack", &Foo::to_msg_pack)
        .def_static("from_msg_pack", &Foo::from_msg_pack<Baz>)
        .def_readwrite("my_bool", &Foo::my_bool)
        .def_readwrite("my_day", &Foo::my_day);

    py::class_<Bar>(m, "Bar")
        .def(py::init<>())
        .def(py::init<std::string, bool, Weekday, int, std::optional<std::string>>(), py::arg("my_string"), py::arg("my_bool")=true,
            py::arg("my_day")=SUNDAY, py::arg("my_int")=123, py::arg("my_optional_string")=std::nullopt)
        .def("to_msg_pack", &Bazr:to_msg_pack)
        .def_static("from_msg_pack", &Bar::from_msg_pack<Baz>)
        .def_readwrite("my_string", &Bar::my_string)
        .def_readwrite("my_int", &Bar::my_int)
        .def_readwrite("my_optional_string", &Bar::my_optional_string);

    py::class_<Baz>(m, "Baz")
        .def(py::init<>())
        .def(py::init<DCFoo, Foo, std::chrono::system_clock::time_point, std::variant<std::string, double>>(), py::arg("my_dc_foo"),
            py::arg("my_foo"), py::arg("my_date"), py::arg("my_variant")=123.0)
        .def("to_msg_pack", &Baz::to_msg_pack)
        .def_static("from_msg_pack", &Baz::from_msg_pack<Baz>)
        .def_readwrite("my_dc_foo", &Baz::my_dc_foo)
        .def_readwrite("my_foo", &Baz::my_foo)
        .def_readwrite("my_date", &Baz::my_date)
        .def_readwrite("my_variant", &Baz::my_variant);
}