QGIS-2.8-PyQGISDeveloperCookbook-en

为开发人员准备的文档,涵盖QGIS的多种功能实现和代码。

PyQGISdevelopercookbook

Release2.8

QGISProject

August24,2015

为开发人员准备的文档,涵盖QGIS的多种功能实现和代码。

为开发人员准备的文档,涵盖QGIS的多种功能实现和代码。

Contents

1Introduction

1.1RunPythoncodewhenQGISstarts

1.2PythonConsole...........

1.3PythonPlugins...........

1.4PythonApplications............................................................................................................................................11223

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362LoadingProjects3LoadingLayers3.1VectorLayers.............................................3.2RasterLayers.............................................3.3MapLayerRegistry.....................................................................................................................................................................................................................................................................................................................................................................................................................................................................................4UsingRasterLayers4.1LayerDetails...4.2DrawingStyle..4.3RefreshingLayers4.4QueryValues...5UsingVectorLayers5.1Retrievinginformationsaboutattributes.....5.2Selectingfeatures.................5.3IteratingoverVectorLayer............5.4ModifyingVectorLayers.............5.5ModifyingVectorLayerswithanEditingBuffer5.6UsingSpatialIndex...............5.7WritingVectorLayers..............5.8MemoryProvider.................5.9Appearance(Symbology)ofVectorLayers...5.10FurtherTopics..................6GeometryHandling6.1GeometryConstruction.......................................6.2AccesstoGeometry.........................................6.3GeometryPredicatesandOperations................................7ProjectionsSupport7.1Coordinatereferencesystems....................................7.2Projections..............................................8UsingMapCanvas8.1EmbeddingMapCanvas.......................................8.2UsingMapToolswithCanvas....................................

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8.3

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9RubberBandsandVertexMarkers..................................WritingCustomMapTools.....................................WritingCustomMapCanvasItems.................................37383941

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85MapRenderingandPrinting9.1SimpleRendering..........................................9.2RenderinglayerswithdifferentCRS.................................9.3OutputusingMapComposer.....................................10Expressions,FilteringandCalculatingValues10.1ParsingExpressions.........................................10.2EvaluatingExpressions........................................10.3Examples...............................................11ReadingAndStoringSettings12Communicatingwiththeuser12.1Showingmessages.TheQgsMessageBarclass.........................12.2Showingprogress..........................................12.3Logging................................................13DevelopingPythonPlugins13.1Writingaplugin...........................................13.2Plugincontent............................................13.3Documentation............................................14IDEsettingsforwritinganddebuggingplugins14.1Anoteoncon guringyourIDEonWindows............................14.2DebuggingusingEclipseandPyDev................................14.3DebuggingusingPDB........................................15UsingPluginLayers15.1SubclassingQgsPluginLayer.....................................16CompatibilitywitholderQGISversions16.1Pluginmenu.............................................17Releasingyourplugin17.1Metadataandnames.........................................17.2Codeandhelp............................................17.3Of cialpythonpluginrepository..................................18CodeSnippets18.1Howtocallamethodbyakeyshortcut...............................18.2HowtotoggleLayers........................................18.3Howtoaccessattributetableofselectedfeatures..........................19Networkanalysislibrary19.1Generalinformation.........................................19.2Buildingagraph...........................................19.3Graphanalysis............................................Index

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CHAPTER1Introduction

Thisdocumentisintendedtoworkbothasatutorialandareferenceguide.Whileitdoesnotlistallpossibleuse

cases,itshouldgiveagoodoverviewoftheprincipalfunctionality.

Startingfrom0.9release,QGIShasoptionalscriptingsupportusingPythonlanguage.We’vedecidedforPython

asit’soneofthemostfavouritelanguagesforscripting.PyQGISbindingsdependonSIPandPyQt4.Thereason

forusingSIPinsteadofmorewidelyusedSWIGisthatthewholeQGIScodedependsonQtlibraries.Python

bindingsforQt(PyQt)aredonealsousingSIPandthisallowsseamlessintegrationofPyQGISwithPyQt.

TODO:GettingPyQGIStowork(Manualcompilation,Troubleshooting)

ThereareseveralwayshowtouseQGISpythonbindings,theyarecoveredindetailinthefollowingsections:

automaticallyrunPythoncodewhenQGISstarts

issuecommandsinPythonconsolewithinQGIS

createandusepluginsinPython

createcustomapplicationsbasedonQGISAPI

ThereisacompleteQGISAPIreferencethatdocumentstheclassesfromtheQGISlibraries.PythonicQGISAPI

isnearlyidenticaltotheAPIinC++.

TherearesomeresourcesaboutprogrammingwithPyQGISonQGISblog.SeeQGIStutorialportedtoPythonfor

someexamplesofsimple3rdpartyapps.Agoodresourcewhendealingwithpluginsistodownloadsomeplugins

frompluginrepositoryandexaminetheircode.Also,thepython/plugins/folderinyourQGISinstallation

containssomepluginthatyoucanusetolearnhowtodevelopsuchpluginandhowtoperformsomeofthemost

commontasks

1.1RunPythoncodewhenQGISstarts

TherearetwodistinctmethodstorunPythoncodeeverytimeQGISstarts.

1.1.1PYQGIS_STARTUPenvironmentvariable

YoucanrunPythoncodejustbeforeQGISinitializationcompletesbysettingthePYQGIS_STARTUPenviron-

mentvariabletothepathofanexistingPython le.

Thismethodissomethingyouwillprobablyrarelyneed,butworthmentioningherebecauseitisoneoftheseveral

waystorunPythoncodewithinQGISandbecausethiscodewillrunbeforeQGISinitializationiscomplete.This

methodisveryusefulforcleaningsys.path,whichmayhaveundesireablepaths,orforisolating/loadingtheinitial

environwithoutrequiringavirtenv,e.g.homebreworMacPortsinstallsonMac.

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1.1.2Thestartup.py le

EverytimeQGISstarts,theuser’sPythonhomedirectory(usually:.qgis2/python)issearchedfora le

namedstartup.py,ifthat leexists,itisexecutedbytheembeddedPythoninterpreter.

1.2PythonConsole

Forscripting,itispossibletotakeadvantageofintegratedPythonconsole.Itcanbeopenedfrommenu:Plugins

→PythonConsole.Theconsoleopensasanon-modalutility

window:

Figure1.1:QGISPythonconsole

Thescreenshotaboveillustrateshowtogetthelayercurrentlyselectedinthelayerlist,showitsIDandoptionally,

ifitisavectorlayer,showthefeaturecount.ForinteractionwithQGISenvironment,thereisaifacevariable,

whichisaninstanceofQgsInterface.Thisinterfaceallowsaccesstothemapcanvas,menus,toolbarsand

otherpartsoftheQGISapplication.

Forconvenienceoftheuser,thefollowingstatementsareexecutedwhentheconsoleisstarted(infutureitwillbe

possibletosetfurtherinitialcommands)

Forthosewhichusetheconsoleoften,itmaybeusefultosetashortcutfortriggeringtheconsole(withinmenu

Settings→Con gureshortcuts...)

1.3PythonPlugins

QGISallowsenhancementofitsfunctionalityusingplugins.ThiswasoriginallypossibleonlywithC++language.

WiththeadditionofPythonsupporttoQGIS,itisalsopossibletousepluginswritteninPython.Themain

advantageoverC++pluginsisitssimplicityofdistribution(nocompilingforeachplatformneeded)andeasier

development.

ManypluginscoveringvariousfunctionalityhavebeenwrittensincetheintroductionofPythonsupport.Theplu-

gininstallerallowsuserstoeasilyfetch,upgradeandremovePythonplugins.SeethePythonPluginRepositories

pageforvarioussourcesofplugins.

CreatingpluginsinPythonissimple,seeDevelopingPythonPluginsfordetailedinstructions.

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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

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onWindows:setPATH=C:\qgispath;%PATH%

Thesecommandscanbeputintoabootstrapscriptthatwilltakecareofthestartup.Whendeployingcustom

applicationsusingPyQGIS,thereareusuallytwopossibilities:

requireusertoinstallQGISonhisplatformpriortoinstallingyourapplication.Theapplicationinstaller

shouldlookfordefaultlocationsofQGISlibrariesandallowusertosetthepathifnotfound.Thisapproach

hastheadvantageofbeingsimpler,howeveritrequiresusertodomoresteps.

packageQGIStogetherwithyourapplication.Releasingtheapplicationmaybemorechallengingandthe

packagewillbelarger,buttheuserwillbesavedfromtheburdenofdownloadingandinstallingadditional

piecesofsoftware.

Thetwodeploymentmodelscanbemixed-deploystandaloneapplicationonWindowsandMacOSX,forLinux

leavetheinstallationofQGISuptouserandhispackagemanager.

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CHAPTER2

LoadingProjects

Sometimesyouneedtoloadanexistingprojectfromapluginor(moreoften)whendevelopingastand-alone

QGISPythonapplication(see:PythonApplications).

ToloadaprojectintothecurrentQGISaplicationyouneedaQgsProjectinstance()objectandcallits

read()methodpassingtoitaQFileInfoobjectthatcontainsthepathfromwheretheprojectwillbeloaded:

Incaseyouneedtomakesomemodi cationstotheproject(forexampleaddorremovesomelayers)andsave

yourchanges,youcancallthewrite()methodofyourprojectinstance.Thewrite()methodalsoacceptsan

optionalQFileInfothatallowsyoutospecifyapathwheretheprojectwillbesaved:

Bothread()andwrite()funtionsreturnabooleanvaluethatyoucanusetocheckiftheoperationwas

successful.

为开发人员准备的文档,涵盖QGIS的多种功能实现和代码。

PyQGISdevelopercookbook,Release2.8

为开发人员准备的文档,涵盖QGIS的多种功能实现和代码。

CHAPTER3

LoadingLayers

Let’sopensomelayerswithdata.QGISrecognizesvectorandrasterlayers.Additionally,customlayertypesare

available,butwearenotgoingtodiscussthemhere.

3.1VectorLayers

Toloadavectorlayer,specifylayer’sdatasourceidenti er,nameforthelayerandprovider’sname:

Thedatasourceidenti erisastringanditisspeci yer’snameisusedinthe

layerlistwidget.Itisimportanttocheckwhetherthelayerhasbeenloadedsuccessfully.Ifitwasnot,aninvalid

layerinstanceisreturned.

ThequickestwaytoopenanddisplayavectorlayerinQGISistheaddVectorLayerfunctionofthe

QgisInterface:

,"ogr")Thiscreatesanewlayerandaddsittothemaplayerregistry(makingitappearinthelayerlist)inonestep.The

functionreturnsthelayerinstanceorNoneifthelayercouldn’tbeloaded.

Thefollowinglistshowshowtoaccessvariousdatasourcesusingvectordataproviders:

OGRlibrary(shape lesandmanyother leformats)—data

sourceisthepathtothe le

)

PostGISdatabase—datasourceisastringwithallinformationneededtocreateaconnectiontoPostgreSQL

database.QgsDataSourceURIclasscangeneratethisstringforyou.NotethatQGIShastobecompiled

withPostgressupport,otherwisethisproviderisn’tavailable.

CSVorotherdelimitedtext les—toopena lewithasemicolonasadelimiter,with eld“x”forx-

coordinateand eld“y”withy-coordinateyouwouldusesomethinglikethis

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Note:fromQGISversion1.7theproviderstringisstructuredasaURL,sothepathmustbepre xedwith

le://.AlsoitallowsWKT(wellknowntext)formattedgeometriesasanalternativeto“x”and“y” elds,

andallowsthe

coordinatereferencesystemtobespeci ed.Forexample

"shape") GPX les—the“gpx”dataproviderreadstracks,routesandwaypointsfromgpx les.Toopena le,the

type(track/route/waypoint)needstobespeci edaspartoftheurl

SpatiaLitedatabase—supportedfromQGISv1.1.SimilarlytoPostGISdatabases,QgsDataSourceURI

canbeusedforgenerationofdatasourceidenti er

MySQLWKB-basedgeometries,throughOGR—datasourceistheconnectionstringtothetable

WFSconnection:.theconnectionisde nedwithaURIandusingtheWFSprovider

Theuricanbecreatedusingthestandardurllib

library.

3.2RasterLayers

Foraccessingraster les,GDALlibraryisused.Itsupportsawiderangeof leformats.Incaseyouhavetroubles

withopeningsome les,checkwhetheryourGDALhassupportfortheparticularformat(notallformatsare

availablebydefault).Toloadarasterfroma le,specifyits

lenameandbasename

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Similarlytovectorlayers,rasterlayerscanbeloadedusingtheaddRasterLayerfunctionoftheQgisInterface:

Thiscreatesanewlayerandaddsittothemaplayerregistry(makingitappearinthelayerlist)inonestep.

RasterlayerscanalsobecreatedfromaWCSservice.

detailedURIsettingscanbefoundinproviderdocumentation

AlternativelyyoucanloadarasterlayerfromWMSserver.Howevercurrentlyit’snotpossibletoaccessGetCa-

pabilitiesresponsefromAPI—youhaveto

knowwhatlayersyouwant

3.3MapLayerRegistry

Ifyouwouldliketousetheopenedlayersforrendering,donotforgettoaddthemtomaplayerregistry.Themap

layerregistrytakesownershipoflayersandtheycanbelateraccessedfromanypartoftheapplicationby

their

uniqueID.Whenthelayerisremovedfrommaplayerregistry,itgetsdeleted,too.

Addingalayertotheregistry

Layersaredestroyedautomaticallyonexit,howeverifyouwanttodeletethelayer

explicitly,use

Foralistofloadedlayersandlayerids,use

TODO:Moreaboutmaplayerregistry?

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Chapter 3. Loading Layers

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CHAPTER4

UsingRasterLayers

Thissectionslistsvariousoperationsyoucandowithrasterlayers.

4.1LayerDetails

Arasterlayerconsistsofoneormorerasterbands—itisreferredtoaseithersinglebandormultibandraster.

ualcolorimage(e.g.aerialphoto)isarasterconsistingofred,blueand

greenband.Singlebandlayerstypicallyrepresenteithercontinuousvariables(e.g.elevation)ordiscretevariables

(nduse).Insomecases,arasterlayercomeswithapaletteandrastervaluesrefertocolorsstoredinthe

palette:

4.2DrawingStyle

Whenarasterlayerisloaded,itgetsadefaultdrawingstylebasedonitstype.Itcanbealteredeitherinraster

layerpropertiesorprogrammatically.Thefollowingdrawingstylesexist:

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dex

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9QgsRasterLater.XSingleBandPseudoColorPalettedColorPalettedSingleBandGrayPalettedSingleBandPseudo-ColorMultiBandSingleBandGrayMultiBandSingle-BandPseudoColorMultiBandColor

Singlebandimagedrawnusingapseudocoloralgorithm“Palette”imagedrawnusingcolortable“Palette”layerdrawningrayscale“Palette”layerdrawnusingapseudocoloralgorithmLayercontaining2ormorebands,butasinglebanddrawnasarangeofgraycolorsLayercontaining2ormorebands,butasinglebanddrawnusingapseudocoloralgorithmLayercontaining2ormorebands,mappedtoRGBcolorspace.

Toquerythecurrentdrawingstyle:

Singlebandrasterlayerscanbedrawneitheringraycolors(lowvalues=black,highvalues=white)orwitha

pseudocoloralgorithmthatassignscolorsforvaluesfromthesingleband.Singlebandrasterswithapalettecan

beadditionallydrawnusingtheirpalette.MultibandlayersaretypicallydrawnbymappingthebandstoRGB

colors.Otherpossibilityistousejustonebandforgrayorpseudocolordrawing.

Thefollowingsectionsexplainhowtoqueryandmodifythelayerdrawingstyle.Afterdoingthechanges,you

mightwanttoforceupdateofmapcanvas,seeRefreshingLayers.

TODO:contrastenhancements,transparency(nodata),userde nedmin/max,bandstatistics

4.2.1SingleBandRasters

Theyarerenderedingraycolorsbydefault.Tochangethedrawingstyletopseudocolor:

ThePseudoColorShaderisabasicshaderthathighlightslowvaluesinblueandhighvaluesinred.Thereis

alsoColorRampShaderwhichmapsthecolorsasspeci edbyitscolormap.Ithasthreemodesof

interpolationofvalues:

linear(INTERPOLATED):resultingcolorislinearlyinterpolatedfromthecolormapentriesaboveand

belowtheactualpixelvalue

discrete(DISCRETE):colorisusedfromthecolormapentrywithequalorhighervalue

exact(EXACT):colorisnotinterpolated,onlythepixelswithvalueequaltocolormapentriesaredrawn

Tosetaninterpolatedcolorrampshaderrangingfromgreentoyellowcolor(forpixelvaluesfrom0to255):

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Toreturnbacktodefaultgraylevels,use:

4.2.2MultiBandRasters

Bydefault,QGISmapsthe rstthreebandstored,greenandbluevaluestocreateacolorimage(thisisthe

MultiBandColordrawingstyle.Insomecasesyoumightwanttooverridethesesetting.Thefollowingcode

interchangesredband(1)andgreenband(2):

4.3RefreshingLayers

Ifyoudochangelayersymbologyandwouldlikeensurethatthechangesareimmediatelyvisibletotheuser,call

these

methods

The rstcallwillensurethatthecachedimageofrenderedlayeriserasedincaserendercachingisturnedon.

ThisfunctionalityisavailablefromQGIS1.4,inpreviousversionsthisfunctiondoesnotexist—tomakesure

thatthecodeworkswithallversionsofQGIS,we rstcheckwhetherthemethodexists.

Thesecondcallemitssignalthatwillforceanymapcanvascontainingthelayertoissuearefresh.

WithWMSrasterlayers,thesecommandsdonotwork.Inthiscase,

youhavetodoitexplicitly

Incaseyouhavechangedlayersymbology(seesectionsaboutrasterandvectorlayersonhowtodothat),you

mightwanttoforceQGIStoupdatethelayersymbologyinthelayerlist(legend)widget.Thiscanbedoneas

follows(ifaceisaninstanceofQgisInterface)

4.4QueryValues

Todoaqueryonvalueofbandsofrasterlayeratsomespeci edpoint

Theresultsmethodinthiscasereturnsa

dictionary,withbandindicesaskeys,andbandvaluesasvalues.

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PyQ

GIS developer cookbook, Release 2.8

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Chapter 4. Using Raster Layers

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CHAPTER5

UsingVectorLayers

Thissectionsummarizesvariousactionsthatcanbedonewithvectorlayers.

5.1Retrievinginformationsaboutattributes

Youcanretrieveinformationsaboutthe eldsassociatedwithavectorlayerbycallingpendingFields()on

aQgsVectorLayerinstance:

5.2Selectingfeatures

InQGISdesktop,featurescanbeselectedindifferentways,theusercanclickonafeature,drawarectangleon

themapcanvasoruseanexpression lter.Selectedfaturesarenormallyhiglightedinadifferentcolor(defaultis

yellow)todrawuser’sattentionontheselection.Sometimescanbeusefultoprogrammaticallyselectfeaturesor

tochangethedefaultcolor.

TochangetheselectioncoloryoucanusesetSelectionColor()methodof

QgsMapCanvasasshownin

thefollowingexample:

Toaddaddfeaturestotheselectedfeatureslistforagivenlayer,youcancallsetSelectedFeatures()

passingtoitthelistoffeaturesIDs:

Tocleartheselection,justpassanemptylist:

5.3IteratingoverVectorLayer

Iteratingoverthefeaturesinavectorlayerisoneofthemostcommontasks.Belowisanexampleofthesimple

basiccodetoperformthistaskandshowingsomeinformationabouteachfeature.thelayervariableisassumed

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tohaveaQgsVectorLayerobject

5.3.1Accessingattributes

Attributescanbereferredtobytheir

name.

Alternatively,attributescanbereferredtobyindex.Thisiswillbeabitfasterthanusingthename.Forexample,

togetthe rstattribute:

5.3.2Iteratingoverselectedfeatures

ifyouonlyneedselectedfeatures,youcanusetheselectedFeatures()methodfromvectorlayer:

AnotheroptionistheProcessingfeatures()method:

Bydefault,thiswilliterateoverallthefeaturesinthelayer,incasethereisnoselection,orovertheselected

featuresotherwise.NotethatthisbehaviorcanbechangedintheProcessingoptionstoignoreselections.

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5.3.3Iteratingoverasubsetoffeatures

Ifyouwanttoiterateoveragivensubsetoffeaturesinalayer,suchasthosewithinagivenarea,youhavetoadd

aQgsFeatureRequestobjecttothegetFeatures()call.Here’sanexample

Ifyouneedanattribute-based lterinstead(orinaddition)ofaspatialonelikeshownintheexampleabove,you

canbuildanQgsExpressionobjectandpassittotheQgsFeatureRequestconstructor.Here’sanexample

Therequestcanbeusedtode nethedataretrievedforeachfeature,sotheiteratorreturnsallfeatures,butreturns

partialdataforeachofthem.

Tip:Ifyouonlyneedasubsetoftheattributesoryoudon’tneedthegeometryinformations,youcansigni cantlyincreasethespeedofthefeaturesrequestbyusingQgsFeatureRequest.NoGeometry agorspecifyinga

subsetofattributes(possiblyempty)likeshownintheexampleabove.

5.4ModifyingVectorLayers

Mostvectordataproviderssupportediting

oflayerdata.Sometimestheysupportjustasubsetofpossibleediting

ethecapabilities()functionto ndoutwhatsetoffunctionalityissupported

Byusinganyofthefollowingmethodsforvectorlayerediting,thechangesaredirectlycommittedtotheunderly-

ingdatastore(a le,databaseetc).Incaseyouwouldliketodoonlytemporarychanges,skiptothenextsection

thatexplainshowtodomodi cationswitheditingbuffer.

Note:IfyouareworkinginsideQGIS(eitherfromtheconsoleorfromaplugin),itmightbenecessarytoforcearedrawofthemapcanvasinordertoseethechangesyou’vedonetothegeometry,tothestyleortotheattributes:

5.4.1AddFeatures

CreatesomeQgsFeatureinstancesandpassalistofthemtoprovider’saddFeatures()method.Itwill

returntwovalues:result(true/false)andlistofaddedfeatures(theirIDissetbythedatastore)

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