벡터 레이어 사용¶

Retrieving information about attributes¶

You can retrieve information about the fields associated with a vector layer by calling pendingFields() on a QgsVectorLayer instance:

# "layer" is a QgsVectorLayer instance
for field in layer.pendingFields():
    print field.name(), field.typeName()

피처 선택¶

In QGIS desktop, features can be selected in different ways, the user can click on a feature, draw a rectangle on the map canvas or use an expression filter. Selected features are normally highlighted in a different color (default is yellow) to draw user’s attention on the selection. Sometimes can be useful to programmatically select features or to change the default color.

To change the selection color you can use setSelectionColor() method of QgsMapCanvas as shown in the following example:

iface.mapCanvas().setSelectionColor( QColor("red") )

To add add features to the selected features list for a given layer, you can call setSelectedFeatures() passing to it the list of features IDs:

# Get the active layer (must be a vector layer)
layer = iface.activeLayer()
# Get the first feature from the layer
feature = layer.getFeatures().next()
# Add this features to the selected list
layer.setSelectedFeatures([feature.id()])

To clear the selection, just pass an empty list:

layer.setSelectedFeatures([])

벡터 레이어 상의 반복 작업¶

Iterating over the features in a vector layer is one of the most common tasks. Below is an example of the simple basic code to perform this task and showing some information about each feature. the layer variable is assumed to have a QgsVectorLayer object

iter = layer.getFeatures()
for feature in iter:
    # retrieve every feature with its geometry and attributes
    # fetch geometry
    geom = feature.geometry()
    print "Feature ID %d: " % feature.id()

    # show some information about the feature
    if geom.type() == QGis.Point:
        x = geom.asPoint()
        print "Point: " + str(x)
    elif geom.type() == QGis.Line:
        x = geom.asPolyline()
        print "Line: %d points" % len(x)
    elif geom.type() == QGis.Polygon:
        x = geom.asPolygon()
        numPts = 0
        for ring in x:
            numPts += len(ring)
        print "Polygon: %d rings with %d points" % (len(x), numPts)
    else:
        print "Unknown"

    # fetch attributes
    attrs = feature.attributes()

    # attrs is a list. It contains all the attribute values of this feature
    print attrs

print feature['name']

print feature[0]

selection = layer.selectedFeatures()
print len(selection)
for feature in selection:
    # do whatever you need with the feature

import processing
features = processing.features(layer)
for feature in features:
    # do whatever you need with the feature

request = QgsFeatureRequest()
request.setFilterRect(areaOfInterest)
for feature in layer.getFeatures(request):
    # do whatever you need with the feature

request = QgsFeatureRequest()
request.setLimit(2)
for feature in layer.getFeatures(request):
    # loop through only 2 features

# The expression will filter the features where the field "location_name"
# contains the word "Lake" (case insensitive)
exp = QgsExpression('location_name ILIKE \'%Lake%\'')
request = QgsFeatureRequest(exp)

# Only return selected fields
request.setSubsetOfAttributes([0,2])
# More user friendly version
request.setSubsetOfAttributes(['name','id'],layer.pendingFields())
# Don't return geometry objects
request.setFlags(QgsFeatureRequest.NoGeometry)

벡터 레이어 수정¶

Most vector data providers support editing of layer data. Sometimes they support just a subset of possible editing actions. Use the capabilities() function to find out what set of functionality is supported

caps = layer.dataProvider().capabilities()
# Check if a particular capability is supported:
caps & QgsVectorDataProvider.DeleteFeatures
# Print 2 if DeleteFeatures is supported

For a list of all available capabilities, please refer to the API Documentation of QgsVectorDataProvider

To print layer’s capabilities textual description in a comma separated list you can use capabilitiesString() as in the following example:

caps_string = layer.dataProvider().capabilitiesString()
# Print:
# u'Add Features, Delete Features, Change Attribute Values,
# Add Attributes, Delete Attributes, Create Spatial Index,
# Fast Access to Features at ID, Change Geometries,
# Simplify Geometries with topological validation'

벡터 레이어 편집을 위한 다음 방법 가운데 어떤 것을 사용하든, 변경 사항은 기저 데이터 저장소(파일, 데이터베이스 등등)에 직접 반영됩니다. 일시적으로만 변경하고자 할 경우, 편집 버퍼로 수정 하는 방법을 설명하는 다음 단계로 넘어가십시오.

# If caching is enabled, a simple canvas refresh might not be sufficient
# to trigger a redraw and you must clear the cached image for the layer
if iface.mapCanvas().isCachingEnabled():
    layer.setCacheImage(None)
else:
    iface.mapCanvas().refresh()

if caps & QgsVectorDataProvider.AddFeatures:
    feat = QgsFeature(layer.pendingFields())
    feat.setAttributes([0, 'hello'])
    # Or set a single attribute by key or by index:
    feat.setAttribute('name', 'hello')
    feat.setAttribute(0, 'hello')
    feat.setGeometry(QgsGeometry.fromPoint(QgsPoint(123, 456)))
    (res, outFeats) = layer.dataProvider().addFeatures([feat])

if caps & QgsVectorDataProvider.DeleteFeatures:
    res = layer.dataProvider().deleteFeatures([5, 10])

fid = 100   # ID of the feature we will modify

if caps & QgsVectorDataProvider.ChangeAttributeValues:
    attrs = { 0 : "hello", 1 : 123 }
    layer.dataProvider().changeAttributeValues({ fid : attrs })

if caps & QgsVectorDataProvider.ChangeGeometries:
    geom = QgsGeometry.fromPoint(QgsPoint(111,222))
    layer.dataProvider().changeGeometryValues({ fid : geom })

from PyQt4.QtCore import QVariant

if caps & QgsVectorDataProvider.AddAttributes:
    res = layer.dataProvider().addAttributes(
        [QgsField("mytext", QVariant.String),
        QgsField("myint", QVariant.Int)])

if caps & QgsVectorDataProvider.DeleteAttributes:
    res = layer.dataProvider().deleteAttributes([0])

layer.updateFields()

편집 버퍼로 벡터 레이어 수정¶

When editing vectors within QGIS application, you have to first start editing mode for a particular layer, then do some modifications and finally commit (or rollback) the changes. All the changes you do are not written until you commit them — they stay in layer’s in-memory editing buffer. It is possible to use this functionality also programmatically — it is just another method for vector layer editing that complements the direct usage of data providers. Use this option when providing some GUI tools for vector layer editing, since this will allow user to decide whether to commit/rollback and allows the usage of undo/redo. When committing changes, all changes from the editing buffer are saved to data provider.

To find out whether a layer is in editing mode, use isEditable() — the editing functions work only when the editing mode is turned on. Usage of editing functions

from PyQt4.QtCore import QVariant

# add two features (QgsFeature instances)
layer.addFeatures([feat1,feat2])
# delete a feature with specified ID
layer.deleteFeature(fid)

# set new geometry (QgsGeometry instance) for a feature
layer.changeGeometry(fid, geometry)
# update an attribute with given field index (int) to given value (QVariant)
layer.changeAttributeValue(fid, fieldIndex, value)

# add new field
layer.addAttribute(QgsField("mytext", QVariant.String))
# remove a field
layer.deleteAttribute(fieldIndex)

In order to make undo/redo work properly, the above mentioned calls have to be wrapped into undo commands. (If you do not care about undo/redo and want to have the changes stored immediately, then you will have easier work by editing with data provider.) How to use the undo functionality

layer.beginEditCommand("Feature triangulation")

# ... call layer's editing methods ...

if problem_occurred:
  layer.destroyEditCommand()
  return

# ... more editing ...

layer.endEditCommand()

The beginEditCommand() will create an internal “active” command and will record subsequent changes in vector layer. With the call to endEditCommand() the command is pushed onto the undo stack and the user will be able to undo/redo it from GUI. In case something went wrong while doing the changes, the destroyEditCommand() method will remove the command and rollback all changes done while this command was active.

To start editing mode, there is startEditing() method, to stop editing there are commitChanges() and rollBack() — however normally you should not need these methods and leave this functionality to be triggered by the user.

You can also use the with edit(layer)-statement to wrap commit and rollback into a more semantic code block as shown in the example below:

with edit(layer):
  feat = layer.getFeatures().next()
  feat[0] = 5
  layer.updateFeature(feat)

This will automatically call commitChanges() in the end. If any exception occurs, it will rollBack() all the changes. In case a problem is encountered within commitChanges() (when the method returns False) a QgsEditError exception will be raised.

공간 인덱스 사용¶

벡터 레이어에 대해 자주 쿼리를 해야 할 경우 공간 인덱스를 사용하면 코드 실행 속도를 획기적으로 향상시킬 수 있습니다. 여러분이 보간 알고리듬을 작성하는데, 보간값을 계산하기 위해 주어진 위치에서 가장 가까운 포인트 레이어의 포인트 10개를 알아내야 한다고 한번 상상해보십시오. 공간 인덱스가 없다면, QGIS가 그 포인트 10개를 찾는 유일한 방법은 해당 위치에서 모든 포인트까지의 거리를 각각 계산한 다음 그 거리들을 비교하는 것입니다. 특히 이 작업을 몇 군데의 위치에 대해 반복해야 할 경우 시간이 아주 오래 걸릴 수 있습니다. 레이어가 공간 인덱스를 가지고 있다면, 훨씬 효율적으로 작업할 수 있습니다.

공간 인덱스가 없는 레이어를 전화번호가 정렬되지도 색인되지도 않은 전화번호부라고 생각해보십시오. 어떤 사람의 전화번호를 찾으려면 처음부터 그 번호를 찾을 때까지 읽을 수밖에 없습니다.

Spatial indexes are not created by default for a QGIS vector layer, but you can create them easily. This is what you have to do:

• create spatial index — the following code creates an empty index

  index = QgsSpatialIndex()
  

• add features to index — index takes QgsFeature object and adds it to the internal data structure. You can create the object manually or use one from previous call to provider’s nextFeature()

  index.insertFeature(feat)
  

• alternatively, you can load all features of a layer at once using bulk loading

  index = QgsSpatialIndex(layer.getFeatures())
  

• 공간 인덱스가 채워지면 쿼리를 해보십시오.

  # returns array of feature IDs of five nearest features
  nearest = index.nearestNeighbor(QgsPoint(25.4, 12.7), 5)
  
  # returns array of IDs of features which intersect the rectangle
  intersect = index.intersects(QgsRectangle(22.5, 15.3, 23.1, 17.2))
  

Writing Vector Layers¶

You can write vector layer files using QgsVectorFileWriter class. It supports any other kind of vector file that OGR supports (shapefiles, GeoJSON, KML and others).

There are two possibilities how to export a vector layer:

• from an instance of QgsVectorLayer

  error = QgsVectorFileWriter.writeAsVectorFormat(layer, "my_shapes.shp", "CP1250", None, "ESRI Shapefile")
  
  if error == QgsVectorFileWriter.NoError:
      print "success!"
  
  error = QgsVectorFileWriter.writeAsVectorFormat(layer, "my_json.json", "utf-8", None, "GeoJSON")
  if error == QgsVectorFileWriter.NoError:
      print "success again!"
  

  The third parameter specifies output text encoding. Only some drivers need this for correct operation - shapefiles are one of those — however in case you are not using international characters you do not have to care much about the encoding. The fourth parameter that we left as None may specify destination CRS — if a valid instance of QgsCoordinateReferenceSystem is passed, the layer is transformed to that CRS.

  For valid driver names please consult the supported formats by OGR — you should pass the value in the “Code” column as the driver name. Optionally you can set whether to export only selected features, pass further driver-specific options for creation or tell the writer not to create attributes — look into the documentation for full syntax.

• directly from features

  from PyQt4.QtCore import QVariant
  
  # define fields for feature attributes. A QgsFields object is needed
  fields = QgsFields()
  fields.append(QgsField("first", QVariant.Int))
  fields.append(QgsField("second", QVariant.String))
  
  """ create an instance of vector file writer, which will create the vector file.
  Arguments:
  1. path to new file (will fail if exists already)
  2. encoding of the attributes
  3. field map
  4. geometry type - from WKBTYPE enum
  5. layer's spatial reference (instance of
     QgsCoordinateReferenceSystem) - optional
  6. driver name for the output file """
  writer = QgsVectorFileWriter("my_shapes.shp", "CP1250", fields, QGis.WKBPoint, None, "ESRI Shapefile")
  
  if writer.hasError() != QgsVectorFileWriter.NoError:
      print "Error when creating shapefile: ",  w.errorMessage()
  
  # add a feature
  fet = QgsFeature()
  fet.setGeometry(QgsGeometry.fromPoint(QgsPoint(10,10)))
  fet.setAttributes([1, "text"])
  writer.addFeature(fet)
  
  # delete the writer to flush features to disk
  del writer
  

Memory Provider¶

Memory provider is intended to be used mainly by plugin or 3rd party app developers. It does not store data on disk, allowing developers to use it as a fast backend for some temporary layers.

이 제공자는 문자열(string), 정수(int), 더블형 실수(double) 유형의 필드를 지원합니다.

The memory provider also supports spatial indexing, which is enabled by calling the provider’s createSpatialIndex() function. Once the spatial index is created you will be able to iterate over features within smaller regions faster (since it’s not necessary to traverse all the features, only those in specified rectangle).

A memory provider is created by passing "memory" as the provider string to the QgsVectorLayer constructor.

이 생성자 역시 URI 형식으로 지정되며, 지오메트리 유형으로 "Point", "LineString", "Polygon", "MultiPoint", "MultiLineString", 또는 "MultiPolygon" 이 가능합니다.

이 URI로 좌표계, 항목, 메모리기반 제공자의 인덱스 작업도 설정할 수 있습니다. 문법은 다음과 같습니다.

crs=definition

Specifies the coordinate reference system, where definition may be any of the forms accepted by QgsCoordinateReferenceSystem.createFromString()

index=yes

제공자가 공간 인덱스를 사용하도록 설정합니다.

field=name:type(length,precision)

레이어의 속성을 설정합니다. 속성의 명칭은 필수적이며, 선택적으로 유형(정수, 더블형 실수, 문자 스트링), 길이 및 정밀도를 설정할 수 있습니다. 여러 개의 속성 항목을 정의할 수도 있습니다.

다음은 이 모든 옵션들을 포함하는 URI의 예시입니다.

"Point?crs=epsg:4326&field=id:integer&field=name:string(20)&index=yes"

다음은 메모리기반 제공자를 생성하고 값을 채우는 코드의 예시입니다.

from PyQt4.QtCore import QVariant

# create layer
vl = QgsVectorLayer("Point", "temporary_points", "memory")
pr = vl.dataProvider()

# add fields
pr.addAttributes([QgsField("name", QVariant.String),
                    QgsField("age",  QVariant.Int),
                    QgsField("size", QVariant.Double)])
vl.updateFields() # tell the vector layer to fetch changes from the provider

# add a feature
fet = QgsFeature()
fet.setGeometry(QgsGeometry.fromPoint(QgsPoint(10,10)))
fet.setAttributes(["Johny", 2, 0.3])
pr.addFeatures([fet])

# update layer's extent when new features have been added
# because change of extent in provider is not propagated to the layer
vl.updateExtents()

마지막으로, 모든 작업이 성공적이었는지 확인해봅시다.

# show some stats
print "fields:", len(pr.fields())
print "features:", pr.featureCount()
e = layer.extent()
print "extent:", e.xMiniminum(), e.yMinimum(), e.xMaximum(), e.yMaximum()

# iterate over features
f = QgsFeature()
features = vl.getFeatures()
for f in features:
    print "F:", f.id(), f.attributes(), f.geometry().asPoint()

벡터 레이어의 표현(심볼)¶

벡터 레이어를 렌더링할 때, 레이어와 관련된 렌더러 와 심볼 이 데이터의 표현을 결정합니다. 심볼은 피처의 시각적 표현을 그리는 일을 담당하는 클래스이며, 렌더러는 특정 피처에 대해 어떤 심볼을 적용할지 결정합니다.

The renderer for a given layer can obtained as shown below:

renderer = layer.rendererV2()

그리고 이를 참고해 조금 더 나가봅시다.

print "Type:", rendererV2.type()

There are several known renderer types available in QGIS core library:

There might be also some custom renderer types, so never make an assumption there are just these types. You can query QgsRendererV2Registry singleton to find out currently available renderers:

print QgsRendererV2Registry.instance().renderersList()
# Print:
[u'singleSymbol',
u'categorizedSymbol',
u'graduatedSymbol',
u'RuleRenderer',
u'pointDisplacement',
u'invertedPolygonRenderer',
u'heatmapRenderer']

텍스트 형식으로 렌더러의 내용을 덤프받을 수도 있습니다. 디버깅 작업에 유용할 수 있습니다.

print rendererV2.dump()

symbol = QgsMarkerSymbolV2.createSimple({'name': 'square', 'color': 'red'})
layer.rendererV2().setSymbol(symbol)

print layer.rendererV2().symbol().symbolLayers()[0].properties()
# Prints
{u'angle': u'0',
u'color': u'0,128,0,255',
u'horizontal_anchor_point': u'1',
u'name': u'circle',
u'offset': u'0,0',
u'offset_map_unit_scale': u'0,0',
u'offset_unit': u'MM',
u'outline_color': u'0,0,0,255',
u'outline_style': u'solid',
u'outline_width': u'0',
u'outline_width_map_unit_scale': u'0,0',
u'outline_width_unit': u'MM',
u'scale_method': u'area',
u'size': u'2',
u'size_map_unit_scale': u'0,0',
u'size_unit': u'MM',
u'vertical_anchor_point': u'1'}

# You can alter a single property...
layer.rendererV2().symbol().symbolLayer(0).setName('square')
# ... but not all properties are accessible from methods,
# you can also replace the symbol completely:
props = layer.rendererV2().symbol().symbolLayer(0).properties()
props['color'] = 'yellow'
props['name'] = 'square'
layer.rendererV2().setSymbol(QgsMarkerSymbolV2.createSimple(props))

for cat in rendererV2.categories():
    print "%s: %s :: %s" % (cat.value().toString(), cat.label(), str(cat.symbol()))

for ran in rendererV2.ranges():
    print "%f - %f: %s %s" % (
        ran.lowerValue(),
        ran.upperValue(),
        ran.label(),
        str(ran.symbol())
      )

from qgis.core import *

myVectorLayer = QgsVectorLayer(myVectorPath, myName, 'ogr')
myTargetField = 'target_field'
myRangeList = []
myOpacity = 1
# Make our first symbol and range...
myMin = 0.0
myMax = 50.0
myLabel = 'Group 1'
myColour = QtGui.QColor('#ffee00')
mySymbol1 = QgsSymbolV2.defaultSymbol(myVectorLayer.geometryType())
mySymbol1.setColor(myColour)
mySymbol1.setAlpha(myOpacity)
myRange1 = QgsRendererRangeV2(myMin, myMax, mySymbol1, myLabel)
myRangeList.append(myRange1)
#now make another symbol and range...
myMin = 50.1
myMax = 100
myLabel = 'Group 2'
myColour = QtGui.QColor('#00eeff')
mySymbol2 = QgsSymbolV2.defaultSymbol(
     myVectorLayer.geometryType())
mySymbol2.setColor(myColour)
mySymbol2.setAlpha(myOpacity)
myRange2 = QgsRendererRangeV2(myMin, myMax, mySymbol2 myLabel)
myRangeList.append(myRange2)
myRenderer = QgsGraduatedSymbolRendererV2('', myRangeList)
myRenderer.setMode(QgsGraduatedSymbolRendererV2.EqualInterval)
myRenderer.setClassAttribute(myTargetField)

myVectorLayer.setRendererV2(myRenderer)
QgsMapLayerRegistry.instance().addMapLayer(myVectorLayer)

for i in xrange(symbol.symbolLayerCount()):
    lyr = symbol.symbolLayer(i)
    print "%d: %s" % (i, lyr.layerType())

from qgis.core import QgsSymbolLayerV2Registry
myRegistry = QgsSymbolLayerV2Registry.instance()
myMetadata = myRegistry.symbolLayerMetadata("SimpleFill")
for item in myRegistry.symbolLayersForType(QgsSymbolV2.Marker):
    print item

EllipseMarker
FontMarker
SimpleMarker
SvgMarker
VectorField

class FooSymbolLayer(QgsMarkerSymbolLayerV2):

  def __init__(self, radius=4.0):
      QgsMarkerSymbolLayerV2.__init__(self)
      self.radius = radius
      self.color = QColor(255,0,0)

  def layerType(self):
     return "FooMarker"

  def properties(self):
      return { "radius" : str(self.radius) }

  def startRender(self, context):
    pass

  def stopRender(self, context):
      pass

  def renderPoint(self, point, context):
      # Rendering depends on whether the symbol is selected (QGIS >= 1.5)
      color = context.selectionColor() if context.selected() else self.color
      p = context.renderContext().painter()
      p.setPen(color)
      p.drawEllipse(point, self.radius, self.radius)

  def clone(self):
      return FooSymbolLayer(self.radius)

class FooSymbolLayerWidget(QgsSymbolLayerV2Widget):
    def __init__(self, parent=None):
        QgsSymbolLayerV2Widget.__init__(self, parent)

        self.layer = None

        # setup a simple UI
        self.label = QLabel("Radius:")
        self.spinRadius = QDoubleSpinBox()
        self.hbox = QHBoxLayout()
        self.hbox.addWidget(self.label)
        self.hbox.addWidget(self.spinRadius)
        self.setLayout(self.hbox)
        self.connect(self.spinRadius, SIGNAL("valueChanged(double)"), \
            self.radiusChanged)

    def setSymbolLayer(self, layer):
        if layer.layerType() != "FooMarker":
            return
        self.layer = layer
        self.spinRadius.setValue(layer.radius)

    def symbolLayer(self):
        return self.layer

    def radiusChanged(self, value):
        self.layer.radius = value
        self.emit(SIGNAL("changed()"))

class FooSymbolLayerMetadata(QgsSymbolLayerV2AbstractMetadata):

  def __init__(self):
    QgsSymbolLayerV2AbstractMetadata.__init__(self, "FooMarker", QgsSymbolV2.Marker)

  def createSymbolLayer(self, props):
    radius = float(props[QString("radius")]) if QString("radius") in props else 4.0
    return FooSymbolLayer(radius)

  def createSymbolLayerWidget(self):
    return FooSymbolLayerWidget()

QgsSymbolLayerV2Registry.instance().addSymbolLayerType(FooSymbolLayerMetadata())

import random

class RandomRenderer(QgsFeatureRendererV2):
  def __init__(self, syms=None):
    QgsFeatureRendererV2.__init__(self, "RandomRenderer")
    self.syms = syms if syms else [QgsSymbolV2.defaultSymbol(QGis.Point), QgsSymbolV2.defaultSymbol(QGis.Point)]

  def symbolForFeature(self, feature):
    return random.choice(self.syms)

  def startRender(self, context, vlayer):
    for s in self.syms:
      s.startRender(context)

  def stopRender(self, context):
    for s in self.syms:
      s.stopRender(context)

  def usedAttributes(self):
    return []

  def clone(self):
    return RandomRenderer(self.syms)

class RandomRendererWidget(QgsRendererV2Widget):
  def __init__(self, layer, style, renderer):
    QgsRendererV2Widget.__init__(self, layer, style)
    if renderer is None or renderer.type() != "RandomRenderer":
      self.r = RandomRenderer()
    else:
      self.r = renderer
    # setup UI
    self.btn1 = QgsColorButtonV2()
    self.btn1.setColor(self.r.syms[0].color())
    self.vbox = QVBoxLayout()
    self.vbox.addWidget(self.btn1)
    self.setLayout(self.vbox)
    self.connect(self.btn1, SIGNAL("clicked()"), self.setColor1)

  def setColor1(self):
    color = QColorDialog.getColor(self.r.syms[0].color(), self)
    if not color.isValid(): return
    self.r.syms[0].setColor(color);
    self.btn1.setColor(self.r.syms[0].color())

  def renderer(self):
    return self.r

class RandomRendererMetadata(QgsRendererV2AbstractMetadata):
  def __init__(self):
    QgsRendererV2AbstractMetadata.__init__(self, "RandomRenderer", "Random renderer")

  def createRenderer(self, element):
    return RandomRenderer()
  def createRendererWidget(self, layer, style, renderer):
    return RandomRendererWidget(layer, style, renderer)

QgsRendererV2Registry.instance().addRenderer(RandomRendererMetadata())

QgsRendererV2AbstractMetadata.__init__(self,
       "RandomRenderer",
       "Random renderer",
       QIcon(QPixmap("RandomRendererIcon.png", "png")))

남은 이야기들¶

TODO:

• creating/modifying symbols
• working with style (QgsStyleV2)
• working with color ramps (QgsVectorColorRampV2)
• rule-based renderer (see this blogpost)
• exploring symbol layer and renderer registries