QGIS-2.8-PyQGISDeveloperCookbook-en(7)

为开发人员准备的文档,涵盖QGIS的多种功能实现和代码。

1.4PythonApplications

OftenwhenprocessingsomeGISdata,itishandytocreatesomescriptsforautomatingtheprocessinsteadof

doingthesametaskagainandagain.WithPyQGIS,thisisperfectlypossible—importtheqgis.coremodule,

initializeitandyouarereadyfortheprocessing.

OryoumaywanttocreateaninteractiveapplicationthatusessomeGISfunctionality—measuresomedata,

exportamapinPDForanyotherfunctionality.Theqgis.guimoduleadditionallybringsvariousGUIcompo-

nents,mostnotablythemapcanvaswidgetthatcanbeveryeasilyincorporatedintotheapplicationwithsupport

forzooming,panningand/oranyfurthercustommaptools.

1.4.1UsingPyQGISincustomapplication

Note:donotuseqgis.pyasanameforyourtestscript—Pythonwillnotbeabletoimportthebindingsasthe

script’snamewillshadowthem.

Firstofallyouhavetoimportqgismodule,setQGISpathwheretosearchforresources—databaseofprojections,

providersetc.Whenyousetpre xpathwithsecondargumentsetasTrue,QGISwillinitializeallpathswith

standarddirunderthepre xdirectory.CallinginitQgis()functionisimportanttoletQGISsearchforthe

availableproviders.

NowyoucanworkwithQGISAPI—loadlayersanddosomeprocessingor reupaGUIwithamapcanvas.

Thepossibilitiesareendless:-)

Whenyouaredonewithusing

QGISlibrary,callexitQgis()tomakesurethateverythingiscleanedup(e.g.

clearmaplayerregistryanddeletelayers):

1.4.2RunningCustomApplications

YouwillneedtotellyoursystemwheretosearchforQGISlibrariesandappropriate

Pythonmodulesiftheyare

notinawell-knownlocation—otherwisePythonwillcomplain:

Thiscanbe xedbysettingthePYTHONPATHenvironmentvariable.Inthefollowingcommands,qgispath

shouldbereplacedwithyouractualQGISinstallationpath:

onLinux:exportPYTHONPATH=/qgispath/share/qgis/python

onWindows:setPYTHONPATH=c:\qgispath\python

ThepathtothePyQGISmodulesisnowknown,howevertheydependonqgis_coreandqgis_guilibraries

(thePythonmodulesserveonlyaswrappers).Pathtotheselibrariesistypicallyunknownfortheoperatingsystem,

soyougetanimporterroragain

(themessagemightvarydependingonthesystem):

FixthisbyaddingthedirectorieswheretheQGISlibrariesresidetosearchpathofthedynamiclinker:

onLinux:exportLD_LIBRARY_PATH=/qgispath/lib