扩展和嵌入Python 解释器(Extending and Embedding the Python Interpreter)--第一章

1.ExtendingPythonwithCorC++

Itisquiteeasytoaddnewbuilt-inmodulestoPython,ifyouknowhowtoprograminC.Suchextensionmodulescandotwothingsthatcan'tbedonedirectlyinPython:theycanimplementnewbuilt-inobjecttypes,andtheycancallClibraryfunctionsandsystemcalls.

如果知道如何使用C编程，很容易建立Python新模块。
Tosupportextensions,thePythonAPI(ApplicationProgrammersInterface)definesasetoffunctions,macrosandvariablesthatprovideaccesstomostaspectsofthePythonrun-timesystem.ThePythonAPIisincorporatedinaCsourcefilebyincludingtheheader

"Python.h"

.

为支持扩展，PythonAPI定义了一些函数，宏和变量。使用PythonAPI，需要在C源文件里包含

"Python.h"头文件。

Thecompilationofanextensionmoduledependsonitsintendeduseaswellasonyoursystemsetup;detailsaregiveninlaterchapters.

1.1ASimpleExample

一个简单的例子
Let'screateanextensionmodulecalled"spam"(thefavoritefoodofMontyPythonfans...)andlet'ssaywewanttocreateaPythoninterfacetotheClibraryfunctionsystem().1.1Thisfunctiontakesanull-terminatedcharacterstringasargumentandreturnsaninteger.WewantthisfunctiontobecallablefromPythonasfollows:

建一个"spam"的扩张模块

>>>importspam

>>>status=spam.system("ls-l")

Beginbycreatingafilespammodule.c.(Historically,ifamoduleiscalled"spam",theCfilecontainingitsimplementationiscalledspammodule.c;ifthemodulenameisverylong,like"spammify",themodulenamecanbejustspammify.c.)

开始创建一个spammodule.c

Thefirstlineofourfilecanbe:

#include<Python.h>

whichpullsinthePythonAPI(youcanaddacommentdescribingthepurposeofthemoduleandacopyrightnoticeifyoulike).

Warning:SincePythonmaydefinesomepre-processordefinitionswhichaffectthestandardheadersonsomesystems,youmustincludePython.hbeforeanystandardheadersareincluded.
第一行应该是#include<Python.h>，否则可能会影响到其它
Alluser-visiblesymbolsdefinedbyPython.hhaveaprefixof"Py"or"PY",exceptthosedefinedinstandardheaderfiles.Forconvenience,andsincetheyareusedextensivelybythePythoninterpreter,

"Python.h"

includesafewstandardheaderfiles:

<stdio.h>

,

<string.h>

,

<errno.h>

,and

<stdlib.h>

.Ifthelatterheaderfiledoesnotexistonyoursystem,itdeclaresthefunctionsmalloc(),free()andrealloc()directly.

ThenextthingweaddtoourmodulefileistheCfunctionthatwillbecalledwhenthePythonexpression"spam.system(string)"isevaluated(we'llseeshortlyhowitendsupbeingcalled):

添加C函数：

staticPyObject*

spam_system(PyObject*self,PyObject*args)

{

constchar*command;

intsts;

if(!PyArg_ParseTuple(args,"s",&command))

returnNULL;

sts=system(command);

returnPy_BuildValue("i",sts);

}

ThereisastraightforwardtranslationfromtheargumentlistinPython(forexample,thesingleexpression

"ls-l"

)totheargumentspassedtotheCfunction.TheCfunctionalwayshastwoarguments,conventionallynamedselfandargs.

C函数总是有两个参数，分别是selfandargs.

TheselfargumentisonlyusedwhentheCfunctionimplementsabuilt-inmethod,notafunction.Intheexample,selfwillalwaysbeaNULLpointer,sincewearedefiningafunction,notamethod.(Thisisdonesothattheinterpreterdoesn'thavetounderstandtwodifferenttypesofCfunctions.)

Self参数在这个例子里是NULL指针

TheargsargumentwillbeapointertoaPythontupleobjectcontainingthearguments.Eachitemofthetuplecorrespondstoanargumentinthecall'sargumentlist.TheargumentsarePythonobjects--inordertodoanythingwiththeminourCfunctionwehavetoconvertthemtoCvalues.ThefunctionPyArg_ParseTuple()inthePythonAPIcheckstheargumenttypesandconvertsthemtoCvalues.ItusesatemplatestringtodeterminetherequiredtypesoftheargumentsaswellasthetypesoftheCvariablesintowhichtostoretheconvertedvalues.Moreaboutthislater.

Args参数
PyArg_ParseTuple()returnstrue(nonzero)ifallargumentshavetherighttypeanditscomponentshavebeenstoredinthevariableswhoseaddressesarepassed.Itreturnsfalse(zero)ifaninvalidargumentlistwaspassed.InthelattercaseitalsoraisesanappropriateexceptionsothecallingfunctioncanreturnNULLimmediately(aswesawintheexample).

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