Using Vector Layers

This section summarizes various actions that can be done with vector layers.

Most work here is based on the methods of the QgsVectorLayer class.

Retrieving information about attributes

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

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

Iterating over Vector Layer

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.

layer = iface.activeLayer()
features = layer.getFeatures()

for feature in features:
    # retrieve every feature with its geometry and attributes
    print("Feature ID: ", feature.id())
    # fetch geometry
    # show some information about the feature geometry
    geom = feature.geometry()
    geomSingleType = QgsWkbTypes.isSingleType(geom.wkbType())
    if geom.type() == QgsWkbTypes.PointGeometry:
        # the geometry type can be of single or multi type
        if geomSingleType:
            x = geom.asPoint()
            print("Point: ", x)
        else:
            x = geom.asMultiPoint()
            print("MultiPoint: ", x)
    elif geom.type() == QgsWkbTypes.LineGeometry:
        if geomSingleType:
            x = geom.asPolyline()
            print("Line: ", x, "length: ", geom.length())
        else:
            x = geom.asMultiPolyline()
            print("MultiLine: ", x, "length: ", geom.length())
    elif geom.type() == QgsWkbTypes.PolygonGeometry:
        if geomSingleType:
            x = geom.asPolygon()
            print("Polygon: ", x, "Area: ", geom.area())
        else:
            x = geom.asMultiPolygon()
            print("MultiPolygon: ", x, "Area: ", geom.area())
    else:
        print("Unknown or invalid geometry")
    # fetch attributes
    attrs = feature.attributes()
    # attrs is a list. It contains all the attribute values of this feature
    print(attrs)

Selecting features

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 it can be useful to programmatically select features or to change the default color.

To select all the features, the selectAll() method can be used:

# Get the active layer (must be a vector layer)
layer = iface.activeLayer()
layer.selectAll()

To select using an expression, use the selectByExpression() method:

# Assumes that the active layer is points.shp file from the QGIS test suite
# (Class (string) and Heading (number) are attributes in points.shp)
layer = iface.activeLayer()
layer.selectByExpression('"Class"=\'B52\' and "Heading" > 10 and "Heading" <70', QgsVectorLayer.SetSelection)

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

selected_fid = []

# Get the first feature id from the layer
for feature in layer.getFeatures():
    selected_fid.append(feature.id())
    break

# Add these features to the selected list
layer.select(selected_fid)

To clear the selection:

layer.removeSelection()

Accessing attributes

Attributes can be referred to by their name:

print(feature['name'])

Alternatively, attributes can be referred to by index. This is a bit faster than using the name. For example, to get the first attribute:

print(feature[0])

Iterating over selected features

If you only need selected features, you can use the selectedFeatures() method from the vector layer:

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

Iterating over a subset of features

If you want to iterate over a given subset of features in a layer, such as those within a given area, you have to add a QgsFeatureRequest object to the getFeatures() call. Here's an example:

areaOfInterest = QgsRectangle(450290,400520, 450750,400780)

request = QgsFeatureRequest().setFilterRect(areaOfInterest)

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

For the sake of speed, the intersection is often done only using feature’s bounding box. There is however a flag ExactIntersect that makes sure that only intersecting features will be returned:

request = QgsFeatureRequest().setFilterRect(areaOfInterest).setFlags(QgsFeatureRequest.ExactIntersect)

With setLimit() you can limit the number of requested features. Here's an example:

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

If you need an attribute-based filter instead (or in addition) of a spatial one like shown in the examples above, you can build a QgsExpression object and pass it to the QgsFeatureRequest constructor. Here's an example:

# 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)

See Expressions, Filtering and Calculating Values for the details about the syntax supported by QgsExpression.

The request can be used to define the data retrieved for each feature, so the iterator returns all features, but returns partial data for each of them.

# Only return selected fields to increase the "speed" of the request
request.setSubsetOfAttributes([0,2])

# More user friendly version
request.setSubsetOfAttributes(['name','id'],layer.fields())

# Don't return geometry objects to increase the "speed" of the request
request.setFlags(QgsFeatureRequest.NoGeometry)

# Fetch only the feature with id 45
request.setFilterFid(45)

# The options may be chained
request.setFilterRect(areaOfInterest).setFlags(QgsFeatureRequest.NoGeometry).setFilterFid(45).setSubsetOfAttributes([0,2])

Modifying Vector Layers

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:
if caps & QgsVectorDataProvider.DeleteFeatures:
    print('The layer supports DeleteFeatures')

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:
# 'Add Features, Delete Features, Change Attribute Values, Add Attributes,
# Delete Attributes, Rename Attributes, Fast Access to Features at ID,
# Presimplify Geometries, Presimplify Geometries with Validity Check,
# Transactions, Curved Geometries'

By using any of the following methods for vector layer editing, the changes are directly committed to the underlying data store (a file, database etc). In case you would like to do only temporary changes, skip to the next section that explains how to do modifications with editing buffer.

注解

If you are working inside QGIS (either from the console or from a plugin), it might be necessary to force a redraw of the map canvas in order to see the changes you've done to the geometry, to the style or to the attributes:

# 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.triggerRepaint()
else:
    iface.mapCanvas().refresh()

Add Features

Create some QgsFeature instances and pass a list of them to provider's addFeatures() method. It will return two values: result (true/false) and list of added features (their ID is set by the data store).

To set up the attributes of the feature, you can either initialize the feature passing a QgsFields object (you can obtain that from the fields() method of the vector layer) or call initAttributes() passing the number of fields you want to be added.

if caps & QgsVectorDataProvider.AddFeatures:
    feat = QgsFeature(layer.fields())
    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.fromPointXY(QgsPointXY(123, 456)))
    (res, outFeats) = layer.dataProvider().addFeatures([feat])

Delete Features

To delete some features, just provide a list of their feature IDs.

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

Modify Features

It is possible to either change feature's geometry or to change some attributes. The following example first changes values of attributes with index 0 and 1, then it changes the feature's geometry.

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.fromPointXY(QgsPointXY(111,222))
    layer.dataProvider().changeGeometryValues({ fid : geom })

小技巧

Favor QgsVectorLayerEditUtils class for geometry-only edits

If you only need to change geometries, you might consider using the QgsVectorLayerEditUtils which provides some useful methods to edit geometries (translate, insert or move vertex, etc.).

Modifying Vector Layers with an Editing Buffer

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 make 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 changes are commited, all changes from the editing buffer are saved to data provider.

The methods are similar to the ones we have seen in the provider, but they are called on the QgsVectorLayer object instead.

For these methods to work, the layer must be in editing mode. To start the editing mode, use the startEditing() method. To stop editing, use the commitChanges() or rollBack() methods. The first one will commit all your changes to the data source, while the second one will discard them and will not modify the data source at all.

To find out whether a layer is in editing mode, use the isEditable() method.

Here you have some examples that demonstrate how to use these editing methods.

from qgis.PyQt.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 a given value
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.)

Here is how you can 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() method 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.

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 = next(layer.getFeatures())
  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.

Adding and Removing Fields

To add fields (attributes), you need to specify a list of field definitions. For deletion of fields just provide a list of field indexes.

from qgis.PyQt.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])

After adding or removing fields in the data provider the layer's fields need to be updated because the changes are not automatically propagated.

layer.updateFields()

小技巧

Directly save changes using with based command

Using with edit(layer): the changes will be commited automatically calling commitChanges() at the end. If any exception occurs, it will rollBack() all the changes. See Modifying Vector Layers with an Editing Buffer.

Using Spatial Index

Spatial indexes can dramatically improve the performance of your code if you need to do frequent queries to a vector layer. Imagine, for instance, that you are writing an interpolation algorithm, and that for a given location you need to know the 10 closest points from a points layer, in order to use those point for calculating the interpolated value. Without a spatial index, the only way for QGIS to find those 10 points is to compute the distance from each and every point to the specified location and then compare those distances. This can be a very time consuming task, especially if it needs to be repeated for several locations. If a spatial index exists for the layer, the operation is much more effective.

Think of a layer without a spatial index as a telephone book in which telephone numbers are not ordered or indexed. The only way to find the telephone number of a given person is to read from the beginning until you find it.

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 using the QgsSpatialIndex() class:

  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 a previous call to the provider's getFeatures() method.

  index.insertFeature(feat)
  

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

  index = QgsSpatialIndex(layer.getFeatures())
  

• once spatial index is filled with some values, you can do some queries

  # returns array of feature IDs of five nearest features
  nearest = index.nearestNeighbor(QgsPointXY(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))
  

Creating Vector Layers

There are several ways to generate a vector layer dataset:

• the QgsVectorFileWriter class: A convenient class for writing vector files to disk, using either a static call to writeAsVectorFormat() which saves the whole vector layer or creating an instance of the class and issue calls to addFeature(). This class supports all the vector formats that OGR supports (GeoPackage, Shapefile, GeoJSON, KML and others).

• the QgsVectorLayer class: instantiates a data provider that interprets the supplied path (url) of the data source to connect to and access the data. It can be used to create temporary, memory-based layers (memory) and connect to OGR datasets (ogr), databases (postgres, spatialite, mysql, mssql) and more (wfs, gpx, delimitedtext...).

From an instance of QgsVectorFileWriter

# Write to a GeoPackage (default)
error = QgsVectorFileWriter.writeAsVectorFormat(layer,
                                                "/path/to/folder/my_data",
                                                "")
if error[0] == QgsVectorFileWriter.NoError:
    print("success!")

# Write to an ESRI Shapefile format dataset using UTF-8 text encoding
error = QgsVectorFileWriter.writeAsVectorFormat(layer,
                                                "/path/to/folder/my_esridata",
                                                "UTF-8",
                                                driverName="ESRI Shapefile")
if error[0] == QgsVectorFileWriter.NoError:
    print("success again!")

The third (mandatory) parameter specifies output text encoding. Only some drivers need this for correct operation - Shapefile is one of them (other drivers will ignore this parameter). Specifying the correct encoding is important if you are using international (non US-ASCII) characters.

# Write to an ESRI GDB file
opts = QgsVectorFileWriter.SaveVectorOptions()
opts.driverName = "FileGDB"
# if no geometry
opts.overrideGeometryType = QgsWkbTypes.NullGeometry
opts.actionOnExistingFile = QgsVectorFileWriter.ActionOnExistingFile.CreateOrOverwriteLayer
opts.layerName = 'my_new_layer_name'
error = QgsVectorFileWriter.writeAsVectorFormat(layer=vlayer,
                                                fileName=gdb_path,
                                                options=opts)
if error[0] == QgsVectorFileWriter.NoError:
  print("success!")
else:
  print(error)

You can also convert fields to make them compatible with different formats by using the FieldValueConverter. For example, to convert array variable types (e.g. in Postgres) to a text type, you can do the following:

LIST_FIELD_NAME = 'xxxx'

class ESRIValueConverter(QgsVectorFileWriter.FieldValueConverter):

  def __init__(self, layer, list_field):
    QgsVectorFileWriter.FieldValueConverter.__init__(self)
      self.layer = layer
      self.list_field_idx = self.layer.fields().indexFromName(list_field)

  def convert(self, fieldIdxInLayer, value):
    if fieldIdxInLayer == self.list_field_idx:
      return QgsListFieldFormatter().representValue(layer=vlayer,
                                                    fieldIndex=self.list_field_idx,
                                                    config={},
                                                    cache=None,
                                                    value=value)
    else:
      return value

  def fieldDefinition(self, field):
    idx = self.layer.fields().indexFromName(field.name())
    if idx == self.list_field_idx:
      return QgsField(LIST_FIELD_NAME, QVariant.String)
    else:
      return self.layer.fields()[idx]

converter = ESRIValueConverter(vlayer, LIST_FIELD_NAME)
#opts is a QgsVectorFileWriter.SaveVectorOptions as above
opts.fieldValueConverter = converter

A destination CRS may also be specified --- if a valid instance of QgsCoordinateReferenceSystem is passed as the fourth parameter, the layer is transformed to that CRS.

For valid driver names please call the supportedFiltersAndFormats method or 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... There are a number of other (optional) parameters; see the QgsVectorFileWriter documentation for details.

Directly from features

from qgis.PyQt.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", "UTF-8", fields, QgsWkbTypes.Point, driverName="ESRI Shapefile")

if writer.hasError() != QgsVectorFileWriter.NoError:
    print("Error when creating shapefile: ",  w.errorMessage())

# add a feature
fet = QgsFeature()

fet.setGeometry(QgsGeometry.fromPointXY(QgsPointXY(10,10)))
fet.setAttributes([1, "text"])
writer.addFeature(fet)

# delete the writer to flush features to disk
del writer

From an instance of QgsVectorLayer

Among all the data providers supported by the QgsVectorLayer class, let's focus on the memory-based layers. 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.

The provider supports string, int and double fields.

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.

The constructor also takes a URI defining the geometry type of the layer, one of: "Point", "LineString", "Polygon", "MultiPoint", "MultiLineString", "MultiPolygon" or "None".

The URI can also specify the coordinate reference system, fields, and indexing of the memory provider in the URI. The syntax is:

crs=definition

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

index=yes

Specifies that the provider will use a spatial index

field=name:type(length,precision)

Specifies an attribute of the layer. The attribute has a name, and optionally a type (integer, double, or string), length, and precision. There may be multiple field definitions.

The following example of a URI incorporates all these options

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

The following example code illustrates creating and populating a memory provider

from qgis.PyQt.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.fromPointXY(QgsPointXY(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()

Finally, let's check whether everything went well

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

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

Appearance (Symbology) of Vector Layers

When a vector layer is being rendered, the appearance of the data is given by renderer and symbols associated with the layer. Symbols are classes which take care of drawing of visual representation of features, while renderers determine what symbol will be used for a particular feature.

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

renderer = layer.renderer()

And with that reference, let us explore it a bit

print("Type:", renderer.type())

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

Type	類	說明
singleSymbol	QgsSingleSymbolRenderer	Renders all features with the same symbol
categorizedSymbol	QgsCategorizedSymbolRenderer	Renders features using a different symbol for each category
graduatedSymbol	QgsGraduatedSymbolRenderer	Renders features using a different symbol for each range of values

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

print(QgsApplication.rendererRegistry().renderersList())
# Print:
['nullSymbol',
'singleSymbol',
'categorizedSymbol',
'graduatedSymbol',
'RuleRenderer',
'pointDisplacement',
'pointCluster',
'invertedPolygonRenderer',
'heatmapRenderer',
'25dRenderer']

It is possible to obtain a dump of a renderer contents in text form --- can be useful for debugging

print(renderer.dump())

Single Symbol Renderer

You can get the symbol used for rendering by calling symbol() method and change it with setSymbol() method (note for C++ devs: the renderer takes ownership of the symbol.)

You can change the symbol used by a particular vector layer by calling setSymbol() passing an instance of the appropriate symbol instance. Symbols for point, line and polygon layers can be created by calling the createSimple() function of the corresponding classes QgsMarkerSymbol, QgsLineSymbol and QgsFillSymbol.

The dictionary passed to createSimple() sets the style properties of the symbol.

For example you can replace the symbol used by a particular point layer by calling setSymbol() passing an instance of a QgsMarkerSymbol, as in the following code example:

symbol = QgsMarkerSymbol.createSimple({'name': 'square', 'color': 'red'})
layer.renderer().setSymbol(symbol)
# show the change
layer.triggerRepaint()

name indicates the shape of the marker, and can be any of the following:

• circle

• square

• cross

• rectangle

• diamond

• pentagon

• triangle

• equilateral_triangle

• star

• regular_star

• arrow

• filled_arrowhead

• x

To get the full list of properties for the first symbol layer of a symbol instance you can follow the example code:

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

This can be useful if you want to alter some properties:

# You can alter a single property...
layer.renderer().symbol().symbolLayer(0).setSize(3)
# ... but not all properties are accessible from methods,
# you can also replace the symbol completely:
props = layer.renderer().symbol().symbolLayer(0).properties()
props['color'] = 'yellow'
props['name'] = 'square'
layer.renderer().setSymbol(QgsMarkerSymbol.createSimple(props))
# show the changes
layer.triggerRepaint()

Categorized Symbol Renderer

When using a categorized renderer, you can query and set the attribute that is used for classification: use the classAttribute() and setClassAttribute() methods.

To get a list of categories

for cat in renderer.categories():
    print("{}: {} :: {}".format(cat.value(), cat.label(), cat.symbol()))

Where value() is the value used for discrimination between categories, label() is a text used for category description and symbol() method returns the assigned symbol.

The renderer usually stores also original symbol and color ramp which were used for the classification: sourceColorRamp() and sourceSymbol() methods.

Graduated Symbol Renderer

This renderer is very similar to the categorized symbol renderer described above, but instead of one attribute value per class it works with ranges of values and thus can be used only with numerical attributes.

To find out more about ranges used in the renderer

for ran in renderer.ranges():
    print("{} - {}: {} {}".format(
        ran.lowerValue(),
        ran.upperValue(),
        ran.label(),
        ran.symbol()
      ))

you can again use the classAttribute (to find the classification attribute name), sourceSymbol and sourceColorRamp methods. Additionally there is the mode method which determines how the ranges were created: using equal intervals, quantiles or some other method.

If you wish to create your own graduated symbol renderer you can do so as illustrated in the example snippet below (which creates a simple two class arrangement)

from qgis.PyQt import QtGui

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 = QgsSymbol.defaultSymbol(myVectorLayer.geometryType())
mySymbol1.setColor(myColour)
mySymbol1.setOpacity(myOpacity)
myRange1 = QgsRendererRange(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 = QgsSymbol.defaultSymbol(
     myVectorLayer.geometryType())
mySymbol2.setColor(myColour)
mySymbol2.setOpacity(myOpacity)
myRange2 = QgsRendererRange(myMin, myMax, mySymbol2, myLabel)
myRangeList.append(myRange2)
myRenderer = QgsGraduatedSymbolRenderer('', myRangeList)
myRenderer.setMode(QgsGraduatedSymbolRenderer.EqualInterval)
myRenderer.setClassAttribute(myTargetField)

myVectorLayer.setRenderer(myRenderer)
QgsProject.instance().addMapLayer(myVectorLayer)

Working with Symbols

For representation of symbols, there is QgsSymbol base class with three derived classes:

• QgsMarkerSymbol --- for point features

• QgsLineSymbol --- for line features

• QgsFillSymbol --- for polygon features

Every symbol consists of one or more symbol layers (classes derived from QgsSymbolLayer). The symbol layers do the actual rendering, the symbol class itself serves only as a container for the symbol layers.

Having an instance of a symbol (e.g. from a renderer), it is possible to explore it: the type method says whether it is a marker, line or fill symbol. There is a dump method which returns a brief description of the symbol. To get a list of symbol layers:

for i in range(symbol.symbolLayerCount()):
    lyr = symbol.symbolLayer(i)
    print("{}: {}".format(i, lyr.layerType()))

To find out symbol's color use color method and setColor to change its color. With marker symbols additionally you can query for the symbol size and rotation with the size and angle methods. For line symbols the width method returns the line width.

Size and width are in millimeters by default, angles are in degrees.

Working with Symbol Layers

As said before, symbol layers (subclasses of QgsSymbolLayer) determine the appearance of the features. There are several basic symbol layer classes for general use. It is possible to implement new symbol layer types and thus arbitrarily customize how features will be rendered. The layerType() method uniquely identifies the symbol layer class --- the basic and default ones are SimpleMarker, SimpleLine and SimpleFill symbol layers types.

You can get a complete list of the types of symbol layers you can create for a given symbol layer class with the following code:

from qgis.core import QgsSymbolLayerRegistry
myRegistry = QgsApplication.symbolLayerRegistry()
myMetadata = myRegistry.symbolLayerMetadata("SimpleFill")
for item in myRegistry.symbolLayersForType(QgsSymbol.Marker):
    print(item)

Output:

EllipseMarker
FilledMarker
FontMarker
GeometryGenerator
SimpleMarker
SvgMarker
VectorField

The QgsSymbolLayerRegistry class manages a database of all available symbol layer types.

To access symbol layer data, use its properties() method that returns a key-value dictionary of properties which determine the appearance. Each symbol layer type has a specific set of properties that it uses. Additionally, there are the generic methods color, size, angle and width, with their setter counterparts. Of course size and angle are available only for marker symbol layers and width for line symbol layers.

Creating Custom Symbol Layer Types

Imagine you would like to customize the way how the data gets rendered. You can create your own symbol layer class that will draw the features exactly as you wish. Here is an example of a marker that draws red circles with specified radius

from qgis.core import QgsMarkerSymbolLayer
from qgis.PyQt.QtGui import QColor

class FooSymbolLayer(QgsMarkerSymbolLayer):

  def __init__(self, radius=4.0):
      QgsMarkerSymbolLayer.__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)

The layerType method determines the name of the symbol layer; it has to be unique among all symbol layers. The properties method is used for persistence of attributes. The clone method must return a copy of the symbol layer with all attributes being exactly the same. Finally there are rendering methods: startRender is called before rendering the first feature, stopRender when the rendering is done, and renderPoint is called to do the rendering. The coordinates of the point(s) are already transformed to the output coordinates.

For polylines and polygons the only difference would be in the rendering method: you would use renderPolyline which receives a list of lines, while renderPolygon receives a list of points on the outer ring as the first parameter and a list of inner rings (or None) as a second parameter.

Usually it is convenient to add a GUI for setting attributes of the symbol layer type to allow users to customize the appearance: in case of our example above we can let user set circle radius. The following code implements such widget

from qgis.gui import QgsSymbolLayerWidget

class FooSymbolLayerWidget(QgsSymbolLayerWidget):
    def __init__(self, parent=None):
        QgsSymbolLayerWidget.__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()"))

This widget can be embedded into the symbol properties dialog. When the symbol layer type is selected in symbol properties dialog, it creates an instance of the symbol layer and an instance of the symbol layer widget. Then it calls the setSymbolLayer method to assign the symbol layer to the widget. In that method the widget should update the UI to reflect the attributes of the symbol layer. The symbolLayer method is used to retrieve the symbol layer again by the properties dialog to use it for the symbol.

On every change of attributes, the widget should emit the changed() signal to let the properties dialog update the symbol preview.

Now we are missing only the final glue: to make QGIS aware of these new classes. This is done by adding the symbol layer to registry. It is possible to use the symbol layer also without adding it to the registry, but some functionality will not work: e.g. loading of project files with the custom symbol layers or inability to edit the layer's attributes in GUI.

We will have to create metadata for the symbol layer

from qgis.core import QgsSymbol, QgsSymbolLayerAbstractMetadata, QgsSymbolLayerRegistry

class FooSymbolLayerMetadata(QgsSymbolLayerAbstractMetadata):

  def __init__(self):
    QgsSymbolLayerAbstractMetadata.__init__(self, "FooMarker", QgsSymbol.Marker)

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

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

QgsApplication.symbolLayerRegistry().addSymbolLayerType(FooSymbolLayerMetadata())

You should pass layer type (the same as returned by the layer) and symbol type (marker/line/fill) to the constructor of the parent class. The createSymbolLayer() method takes care of creating an instance of symbol layer with attributes specified in the props dictionary. And there is the createSymbolLayerWidget() method which returns the settings widget for this symbol layer type.

The last step is to add this symbol layer to the registry --- and we are done.

Creating Custom Renderers

It might be useful to create a new renderer implementation if you would like to customize the rules how to select symbols for rendering of features. Some use cases where you would want to do it: symbol is determined from a combination of fields, size of symbols changes depending on current scale etc.

The following code shows a simple custom renderer that creates two marker symbols and chooses randomly one of them for every feature

import random
from qgis.core import QgsWkbTypes, QgsSymbol, QgsFeatureRenderer


class RandomRenderer(QgsFeatureRenderer):
  def __init__(self, syms=None):
    QgsFeatureRenderer.__init__(self, "RandomRenderer")
    self.syms = syms if syms else [QgsSymbol.defaultSymbol(QgsWkbTypes.geometryType(QgsWkbTypes.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)

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

The constructor of the parent QgsFeatureRenderer class needs a renderer name (which has to be unique among renderers). The symbolForFeature method is the one that decides what symbol will be used for a particular feature. startRender and stopRender take care of initialization/finalization of symbol rendering. The usedAttributes method can return a list of field names that the renderer expects to be present. Finally, the clone method should return a copy of the renderer.

Like with symbol layers, it is possible to attach a GUI for configuration of the renderer. It has to be derived from QgsRendererWidget. The following sample code creates a button that allows the user to set the first symbol

from qgis.gui import QgsRendererWidget, QgsColorButton

class RandomRendererWidget(QgsRendererWidget):
  def __init__(self, layer, style, renderer):
    QgsRendererWidget.__init__(self, layer, style)
    if renderer is None or renderer.type() != "RandomRenderer":
      self.r = RandomRenderer()
    else:
      self.r = renderer
    # setup UI
    self.btn1 = QgsColorButton()
    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

The constructor receives instances of the active layer (QgsVectorLayer), the global style (QgsStyle) and the current renderer. If there is no renderer or the renderer has different type, it will be replaced with our new renderer, otherwise we will use the current renderer (which has already the type we need). The widget contents should be updated to show current state of the renderer. When the renderer dialog is accepted, the widget's renderer method is called to get the current renderer --- it will be assigned to the layer.

The last missing bit is the renderer metadata and registration in registry, otherwise loading of layers with the renderer will not work and user will not be able to select it from the list of renderers. Let us finish our RandomRenderer example

from qgis.core import QgsRendererAbstractMetadata,QgsRendererRegistry,QgsApplication

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

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

QgsApplication.rendererRegistry().addRenderer(RandomRendererMetadata())

Similarly as with symbol layers, abstract metadata constructor awaits renderer name, name visible for users and optionally name of renderer's icon. The createRenderer method passes a QDomElement instance that can be used to restore the renderer's state from the DOM tree. The createRendererWidget method creates the configuration widget. It does not have to be present or can return None if the renderer does not come with GUI.

To associate an icon with the renderer you can assign it in the QgsRendererAbstractMetadata constructor as a third (optional) argument --- the base class constructor in the RandomRendererMetadata __init__() function becomes

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

The icon can also be associated at any later time using the setIcon method of the metadata class. The icon can be loaded from a file (as shown above) or can be loaded from a Qt resource (PyQt5 includes .qrc compiler for Python).

Further Topics

待辦：

• creating/modifying symbols

• working with style (QgsStyle)

• working with color ramps (QgsColorRamp)

• exploring symbol layer and renderer registries