【Python学习之路】Cython：class 和 cdef class，使用 C++

Cython：class 和 cdef class，使用 C++

class 和 cdef class

class 定义属性变量比较自由，cdef class 可以定义 cdef

class 使用 __init__ 初始化，cdef class 在使用 __init__ 之前用 __cinit__ 对 C 相关的参数进行初始化。

cdef class 中的方法可以是 def, cdef, cpdef 三种，只有 public 的属性才可以被访问，不可以添加新的属性。

__dealloc__ 函数类似析构函数，负责释放申请的内存。

Cython 属性可以使用关键词 property 来定义，然后定义 __get__ 和 __set__ 方法来进行获取和设置：

property name:
    def __get__(self):
        return something
    def __set__(self):
        set_something

使用 C++ 类

使用 C++ 类时要加上 cppclass 关键词，在编译时 setup 中要加上 language="c++" 的选项。

假设我们有这样一个 C++ 类：

%%file particle_extern.h
#ifndef _PARTICLE_EXTERN_H_
#define _PARTICLE_EXTERN_H_

class Particle {

    public:

        Particle() :
            mass(0), charge(0) {}
        
        Particle(float m, float c, float *p, float *v);

        ~Particle() {}

        float getMass() {return mass; }

        void setMass(float m) { mass = m; }

        float getCharge() { return charge; }

        const float *getVel() { return vel; }
        const float *getPos() { return pos; }

        void applyImpulse(float *f, float t);

    private:
        float mass, charge;
        float pos[3], vel[3];
};

#endif

Overwriting particle_extern.h

%%file particle_extern.cpp
#include "particle_extern.h"

Particle::Particle(float m, float c, float *p, float *v) :
    mass(m), charge(c) 
{
    for (int i=0; i<3; ++i) {
        pos[i] = p[i]; vel[i] = v[i];
    }
}

void Particle::applyImpulse(float *f, float t)
{
    float newvi;
    for(int i=0; i<3; ++i) {
        newvi = vel[i] + t / mass * f[i];
        pos[i] = (newvi + vel[i]) * t / 2.;
        vel[i] = newvi;
    }
}

Overwriting particle_extern.cpp

在 Cython 中调用这个类：

%%file particle.pyx
import numpy as np

cdef extern from "particle_extern.h":

    cppclass _Particle "Particle":
        _Particle(float m, float c, float *p, float *v)
        float getMass()
        void setMass(float m)
        float getCharge()
        const float *getVel()
        const float *getPos()
        void applyImpulse(float *f, float t)


cdef class Particle:
    cdef _Particle *thisptr # ptr to C++ instance

    def __cinit__(self, m, c, float[::1] p, float[::1] v):
        if p.shape[0] != 3 or v.shape[0] != 3:
            raise ValueError("...")
        self.thisptr = new _Particle(m, c, &p[0], &v[0])

    def __dealloc__(self):
        del self.thisptr

    def apply_impulse(self, float[::1] v, float t):
        self.thisptr.applyImpulse(&v[0], t)

    def __repr__(self):
        args = ', '.join('%s=%s' % (n, getattr(self, n)) for n in ('mass', 'charge', 'pos', 'vel'))
        return 'particle.Particle(%s)' % args

    property charge:

        def __get__(self):
            return self.thisptr.getCharge()

    property mass:  # Cython-style properties.
        def __get__(self):
            return self.thisptr.getMass()

        def __set__(self, m):
            self.thisptr.setMass(m)

    property vel:

        def __get__(self):
            cdef const float *_vel = self.thisptr.getVel()
            cdef float[::1] arr = np.empty((3,), dtype=np.float32)
            for i in range(3):
                arr[i] = _vel[i]
            return np.asarray(arr)

    property pos:

        def __get__(self):
            cdef const float *_pos = self.thisptr.getPos()
            cdef float[::1] arr = np.empty((3,), dtype=np.float32)
            for i in range(3):
                arr[i] = _pos[i]
            return np.asarray(arr)

Overwriting particle.pyx

首先从头文件声明这个类：

cdef extern from "particle_extern.h":

    cppclass _Particle "Particle":
        _Particle(float m, float c, float *p, float *v)
        float getMass()
        void setMass(float m)
        float getCharge()
        const float *getVel()
        const float *getPos()
        void applyImpulse(float *f, float t)

这里要使用 cppclass 关键词，并且为了方便，我们将 Particle 类的名字在 Cython 中重命名为 _Particle。

cdef class Particle:
    cdef _Particle *thisptr
    def __cinit__(self, m, c, float[::1] p, float[::1] v):
        if p.shape[0] != 3 or v.shape[0] != 3:
            raise ValueError("...")
        self.thisptr = new _Particle(m, c, &p[0], &v[0])

为了使用这个类，我们需要定义一个该类的指针，然后用指针指向一个 _Particle 对象。

%%file setup.py
from distutils.core import setup
from distutils.extension import Extension
from Cython.Distutils import build_ext

ext = Extension("particle", ["particle.pyx", "particle_extern.cpp"], language="c++")

setup(
    cmdclass = {'build_ext': build_ext},
    ext_modules = [ext],
)

Overwriting setup.py

要加上 language="c++" 的选项，然后编译：

!python setup.py build_ext -i

running build_ext
cythoning particle.pyx to particle.cpp
building 'particle' extension
C:\Anaconda\Scripts\gcc.bat -DMS_WIN64 -mdll -O -Wall -IC:\Anaconda\include -IC:\Anaconda\PC -c particle.cpp -o build\temp.win-amd64-2.7\Release\particle.o
C:\Anaconda\Scripts\gcc.bat -DMS_WIN64 -mdll -O -Wall -IC:\Anaconda\include -IC:\Anaconda\PC -c particle_extern.cpp -o build\temp.win-amd64-2.7\Release\particle_extern.o
writing build\temp.win-amd64-2.7\Release\particle.def
C:\Anaconda\Scripts\g++.bat -DMS_WIN64 -shared -s build\temp.win-amd64-2.7\Release\particle.o build\temp.win-amd64-2.7\Release\particle_extern.o build\temp.win-amd64-2.7\Release\particle.def -LC:\Anaconda\libs -LC:\Anaconda\PCbuild\amd64 -lpython27 -lmsvcr90 -o "C:\Users\lijin\Documents\Git\python-tutorial\07. interfacing with other languages\particle.pyd"


particle.cpp: In function 'void __Pyx_RaiseArgtupleInvalid(const char*, int, Py_ssize_t, Py_ssize_t, Py_ssize_t)':
particle.cpp:14931:59: warning: unknown conversion type character 'z' in format [-Wformat]
particle.cpp:14931:59: warning: format '%s' expects argument of type 'char*', but argument 5 has type 'Py_ssize_t {aka long long int}' [-Wformat]
particle.cpp:14931:59: warning: unknown conversion type character 'z' in format [-Wformat]
particle.cpp:14931:59: warning: too many arguments for format [-Wformat-extra-args]
particle.cpp: In function 'int __Pyx_BufFmt_ProcessTypeChunk(__Pyx_BufFmt_Context*)':
particle.cpp:15498:78: warning: unknown conversion type character 'z' in format [-Wformat]
particle.cpp:15498:78: warning: unknown conversion type character 'z' in format [-Wformat]
particle.cpp:15498:78: warning: too many arguments for format [-Wformat-extra-args]
particle.cpp:15550:67: warning: unknown conversion type character 'z' in format [-Wformat]
particle.cpp:15550:67: warning: unknown conversion type character 'z' in format [-Wformat]
particle.cpp:15550:67: warning: too many arguments for format [-Wformat-extra-args]
particle.cpp: In function 'PyObject* __pyx_buffmt_parse_array(__Pyx_BufFmt_Context*, const char**)':
particle.cpp:15612:69: warning: unknown conversion type character 'z' in format [-Wformat]
particle.cpp:15612:69: warning: format '%d' expects argument of type 'int', but argument 3 has type 'size_t {aka long long unsigned int}' [-Wformat]
particle.cpp:15612:69: warning: too many arguments for format [-Wformat-extra-args]
particle.cpp: In function 'int __Pyx_GetBufferAndValidate(Py_buffer*, PyObject*, __Pyx_TypeInfo*, int, int, int, __Pyx_BufFmt_StackElem*)':
particle.cpp:15797:73: warning: unknown conversion type character 'z' in format [-Wformat]
particle.cpp:15797:73: warning: format '%s' expects argument of type 'char*', but argument 3 has type 'Py_ssize_t {aka long long int}' [-Wformat]
particle.cpp:15797:73: warning: unknown conversion type character 'z' in format [-Wformat]
particle.cpp:15797:73: warning: too many arguments for format [-Wformat-extra-args]
particle.cpp: In function 'void __Pyx_RaiseTooManyValuesError(Py_ssize_t)':
particle.cpp:16216:94: warning: unknown conversion type character 'z' in format [-Wformat]
particle.cpp:16216:94: warning: too many arguments for format [-Wformat-extra-args]
particle.cpp: In function 'void __Pyx_RaiseNeedMoreValuesError(Py_ssize_t)':
particle.cpp:16222:48: warning: unknown conversion type character 'z' in format [-Wformat]
particle.cpp:16222:48: warning: format '%s' expects argument of type 'char*', but argument 3 has type 'Py_ssize_t {aka long long int}' [-Wformat]
particle.cpp:16222:48: warning: too many arguments for format [-Wformat-extra-args]
particle.cpp: In function 'int __Pyx_ValidateAndInit_memviewslice(int*, int, int, int, __Pyx_TypeInfo*, __Pyx_BufFmt_StackElem*, __Pyx_memviewslice*, PyObject*)':
particle.cpp:16941:50: warning: unknown conversion type character 'z' in format [-Wformat]
particle.cpp:16941:50: warning: format '%s' expects argument of type 'char*', but argument 3 has type 'Py_ssize_t {aka long long int}' [-Wformat]
particle.cpp:16941:50: warning: unknown conversion type character 'z' in format [-Wformat]
particle.cpp:16941:50: warning: too many arguments for format [-Wformat-extra-args]

import particle

注意这里类型要设成 float32，因为 C++ 程序中接受的是 float 类型，默认是 float64(double) 类型：

import numpy as np

pos = vel = np.arange(3., dtype='float32')
mass = 1.0
charge = 2.0

p = particle.Particle(mass, charge, pos, vel)
p

particle.Particle(mass=1.0, charge=2.0, pos=[ 0.  1.  2.], vel=[ 0.  1.  2.])

调用 apply_impulse 方法：

p.apply_impulse(np.arange(3., dtype='float32'), 1.0)

p

particle.Particle(mass=1.0, charge=2.0, pos=[ 0.   1.5  3. ], vel=[ 0.  2.  4.])

查看质量：

p.mass

1.0

修改质量：

p.mass = 3.0

查看 charge：

p.charge

2.0

因为 charge 没有定义 __set__ 方法，所以它是只读的属性，不能进行修改。

import zipfile

f = zipfile.ZipFile('07-05-particle.zip','w',zipfile.ZIP_DEFLATED)

names = ['particle.pyx',
         'particle_extern.cpp',
         'particle_extern.h',
         'setup.py']
for name in names:
    f.write(name)

f.close()

!rm -f setup*.*
!rm -f particle*.*
!rm -rf build