15.10 用Cython包裝C代碼

問(wèn)題

You want to use Cython to make a Python extension module that wraps around anexisting C library.

解決方案

Making an extension module with Cython looks somewhat similar to writing a hand‐written extension, in that you will be creating a collection of wrapper functions. How‐ever, unlike previous recipes, you won’t be doing this in C—the code will look a lot morelike Python.As preliminaries, assume that the sample code shown in the introduction to this chapterhas been compiled into a C library called libsample. Start by creating a file namedcsample.pxd that looks like this:

csample.pxd## Declarations of “external” C functions and structures

cdef extern from “sample.h”:
int gcd(int, int)bint in_mandel(double, double, int)int divide(int, int, int *)double avg(double *, int) nogil

ctypedef struct Point:double xdouble y
double distance(Point *, Point *)

This file serves the same purpose in Cython as a C header file. The initial declarationcdef extern from “sample.h” declares the required C header file. Declarationsthat follow are taken from that header. The name of this file is csample.pxd, not sam‐ple.pxd—this is important.Next, create a file named sample.pyx. This file will define wrappers that bridge thePython interpreter to the underlying C code declared in the csample.pxd file:

sample.pyx

Import the low-level C declarationscimport csample

Import some functionality from Python and the C stdlibfrom cpython.pycapsule cimport *

from libc.stdlib cimport malloc, free

Wrappersdef gcd(unsigned int x, unsigned int y):

return csample.gcd(x, y)

def in_mandel(x, y, unsigned int n):return csample.in_mandel(x, y, n)def divide(x, y):cdef int remquot = csample.divide(x, y, &rem)return quot, remdef avg(double[:] a):cdef:int szdouble result
sz = a.sizewith nogil:

result = csample.avg(<double *> &a[0], sz)

return result

Destructor for cleaning up Point objectscdef del_Point(object obj):

pt = <csample.Point *> PyCapsule_GetPointer(obj,”P(pán)oint”)free(<void *> pt)

Create a Point object and return as a capsuledef Point(double x,double y):

cdef csample.Point *pp = <csample.Point *> malloc(sizeof(csample.Point))if p == NULL:

raise MemoryError(“No memory to make a Point”)

p.x = xp.y = yreturn PyCapsule_New(<void *>p,”P(pán)oint”,del_Point)

def distance(p1, p2):pt1 = <csample.Point *> PyCapsule_GetPointer(p1,”P(pán)oint”)pt2 = <csample.Point *> PyCapsule_GetPointer(p2,”P(pán)oint”)return csample.distance(pt1,pt2)
Various details of this file will be covered further in the discussion section. Finally, tobuild the extension module, create a setup.py file that looks like this:

from distutils.core import setupfrom distutils.extension import Extensionfrom Cython.Distutils import build_ext

ext_modules = [
Extension(‘sample',

[‘sample.pyx'],libraries=[‘sample'],library_dirs=[‘.'])]

setup(name = ‘Sample extension module',cmdclass = {‘build_ext': build_ext},ext_modules = ext_modules
)

To build the resulting module for experimentation, type this:

bash % python3 setup.py build_ext –inplacerunning build_extcythoning sample.pyx to sample.cbuilding ‘sample' extensiongcc -fno-strict-aliasing -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes

-I/usr/local/include/python3.3m -c sample.c-o build/temp.macosx-10.6-x86_64-3.3/sample.o

gcc -bundle -undefined dynamic_lookup build/temp.macosx-10.6-x86_64-3.3/sample.o-L. -lsample -o sample.so
bash %

If it works, you should have an extension module sample.so that can be used as shownin the following example:

>>> import sample
>>> sample.gcd(42,10)
2
>>> sample.in_mandel(1,1,400)
False
>>> sample.in_mandel(0,0,400)
True
>>> sample.divide(42,10)
(4, 2)
>>> import array
>>> a = array.array('d',[1,2,3])
>>> sample.avg(a)
2.0
>>> p1 = sample.Point(2,3)
>>> p2 = sample.Point(4,5)
>>> p1
<capsule object "Point" at 0x1005d1e70>
>>> p2
<capsule object "Point" at 0x1005d1ea0>
>>> sample.distance(p1,p2)
2.8284271247461903
>>>

討論

This recipe incorporates a number of advanced features discussed in prior recipes, in‐cluding manipulation of arrays, wrapping opaque pointers, and releasing the GIL. Eachof these parts will be discussed in turn, but it may help to review earlier recipes first.At a high level, using Cython is modeled after C. The .pxd files merely contain C defi‐nitions (similar to .h files) and the .pyx files contain implementation (similar to a .c file).The cimport statement is used by Cython to import definitions from a .pxd file. This isdifferent than using a normal Python import statement, which would load a regularPython module.Although .pxd files contain definitions, they are not used for the purpose of automati‐cally creating extension code. Thus, you still have to write simple wrapper functions.For example, even though the csample.pxd file declares int gcd(int, int) as a func‐tion, you still have to write a small wrapper for it in sample.pyx. For instance:

cimport csample

def gcd(unsigned int x, unsigned int y):return csample.gcd(x,y)
For simple functions, you don’t have to do too much. Cython will generate wrapper codethat properly converts the arguments and return value. The C data types attached to thearguments are optional. However, if you include them, you get additional error checkingfor free. For example, if someone calls this function with negative values, an exceptionis generated:

>>> sample.gcd(-10,2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "sample.pyx", line 7, in sample.gcd (sample.c:1284)
    def gcd(unsigned int x,unsigned int y):
OverflowError: can't convert negative value to unsigned int
>>>

If you want to add additional checking to the wrapper, just use additional wrapper code.For example:

def gcd(unsigned int x, unsigned int y):if x <= 0:raise ValueError(“x must be > 0”)if y <= 0:raise ValueError(“y must be > 0”)
return csample.gcd(x,y)

The declaration of in_mandel() in the csample.pxd file has an interesting, but subtledefinition. In that file, the function is declared as returning a bint instead of an int.This causes the function to create a proper Boolean value from the result instead of asimple integer. So, a return value of 0 gets mapped to False and 1 to True.

Within the Cython wrappers, you have the option of declaring C data types in additionto using all of the usual Python objects. The wrapper for divide() shows an exampleof this as well as how to handle a pointer argument.

def divide(x,y):cdef int remquot = csample.divide(x,y,&rem)return quot, rem
Here, the rem variable is explicitly declared as a C int variable. When passed to theunderlying divide() function, &rem makes a pointer to it just as in C.The code for the avg() function illustrates some more advanced features of Cython.First the declaration def avg(double[:] a) declares avg() as taking a one-dimensionalmemoryview of double values. The amazing part about this is that the resulting functionwill accept any compatible array object, including those created by libraries such asnumpy. For example:>>> import array>>> a = array.array(‘d',[1,2,3])>>> import numpy>>> b = numpy.array([1., 2., 3.])>>> import sample>>> sample.avg(a)2.0>>> sample.avg(b)2.0>>>

In the wrapper, a.size and &a[0] refer to the number of array items and underlyingpointer, respectively. The syntax <double *> &a[0] is how you type cast pointers to adifferent type if necessary. This is needed to make sure the C avg() receives a pointerof the correct type. Refer to the next recipe for some more advanced usage of Cythonmemoryviews.In addition to working with general arrays, the avg() example also shows how to workwith the global interpreter lock. The statement with nogil: declares a block of code asexecuting without the GIL. Inside this block, it is illegal to work with any kind of normalPython object—only objects and functions declared as cdef can be used. In addition tothat, external functions must explicitly declare that they can execute without the GIL.Thus, in the csample.pxd file, the avg() is declared as double avg(double *, int)nogil.The handling of the Point structure presents a special challenge. As shown, this recipetreats Point objects as opaque pointers using capsule objects, as described inRecipe 15.4. However, to do this, the underlying Cython code is a bit more complicated.First, the following imports are being used to bring in definitions of functions from theC library and Python C API:

from cpython.pycapsule cimport *from libc.stdlib cimport malloc, free

The function del_Point() and Point() use this functionality to create a capsule objectthat wraps around a Point * pointer. The declaration cdef del_Point() declaresdel_Point() as a function that is only accessible from Cython and not Python. Thus,this function will not be visible to the outside—instead, it’s used as a callback functionto clean up memory allocated by the capsule. Calls to functions such as PyCapsule_New(), PyCapsule_GetPointer() are directly from the Python C API and are usedin the same way.The distance() function has been written to extract pointers from the capsule objectscreated by Point(). One notable thing here is that you simply don’t have to worry aboutexception handling. If a bad object is passed, PyCapsule_GetPointer() raises an ex‐ception, but Cython already knows to look for it and propagate it out of the distance() function if it occurs.A downside to the handling of Point structures is that they will be completely opaquein this implementation. You won’t be able to peek inside or access any of their attributes.There is an alternative approach to wrapping, which is to define an extension type, asshown in this code:

sample.pyx

cimport csamplefrom libc.stdlib cimport malloc, free...

cdef class Point:
cdef csample.Point *_c_pointdef cinit(self, double x, double y):

self._c_point = <csample.Point *> malloc(sizeof(csample.Point))self._c_point.x = xself._c_point.y = y

def dealloc(self):free(self._c_point)property x:def get(self):return self._c_point.xdef set(self, value):self._c_point.x = valueproperty y:def get(self):return self._c_point.ydef set(self, value):self._c_point.y = valuedef distance(Point p1, Point p2):return csample.distance(p1._c_point, p2._c_point)
Here, the cdef class Point is declaring Point as an extension type. The class variablecdef csample.Point *_c_point is declaring an instance variable that holds a pointerto an underlying Point structure in C. The cinit() and dealloc() methodscreate and destroy the underlying C structure using malloc() and free() calls. Theproperty x and property y declarations give code that gets and sets the underlyingstructure attributes. The wrapper for distance() has also been suitably modified toaccept instances of the Point extension type as arguments, but pass the underlyingpointer to the C function.Making this change, you will find that the code for manipulating Point objects is morenatural:

>>> import sample
>>> p1 = sample.Point(2,3)
>>> p2 = sample.Point(4,5)
>>> p1
<sample.Point object at 0x100447288>
>>> p2
<sample.Point object at 0x1004472a0>
>>> p1.x
2.0
>>> p1.y
3.0
>>> sample.distance(p1,p2)
2.8284271247461903
>>>

This recipe has illustrated many of Cython’s core features that you might be able toextrapolate to more complicated kinds of wrapping. However, you will definitely wantto read more of the official documentation to do more.The next few recipes also illustrate a few additional Cython features.