Advanced pybind11 features

You can find the corresponding Medium article here.

This repo details three advanced pybind11 features:

• Shared pointers.
• Enum.
• Abstract base classes (ABC) and pure virtual methods.

The starting point for this project is a previous project found here.

Image source.

Requirements

Obviously, you will need pybind11. On Mac:

brew install pybind11

will do it.

Setup with CMake

We will start with the CMake based setup from a previous introduction found here. Are you gonna check it out? Of course not. No time for that! Here is the setup:

The directory structure is as follows:

cpp/CMakeLists.txt
cpp/include/automobile
cpp/include/automobile_bits/motorcycle.hpp
cpp/src/motorcycle.cpp
python/automobile.cpp
python/automobile.hpp
CMakeLists.txt

The idea here is:

1. The inner cpp folder contains a C++ project for a library. It can be built using the CMakeLists.txt as follows:

   cd cpp
   mkdir build
   cd build
   cmake ..
   make
   make install
   

2. The outer folder contains the wrapping code in the python library, and a second CMakeLists.txt for building the python library as follows:

   mkdir build
   cd build
   cmake .. -DPYTHON_LIBRARY_DIR="/path/to/site-packages" -DPYTHON_EXECUTABLE="/path/to/executable/python3"
   make
   make install
   

   My paths are:

   DPYTHON_LIBRARY_DIR="/Users/USERNAME/opt/anaconda3/lib/python3.7/site-packages"
   DPYTHON_EXECUTABLE="/Users/USERNAME/opt/anaconda3/bin/python3"
   

I won't review all the files here - you can find them in this repo.

Shared pointers

C++ standard 11 introduced shared and unique pointers which do not require manual memory cleanup. This is highly parallel to Python, where garbage collection is automatic. Wrapping shared pointers into Python is therefore only natural.

We will add a static constructor method that returns a shared_ptr to a Motorcycle. Add to cpp/include/automobile_bits/motorcycle.hpp above the constructor:

/// Shared pointer constructor
static std::shared_ptr<Motorcycle> create(std::string name);

and the implementation in cpp/src/motorcycle.cpp:

std::shared_ptr<Motorcycle> Motorcycle::create(std::string name) {
    return std::make_shared<Motorcycle>(name);
}

There are two parts now: (1) we must allow a shared_ptr<Motorcycle> to be accessible by Python, and (2) we need to expose the create method.

For the first part, we will modify the glue code in python/motorcycle.cpp:

// Old:
// py::class_<autos::Motorcycle>(m, "Motorcycle")
// New:
py::class_<autos::Motorcycle, std::shared_ptr<autos::Motorcycle>>(m, "Motorcycle")

For the second part, to wrap the static method, we will also add:

.def_static("create",
        py::overload_cast<std::string>( &autos::Motorcycle::create),
        py::arg("name"))

Notice that we used def_static instead of def for a static method.

Build and install the library as before. The test python code:

import automobile
bike = automobile.Motorcycle.create("yamaha")
bike.ride("mullholland")

works as expected with output:

Zoom Zoom on road: mullholland

Enum

Enum are great for setting flags or options in a more verbose way than simply true/false or 1/2/3/4.... They are supported in both Python and C++.

Let's create an enum in C++. In the header cpp/include/automobile_bits/motorcycle.hpp add:

enum EngineType {
    TWO_STROKE = 0,
    FOUR_STROKE = 1
};

above the Motorcycle class, as well as the public method of the Motorcycle class:

/// Get engine type
/// @return Engine type
EngineType get_engine_type() const;

and it's implementation in cpp/src/motorcycle.cpp:

EngineType Motorcycle::get_engine_type() const {
    return EngineType::TWO_STROKE;
}

Add the following glue code in python/motorcycle.cpp below the Motorcycle wrapper:

py::enum_<autos::EngineType>(m, "EngineType")
.value("TWO_STROKE", autos::EngineType::TWO_STROKE)
.value("FOUR_STROKE", autos::EngineType::FOUR_STROKE)
.export_values();

and expose the method in the Motorcycle class:

.def("get_engine_type", py::overload_cast<>( &autos::Motorcycle::get_engine_type, py::const_ ))

Finally, the proof in Python is:

import automobile
bike = automobile.Motorcycle("yamaha")
bike.get_engine_type()

returns

EngineType.TWO_STROKE

Abstract base classes

Python also has an abc module that is entirely underused. The basic principles of abstract base classes will translate nicely from C++ to Python, but as we shall see some behavior is missing.

Add to the header cpp/include/automobile_bits/motorcycle.hpp:

class Photograph {
    
public:
    
    /// Constructor/destructor
    virtual ~Photograph() {};
    
    /// Pure virtual method
    /// @param bike Bike
    /// @return true if beautiful
    virtual bool is_beautiful(std::shared_ptr<Motorcycle> bike) const = 0;
};

and of course, no implementation for is_beautiful (although you could have one!).

What happens when we try to add the glue code in python/motorcycle.cpp? If we try:

py::class_<autos::Photograph>(m, "Photograph")
.def(py::init<>())
.def("is_beautiful", py::overload_cast<std::shared_ptr<autos::Motorcycle>>( &autos::Photograph::is_beautiful, py::const_ ), py::arg("bike"));

we'll get the error

Allocating an object of abstract class type 'autos::Photograph'

Uh oh! It looks like it is unhappy with the constructor. Of course, we could simply eliminate the constructor:

py::class_<autos::Photograph>(m, "Photograph")
.def("is_beautiful", py::overload_cast<std::shared_ptr<autos::Motorcycle>>( &autos::Photograph::is_beautiful, py::const_ ), py::arg("bike"));

This compiles - but now consider the following example in Python:

import automobile

class YamahaPhoto(automobile.Photograph):
    
    def __init__(self):
        super().__init__()
        
    def is_beautiful(self, bike):
        return True

bike = automobile.Motorcycle.create("yamaha")

photo = YamahaPhoto()
print(photo.is_beautiful(bike))

This gives the error:

TypeError: YamahaPhoto: No constructor defined!

because of course, we deleted the constructor! So abstract base classes are no longer extensible.

The solution is to define what pybind11 refers to as a "trampoline" class. In python/motorcycle.cpp, define the trampoline at the top:

namespace autos {

class PhotographTrampoline : public Photograph {

public:
    
    using Photograph::Photograph;
    
    bool is_beautiful(std::shared_ptr<Motorcycle> bike) const override {
        PYBIND11_OVERLOAD_PURE(
            bool, /* Return type */
            Photograph,      /* Parent class */
            is_beautiful,          /* Name of function in C++ (must match Python name) */
            bike   /* args */
        );
    }
};

}

and change the glue code to:

py::class_<autos::Photograph, autos::PhotographTrampoline>(m, "Photograph")
.def(py::init<>())
.def("is_beautiful", py::overload_cast<std::shared_ptr<autos::Motorcycle>>( &autos::Photograph::is_beautiful, py::const_ ), py::arg("bike"));

Notice here the order in py::class_<autos::Photograph, autos::PhotographTrampoline> - first the parent class (the ABC), then the trampoline. Everywhere else, we use just the name of the ABC, i.e. Photograph::is_beautiful, not PhotographTrampoline::is_beautiful. Now we could also add the constructor without an error.

The python example will now run and produce a resounding True.

A limitation here is that the Photograh class in Python is no longer an abstract base class. That means, we can actually run the following:

import automobile

class YamahaPhoto(automobile.Photograph):
    
    def __init__(self):
        super().__init__()

bike = automobile.Motorcycle.create("yamaha")

photo = YamahaPhoto()

which will construct a YamahaPhoto object, despite the fact that we did not implement the is_beautiful method. This is unfortunate, as it breaks some of the design principles enforced in C++. At the moment, it seems we just cannot have everything - but maybe one day!

Final thoughts

That's three advanced features of pybind11 - some things are not so obvious, but it seems just about everything is possible!