pybind11The package pybind11 is provides an elegant way to wrap C++ code for Python, including automatic conversions for numpy arrays and the C++ Eigen linear algebra library. Used with the cppimport package, this provides a very nice work flow for integrating C++ and Python:
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! pip3 install pybind11
! pip3 install cppimport
Clone the Eigen library - no installation is required as Eigen is a header only library.
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! git clone https://github.com/RLovelett/eigen.git
First write the C++ header and implementation files
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%mkdir example1
%cd example1
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%%file funcs.hpp
int add(int i, int j);
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%%file funcs.cpp
int add(int i, int j) {
return i + j;
};
Next write the C++ wrapper code using pybind11 in wrap.cpp. The arguments "i"_a=1, "j"_a=2 in the exported function definition tells pybind11 to generate variables named i with default value 1 and j with default value 2 for the add function.
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%%file wrap1.cpp
#include <pybind11/pybind11.h>
#include "funcs.hpp"
namespace py = pybind11;
using namespace pybind11::literals;
PYBIND11_PLUGIN(wrap1) {
py::module m("wrap1", "pybind11 example plugin");
m.def("add", &add, "A function which adds two numbers",
"i"_a=1, "j"_a=2);
return m.ptr();
}
Finally, write the setup.py file to compile the extension module. This is mostly boilerplate.
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%%file setup.py
import os, sys
from distutils.core import setup, Extension
from distutils import sysconfig
cpp_args = ['-std=c++11']
ext_modules = [
Extension(
'wrap1',
['funcs.cpp', 'wrap1.cpp'],
include_dirs=['pybind11/include'],
language='c++',
extra_compile_args = cpp_args,
),
]
setup(
name='wrap1',
version='0.0.1',
author='Cliburn Chan',
author_email='cliburn.chan@duke.edu',
description='Example',
ext_modules=ext_modules,
)
Now build the extension module in the subdirectory with these files
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%%bash
python setup.py build_ext -i
And if you are successful, you should now see a new funcs.so extension module. We can write a test_funcs.py file test the extension module:
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%%file test_funcs.py
import wrap1
def test_add():
print(wrap1.add(3, 4))
assert(wrap1.add(3, 4) == 7)
if __name__ == '__main__':
test_add()
And finally, running the test should not generate any error messages:
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%%bash
python test_funcs.py
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%cd ..
cppimportIn the development stage, it can be distracting to have to repeatedly rebuild the extension module by running
python setup.py clean
python setup.py build_ext -i
every single time you modify the C++ code. The cppimport package does this for you.
Create a new sub-directory exaample2 and copy the files func.hpp, funcs.cpp and wrap.cpp from example1 over.
For the previous example, we just need to add some annotation (between <% and %> delimiters) to the top of the wrap.cpp file
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%mkdir example2
%cp example1/funcs.* example2/
%cd example2
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%%file wrap2.cpp
<%
cfg['compiler_args'] = ['-std=c++11']
cfg['sources'] = ['funcs.cpp']
setup_pybind11(cfg)
%>
#include "funcs.hpp"
#include <pybind11/pybind11.h>
namespace py = pybind11;
PYBIND11_PLUGIN(wrap2) {
py::module m("wrap2", "pybind11 example plugin");
m.def("add", &add, "A function which adds two numbers");
return m.ptr();
}
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%%file test_funcs.py
import cppimport
funcs = cppimport.imp("wrap2")
def test_add():
assert(funcs.add(3, 4) == 7)
if __name__ == '__main__':
print(funcs.add(3,4))
test_add()
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%%bash
python test_funcs.py
Or just call from notebook
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import cppimport
funcs = cppimport.imp("wrap2")
funcs.add(3, 4)
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without any need to manually build the extension module. Any updates will be detected by cppimport and it will automatically trigger a re-build.
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%cd ..
numpy arraysExample showing how to vectorize a square function. Note that from here on, we don't bother to use separate header and implementation files for these code snippets, and just write them together with the wrapping code in a code.cpp file. This means that with cppimport, there are only two files that we actually code for, a C++ code.cpp file and a python test file.
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%mkdir example3
%cd example3
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%%file wrap3.cpp
<%
cfg['compiler_args'] = ['-std=c++11']
setup_pybind11(cfg)
%>
#include <pybind11/pybind11.h>
#include <pybind11/numpy.h>
namespace py = pybind11;
double square(double x) {
return x * x;
}
PYBIND11_PLUGIN(wrap3) {
py::module m("wrap3", "pybind11 example plugin");
m.def("square", py::vectorize(square), "A vectroized square function.");
return m.ptr();
}
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import cppimport
wrap3 = cppimport.imp("wrap3")
wrap3.square([1,2,3])
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Once the shared libary is built, you can use it as a regular Python module.
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! ls
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import wrap3
wrap3.square([2,4,6])
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%cd ..
numpy arrays as function arguments and return valuesExample showing how to pass numpy arrays in and out of functions. These numpy array arguments can either be generic py:array or typed py:array_t<double>. The properties of the numpy array can be obtained by calling its request method. This returns a struct of the following form:
struct buffer_info {
void *ptr;
size_t itemsize;
std::string format;
int ndim;
std::vector<size_t> shape;
std::vector<size_t> strides;
};
Here is C++ code for two functions - the function twice shows how to change a passed in numpy array in-place using pointers; the function sum shows how to sum the elements of a numpy array. By taking advantage of the information in buffer_info, the code will work for arbitrary n-d arrays.
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%mkdir example4
%cd example4
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%%file wrap4.cpp
<%
cfg['compiler_args'] = ['-std=c++11']
setup_pybind11(cfg)
%>
#include <pybind11/pybind11.h>
#include <pybind11/numpy.h>
namespace py = pybind11;
// Passing in an array of doubles
void twice(py::array_t<double> xs) {
py::buffer_info info = xs.request();
auto ptr = static_cast<double *>(info.ptr);
int n = 1;
for (auto r: info.shape) {
n *= r;
}
for (int i = 0; i <n; i++) {
*ptr++ *= 2;
}
}
// Passing in a generic array
double sum(py::array xs) {
py::buffer_info info = xs.request();
auto ptr = static_cast<double *>(info.ptr);
int n = 1;
for (auto r: info.shape) {
n *= r;
}
double s = 0.0;
for (int i = 0; i <n; i++) {
s += *ptr++;
}
return s;
}
PYBIND11_PLUGIN(wrap4) {
pybind11::module m("wrap4", "auto-compiled c++ extension");
m.def("sum", &sum);
m.def("twice", &twice);
return m.ptr();
}
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%%file test_code.py
import cppimport
import numpy as np
code = cppimport.imp("wrap4")
if __name__ == '__main__':
xs = np.arange(12).reshape(3,4).astype('float')
print(xs)
print("np :", xs.sum())
print("cpp:", code.sum(xs))
print()
code.twice(xs)
print(xs)
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%%bash
python test_code.py
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%cd ..
numpy arraysThis example shows how to use array access for numpy arrays within the C++ function. It is taken from the pybind11 documentation, but fixes a small bug in the official version. As noted in the documentation, the function would be more easily coded using py::vectorize.
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%mkdir example5
%cd example5
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%%file wrap5.cpp
<%
cfg['compiler_args'] = ['-std=c++11']
setup_pybind11(cfg)
%>
#include <pybind11/pybind11.h>
#include <pybind11/numpy.h>
namespace py = pybind11;
py::array_t<double> add_arrays(py::array_t<double> input1, py::array_t<double> input2) {
auto buf1 = input1.request(), buf2 = input2.request();
if (buf1.ndim != 1 || buf2.ndim != 1)
throw std::runtime_error("Number of dimensions must be one");
if (buf1.shape[0] != buf2.shape[0])
throw std::runtime_error("Input shapes must match");
auto result = py::array(py::buffer_info(
nullptr, /* Pointer to data (nullptr -> ask NumPy to allocate!) */
sizeof(double), /* Size of one item */
py::format_descriptor<double>::value, /* Buffer format */
buf1.ndim, /* How many dimensions? */
{ buf1.shape[0] }, /* Number of elements for each dimension */
{ sizeof(double) } /* Strides for each dimension */
));
auto buf3 = result.request();
double *ptr1 = (double *) buf1.ptr,
*ptr2 = (double *) buf2.ptr,
*ptr3 = (double *) buf3.ptr;
for (size_t idx = 0; idx < buf1.shape[0]; idx++)
ptr3[idx] = ptr1[idx] + ptr2[idx];
return result;
}
PYBIND11_PLUGIN(wrap5) {
py::module m("wrap5");
m.def("add_arrays", &add_arrays, "Add two NumPy arrays");
return m.ptr();
}
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import cppimport
import numpy as np
code = cppimport.imp("wrap5")
xs = np.arange(12)
print(xs)
print(code.add_arrays(xs, xs))
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%cd ..
eigen library to calculate matrix inverse and determinantExample showing how Eigen vectors and matrices can be passed in and out of C++ functions. Note that Eigen arrays are automatically converted to/from numpy arrays simply by including the pybind/eigen.h header. Because of this, it is probably simplest in most cases to work with Eigen vectors and matrices rather than py::buffer or py::array where py::vectorize is insufficient.
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%mkdir example6
%cd example6
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%%file wrap6.cpp
<%
cfg['compiler_args'] = ['-std=c++11']
cfg['include_dirs'] = ['../eigen']
setup_pybind11(cfg)
%>
#include <pybind11/pybind11.h>
#include <pybind11/eigen.h>
#include <Eigen/LU>
namespace py = pybind11;
// convenient matrix indexing comes for free
double get(Eigen::MatrixXd xs, int i, int j) {
return xs(i, j);
}
// takes numpy array as input and returns double
double det(Eigen::MatrixXd xs) {
return xs.determinant();
}
// takes numpy array as input and returns another numpy array
Eigen::MatrixXd inv(Eigen::MatrixXd xs) {
return xs.inverse();
}
PYBIND11_PLUGIN(wrap6) {
pybind11::module m("wrap6", "auto-compiled c++ extension");
m.def("inv", &inv);
m.def("det", &det);
return m.ptr();
}
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import cppimport
import numpy as np
code = cppimport.imp("wrap6")
A = np.array([[1,2,1],
[2,1,0],
[-1,1,2]])
print(A)
print(code.det(A))
print(code.inv(A))
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%cd ..
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%mkdir example7
%cd example7
Here is an example of using OpenMP to integrate the value of $\pi$ written using pybind11.
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%%file wrap7.cpp
/*
<%
cfg['compiler_args'] = ['-std=c++11', '-fopenmp']
cfg['linker_args'] = ['-lgomp']
setup_pybind11(cfg)
%>
*/
#include <cmath>
#include <omp.h>
#include <pybind11/pybind11.h>
#include <pybind11/numpy.h>
namespace py = pybind11;
// Passing in an array of doubles
void twice(py::array_t<double> xs) {
py::gil_scoped_acquire acquire;
py::buffer_info info = xs.request();
auto ptr = static_cast<double *>(info.ptr);
int n = 1;
for (auto r: info.shape) {
n *= r;
}
#pragma omp parallel for
for (int i = 0; i <n; i++) {
*ptr++ *= 2;
}
}
PYBIND11_PLUGIN(wrap7) {
pybind11::module m("wrap7", "auto-compiled c++ extension");
m.def("twice", [](py::array_t<double> xs) {
/* Release GIL before calling into C++ code */
py::gil_scoped_release release;
return twice(xs);
});
return m.ptr();
}
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import cppimport
import numpy as np
code = cppimport.imp("wrap7")
xs = np.arange(10).astype('double')
code.twice(xs)
xs
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In [66]:
%cd ..