6. Using Vector Layers
Hint
The code snippets on this page need the following imports if you’re outside the pyqgis console:
1from qgis.core import (
2 QgsApplication,
3 QgsDataSourceUri,
4 QgsCategorizedSymbolRenderer,
5 QgsClassificationRange,
6 QgsPointXY,
7 QgsProject,
8 QgsExpression,
9 QgsField,
10 QgsFields,
11 QgsFeature,
12 QgsFeatureRequest,
13 QgsFeatureRenderer,
14 QgsGeometry,
15 QgsGraduatedSymbolRenderer,
16 QgsMarkerSymbol,
17 QgsMessageLog,
18 QgsRectangle,
19 QgsRendererCategory,
20 QgsRendererRange,
21 QgsSymbol,
22 QgsVectorDataProvider,
23 QgsVectorLayer,
24 QgsVectorFileWriter,
25 QgsWkbTypes,
26 QgsSpatialIndex,
27 QgsVectorLayerUtils
28)
29
30from qgis.core.additions.edit import edit
31
32from qgis.PyQt.QtGui import (
33 QColor,
34)
This section summarizes various actions that can be done with vector layers.
Most work here is based on the methods of the QgsVectorLayer
class.
6.1. Retrieving information about attributes
You can retrieve information about the fields associated with a vector layer
by calling fields()
on a
QgsVectorLayer
object:
vlayer = QgsVectorLayer("testdata/airports.shp", "airports", "ogr")
for field in vlayer.fields():
print(field.name(), field.typeName())
1ID Integer64
2fk_region Integer64
3ELEV Real
4NAME String
5USE String
The displayField()
and
mapTipTemplate()
methods provide
information on the field and template used in the Display Properties tab.
When you load a vector layer, a field is always chosen by QGIS as the
Display Name
, while the HTML Map Tip
is empty by default. With these
methods you can easily get both:
vlayer = QgsVectorLayer("testdata/airports.shp", "airports", "ogr")
print(vlayer.displayField())
NAME
Note
If you change the Display Name
from a field to an expression, you have to
use displayExpression()
instead of displayField()
.
6.2. 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.
1# "layer" is a QgsVectorLayer instance
2layer = iface.activeLayer()
3features = layer.getFeatures()
4
5for feature in features:
6 # retrieve every feature with its geometry and attributes
7 print("Feature ID: ", feature.id())
8 # fetch geometry
9 # show some information about the feature geometry
10 geom = feature.geometry()
11 geomSingleType = QgsWkbTypes.isSingleType(geom.wkbType())
12 if geom.type() == QgsWkbTypes.PointGeometry:
13 # the geometry type can be of single or multi type
14 if geomSingleType:
15 x = geom.asPoint()
16 print("Point: ", x)
17 else:
18 x = geom.asMultiPoint()
19 print("MultiPoint: ", x)
20 elif geom.type() == QgsWkbTypes.LineGeometry:
21 if geomSingleType:
22 x = geom.asPolyline()
23 print("Line: ", x, "length: ", geom.length())
24 else:
25 x = geom.asMultiPolyline()
26 print("MultiLine: ", x, "length: ", geom.length())
27 elif geom.type() == QgsWkbTypes.PolygonGeometry:
28 if geomSingleType:
29 x = geom.asPolygon()
30 print("Polygon: ", x, "Area: ", geom.area())
31 else:
32 x = geom.asMultiPolygon()
33 print("MultiPolygon: ", x, "Area: ", geom.area())
34 else:
35 print("Unknown or invalid geometry")
36 # fetch attributes
37 attrs = feature.attributes()
38 # attrs is a list. It contains all the attribute values of this feature
39 print(attrs)
40 # for this test only print the first feature
41 break
Feature ID: 1
Point: <QgsPointXY: POINT(7 45)>
[1, 'First feature']
6.3. 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:
1selected_fid = []
2
3# Get the first feature id from the layer
4feature = next(layer.getFeatures())
5if feature:
6 selected_fid.append(feature.id())
7
8# Add that features to the selected list
9layer.select(selected_fid)
To clear the selection:
layer.removeSelection()
6.3.1. Accessing attributes
Attributes can be referred to by their name:
print(feature['name'])
First feature
Alternatively, attributes can be referred to by index. This is a bit faster than using the name. For example, to get the second attribute:
print(feature[1])
First feature
6.3.2. Iterating over selected features
If you only need selected features, you can use the selectedFeatures()
method from the vector layer:
selection = layer.selectedFeatures()
for feature in selection:
# do whatever you need with the feature
pass
6.3.3. 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:
1areaOfInterest = QgsRectangle(450290,400520, 450750,400780)
2
3request = QgsFeatureRequest().setFilterRect(areaOfInterest)
4
5for feature in layer.getFeatures(request):
6 # do whatever you need with the feature
7 pass
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):
print(feature)
<qgis._core.QgsFeature object at 0x7f9b78590948>
<qgis._core.QgsFeature object at 0x7faef5881670>
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.
1# Only return selected fields to increase the "speed" of the request
2request.setSubsetOfAttributes([0,2])
3
4# More user friendly version
5request.setSubsetOfAttributes(['name','id'],layer.fields())
6
7# Don't return geometry objects to increase the "speed" of the request
8request.setFlags(QgsFeatureRequest.NoGeometry)
9
10# Fetch only the feature with id 45
11request.setFilterFid(45)
12
13# The options may be chained
14request.setFilterRect(areaOfInterest).setFlags(QgsFeatureRequest.NoGeometry).setFilterFid(45).setSubsetOfAttributes([0,2])
6.4. 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')
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:
1caps_string = layer.dataProvider().capabilitiesString()
2# Print:
3# 'Add Features, Delete Features, Change Attribute Values, Add Attributes,
4# Delete Attributes, Rename Attributes, Fast Access to Features at ID,
5# Presimplify Geometries, Presimplify Geometries with Validity Check,
6# 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.
Note
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:
1# If caching is enabled, a simple canvas refresh might not be sufficient
2# to trigger a redraw and you must clear the cached image for the layer
3if iface.mapCanvas().isCachingEnabled():
4 layer.triggerRepaint()
5else:
6 iface.mapCanvas().refresh()
6.4.1. Add Features
Create some QgsFeature
instances and pass a list of them to the provider
QgsVectorDataProvider
addFeatures()
method. It will return two values:
result (True
or 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.
1if caps & QgsVectorDataProvider.AddFeatures:
2 feat = QgsFeature(layer.fields())
3 feat.setAttributes([0, 'hello'])
4 # Or set a single attribute by key or by index:
5 feat.setAttribute('name', 'hello')
6 feat.setAttribute(0, 'hello')
7 feat.setGeometry(QgsGeometry.fromPointXY(QgsPointXY(123, 456)))
8 (res, outFeats) = layer.dataProvider().addFeatures([feat])
6.4.2. Delete Features
To delete some features, just provide a list of their feature IDs.
if caps & QgsVectorDataProvider.DeleteFeatures:
res = layer.dataProvider().deleteFeatures([5, 10])
6.4.3. 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.
1fid = 100 # ID of the feature we will modify
2
3if caps & QgsVectorDataProvider.ChangeAttributeValues:
4 attrs = { 0 : "hello", 1 : 123 }
5 layer.dataProvider().changeAttributeValues({ fid : attrs })
6
7if caps & QgsVectorDataProvider.ChangeGeometries:
8 geom = QgsGeometry.fromPointXY(QgsPointXY(111,222))
9 layer.dataProvider().changeGeometryValues({ fid : geom })
Tip
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.).
6.4.4. 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 committed, 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.
1from qgis.PyQt.QtCore import QMetaType
2
3feat1 = feat2 = QgsFeature(layer.fields())
4fid = 99
5feat1.setId(fid)
6
7# add two features (QgsFeature instances)
8layer.addFeatures([feat1,feat2])
9# delete a feature with specified ID
10layer.deleteFeature(fid)
11
12# set new geometry (QgsGeometry instance) for a feature
13geometry = QgsGeometry.fromWkt("POINT(7 45)")
14layer.changeGeometry(fid, geometry)
15# update an attribute with given field index (int) to a given value
16fieldIndex =1
17value ='My new name'
18layer.changeAttributeValue(fid, fieldIndex, value)
19
20# add new field
21layer.addAttribute(QgsField("mytext", QMetaType.Type.QString))
22# remove a field
23layer.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:
1layer.beginEditCommand("Feature triangulation")
2
3# ... call layer's editing methods ...
4
5if problem_occurred:
6 layer.destroyEditCommand()
7 # ... tell the user that there was a problem
8 # and return
9
10# ... more editing ...
11
12layer.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.
6.4.5. 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.
1from qgis.PyQt.QtCore import QMetaType
2
3if caps & QgsVectorDataProvider.AddAttributes:
4 res = layer.dataProvider().addAttributes(
5 [QgsField("mytext", QMetaType.Type.QString),
6 QgsField("myint", QMetaType.Type.Int)])
7
8if caps & QgsVectorDataProvider.DeleteAttributes:
9 res = layer.dataProvider().deleteAttributes([0])
1# Alternate methods for removing fields
2# first create temporary fields to be removed (f1-3)
3layer.dataProvider().addAttributes([QgsField("f1", QMetaType.Type.Int),
4 QgsField("f2", QMetaType.Type.Int),
5 QgsField("f3", QMetaType.Type.Int)])
6layer.updateFields()
7count=layer.fields().count() # count of layer fields
8ind_list=list((count-3, count-2)) # create list
9
10# remove a single field with an index
11layer.dataProvider().deleteAttributes([count-1])
12
13# remove multiple fields with a list of indices
14layer.dataProvider().deleteAttributes(ind_list)
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()
Tip
Directly save changes using with
based command
Using with edit(layer):
the changes will be committed automatically
calling commitChanges()
at the end. If any exception occurs, it will
rollBack()
all the changes. See Modifying Vector Layers with an Editing Buffer.
6.5. 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’sgetFeatures()
method.index.addFeature(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
1# returns array of feature IDs of five nearest features 2nearest = index.nearestNeighbor(QgsPointXY(25.4, 12.7), 5) 3 4# returns array of IDs of features which intersect the rectangle 5intersect = index.intersects(QgsRectangle(22.5, 15.3, 23.1, 17.2))
You can also use the QgsSpatialIndexKDBush
spatial index. This index is similar to the standard QgsSpatialIndex
but:
supports only single point features
is static (no additional features can be added to the index after the construction)
is much faster!
allows direct retrieval of the original feature’s points, without requiring additional feature requests
supports true distance based searches, i.e. return all points within a radius from a search point
6.6. The QgsVectorLayerUtils class
The QgsVectorLayerUtils
class contains
some very useful methods that you can use with vector layers.
For example the createFeature()
method prepares a QgsFeature
to be added to
a vector layer keeping all the eventual constraints and default values of each
field:
vlayer = QgsVectorLayer("testdata/airports.shp", "airports", "ogr")
feat = QgsVectorLayerUtils.createFeature(vlayer)
The getValues()
method allows
you to quickly get the values of a field or expression:
1vlayer = QgsVectorLayer("testdata/airports.shp", "airports", "ogr")
2# select only the first feature to make the output shorter
3vlayer.selectByIds([1])
4val = QgsVectorLayerUtils.getValues(vlayer, "NAME", selectedOnly=True)
5print(val)
(['AMBLER'], True)
6.7. 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 towriteAsVectorFormatV3()
which saves the whole vector layer or creating an instance of the class and issue calls to inheritedaddFeature()
. This class supports all the vector formats that GDAL 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 GDAL vector datasets (ogr
), databases (postgres
,spatialite
,mysql
,mssql
) and more (wfs
,gpx
,delimitedtext
…).
6.7.1. From an instance of QgsVectorFileWriter
1# SaveVectorOptions contains many settings for the writer process
2save_options = QgsVectorFileWriter.SaveVectorOptions()
3transform_context = QgsProject.instance().transformContext()
4# Write to a GeoPackage (default)
5error = QgsVectorFileWriter.writeAsVectorFormatV3(layer,
6 "testdata/my_new_file.gpkg",
7 transform_context,
8 save_options)
9if error[0] == QgsVectorFileWriter.NoError:
10 print("success!")
11else:
12 print(error)
1# Write to an ESRI Shapefile format dataset using UTF-8 text encoding
2save_options = QgsVectorFileWriter.SaveVectorOptions()
3save_options.driverName = "ESRI Shapefile"
4save_options.fileEncoding = "UTF-8"
5transform_context = QgsProject.instance().transformContext()
6error = QgsVectorFileWriter.writeAsVectorFormatV3(layer,
7 "testdata/my_new_shapefile",
8 transform_context,
9 save_options)
10if error[0] == QgsVectorFileWriter.NoError:
11 print("success again!")
12else:
13 print(error)
1# Write to an ESRI GDB file
2save_options = QgsVectorFileWriter.SaveVectorOptions()
3save_options.driverName = "FileGDB"
4# if no geometry
5save_options.overrideGeometryType = QgsWkbTypes.Unknown
6save_options.actionOnExistingFile = QgsVectorFileWriter.CreateOrOverwriteLayer
7save_options.layerName = 'my_new_layer_name'
8transform_context = QgsProject.instance().transformContext()
9gdb_path = "testdata/my_example.gdb"
10error = QgsVectorFileWriter.writeAsVectorFormatV3(layer,
11 gdb_path,
12 transform_context,
13 save_options)
14if error[0] == QgsVectorFileWriter.NoError:
15 print("success!")
16else:
17 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:
1LIST_FIELD_NAME = 'xxxx'
2
3class ESRIValueConverter(QgsVectorFileWriter.FieldValueConverter):
4
5 def __init__(self, layer, list_field):
6 QgsVectorFileWriter.FieldValueConverter.__init__(self)
7 self.layer = layer
8 self.list_field_idx = self.layer.fields().indexFromName(list_field)
9
10 def convert(self, fieldIdxInLayer, value):
11 if fieldIdxInLayer == self.list_field_idx:
12 return QgsListFieldFormatter().representValue(layer=vlayer,
13 fieldIndex=self.list_field_idx,
14 config={},
15 cache=None,
16 value=value)
17 else:
18 return value
19
20 def fieldDefinition(self, field):
21 idx = self.layer.fields().indexFromName(field.name())
22 if idx == self.list_field_idx:
23 return QgsField(LIST_FIELD_NAME, QMetaType.Type.QString)
24 else:
25 return self.layer.fields()[idx]
26
27converter = ESRIValueConverter(vlayer, LIST_FIELD_NAME)
28opts = QgsVectorFileWriter.SaveVectorOptions()
29opts.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.
6.7.2. Directly from features
1from qgis.PyQt.QtCore import QMetaType
2
3# define fields for feature attributes. A QgsFields object is needed
4fields = QgsFields()
5fields.append(QgsField("first", QMetaType.Type.Int))
6fields.append(QgsField("second", QMetaType.Type.QString))
7
8""" create an instance of vector file writer, which will create the vector file.
9Arguments:
101. path to new file (will fail if exists already)
112. field map
123. geometry type - from WKBTYPE enum
134. layer's spatial reference (instance of
14 QgsCoordinateReferenceSystem)
155. coordinate transform context
166. save options (driver name for the output file, encoding etc.)
17"""
18
19crs = QgsProject.instance().crs()
20transform_context = QgsProject.instance().transformContext()
21save_options = QgsVectorFileWriter.SaveVectorOptions()
22save_options.driverName = "ESRI Shapefile"
23save_options.fileEncoding = "UTF-8"
24
25writer = QgsVectorFileWriter.create(
26 "testdata/my_new_shapefile.shp",
27 fields,
28 QgsWkbTypes.Point,
29 crs,
30 transform_context,
31 save_options
32)
33
34if writer.hasError() != QgsVectorFileWriter.NoError:
35 print("Error when creating shapefile: ", writer.errorMessage())
36
37# add a feature
38fet = QgsFeature()
39
40fet.setGeometry(QgsGeometry.fromPointXY(QgsPointXY(10,10)))
41fet.setAttributes([1, "text"])
42writer.addFeature(fet)
43
44# delete the writer to flush features to disk
45del writer
6.7.3. 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
1from qgis.PyQt.QtCore import QMetaType
2
3# create layer
4vl = QgsVectorLayer("Point", "temporary_points", "memory")
5pr = vl.dataProvider()
6
7# add fields
8pr.addAttributes([QgsField("name", QMetaType.Type.QString),
9 QgsField("age", QMetaType.Type.Int),
10 QgsField("size", QMetaType.Type.Double)])
11vl.updateFields() # tell the vector layer to fetch changes from the provider
12
13# add a feature
14fet = QgsFeature()
15fet.setGeometry(QgsGeometry.fromPointXY(QgsPointXY(10,10)))
16fet.setAttributes(["Johnny", 2, 0.3])
17pr.addFeatures([fet])
18
19# update layer's extent when new features have been added
20# because change of extent in provider is not propagated to the layer
21vl.updateExtents()
Finally, let’s check whether everything went well
1# show some stats
2print("fields:", len(pr.fields()))
3print("features:", pr.featureCount())
4e = vl.extent()
5print("extent:", e.xMinimum(), e.yMinimum(), e.xMaximum(), e.yMaximum())
6
7# iterate over features
8features = vl.getFeatures()
9for fet in features:
10 print("F:", fet.id(), fet.attributes(), fet.geometry().asPoint())
fields: 3
features: 1
extent: 10.0 10.0 10.0 10.0
F: 1 ['Johnny', 2, 0.3] <QgsPointXY: POINT(10 10)>
6.8. 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())
Type: singleSymbol
There are several known renderer types available in the QGIS core library:
Type |
Class |
Description |
---|---|---|
singleSymbol |
|
Renders all features with the same symbol |
categorizedSymbol |
|
Renders features using a different symbol for each category |
graduatedSymbol |
|
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())
['nullSymbol', 'singleSymbol', 'categorizedSymbol', 'graduatedSymbol', 'RuleRenderer', 'pointDisplacement', 'pointCluster', 'mergedFeatureRenderer', 'invertedPolygonRenderer', 'heatmapRenderer', '25dRenderer', 'embeddedSymbol']
It is possible to obtain a dump of a renderer contents in text form — can be useful for debugging
renderer.dump()
SINGLE: MARKER SYMBOL (1 layers) color 190,207,80,255
6.8.1. 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())
{'angle': '0', 'cap_style': 'square', 'color': '255,0,0,255,rgb:1,0,0,1', 'horizontal_anchor_point': '1', 'joinstyle': 'bevel', 'name': 'square', 'offset': '0,0', 'offset_map_unit_scale': '3x:0,0,0,0,0,0', 'offset_unit': 'MM', 'outline_color': '35,35,35,255,rgb:0.13725490196078433,0.13725490196078433,0.13725490196078433,1', 'outline_style': 'solid', 'outline_width': '0', 'outline_width_map_unit_scale': '3x:0,0,0,0,0,0', 'outline_width_unit': 'MM', 'scale_method': 'diameter', 'size': '2', 'size_map_unit_scale': '3x:0,0,0,0,0,0', 'size_unit': 'MM', 'vertical_anchor_point': '1'}
This can be useful if you want to alter some properties:
1# You can alter a single property...
2layer.renderer().symbol().symbolLayer(0).setSize(3)
3# ... but not all properties are accessible from methods,
4# you can also replace the symbol completely:
5props = layer.renderer().symbol().symbolLayer(0).properties()
6props['color'] = 'yellow'
7props['name'] = 'square'
8layer.renderer().setSymbol(QgsMarkerSymbol.createSimple(props))
9# show the changes
10layer.triggerRepaint()
6.8.2. 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
1categorized_renderer = QgsCategorizedSymbolRenderer()
2# Add a few categories
3cat1 = QgsRendererCategory('1', QgsMarkerSymbol(), 'category 1')
4cat2 = QgsRendererCategory('2', QgsMarkerSymbol(), 'category 2')
5categorized_renderer.addCategory(cat1)
6categorized_renderer.addCategory(cat2)
7
8for cat in categorized_renderer.categories():
9 print("{}: {} :: {}".format(cat.value(), cat.label(), cat.symbol()))
1: category 1 :: <qgis._core.QgsMarkerSymbol object at 0x7f378ffcd9d8>
2: category 2 :: <qgis._core.QgsMarkerSymbol object at 0x7f378ffcd9d8>
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.
6.8.3. 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
1graduated_renderer = QgsGraduatedSymbolRenderer()
2# Add a few categories
3graduated_renderer.addClassRange(QgsRendererRange(QgsClassificationRange('class 0-100', 0, 100), QgsMarkerSymbol()))
4graduated_renderer.addClassRange(QgsRendererRange(QgsClassificationRange('class 101-200', 101, 200), QgsMarkerSymbol()))
5
6for ran in graduated_renderer.ranges():
7 print("{} - {}: {} {}".format(
8 ran.lowerValue(),
9 ran.upperValue(),
10 ran.label(),
11 ran.symbol()
12 ))
0.0 - 100.0: class 0-100 <qgis._core.QgsMarkerSymbol object at 0x7f8bad281b88>
101.0 - 200.0: class 101-200 <qgis._core.QgsMarkerSymbol object at 0x7f8bad281b88>
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)
1from qgis.PyQt import QtGui
2
3myVectorLayer = QgsVectorLayer("testdata/airports.shp", "airports", "ogr")
4myTargetField = 'ELEV'
5myRangeList = []
6myOpacity = 1
7# Make our first symbol and range...
8myMin = 0.0
9myMax = 50.0
10myLabel = 'Group 1'
11myColour = QtGui.QColor('#ffee00')
12mySymbol1 = QgsSymbol.defaultSymbol(myVectorLayer.geometryType())
13mySymbol1.setColor(myColour)
14mySymbol1.setOpacity(myOpacity)
15myRange1 = QgsRendererRange(myMin, myMax, mySymbol1, myLabel)
16myRangeList.append(myRange1)
17#now make another symbol and range...
18myMin = 50.1
19myMax = 100
20myLabel = 'Group 2'
21myColour = QtGui.QColor('#00eeff')
22mySymbol2 = QgsSymbol.defaultSymbol(
23 myVectorLayer.geometryType())
24mySymbol2.setColor(myColour)
25mySymbol2.setOpacity(myOpacity)
26myRange2 = QgsRendererRange(myMin, myMax, mySymbol2, myLabel)
27myRangeList.append(myRange2)
28myRenderer = QgsGraduatedSymbolRenderer('', myRangeList)
29myClassificationMethod = QgsApplication.classificationMethodRegistry().method("EqualInterval")
30myRenderer.setClassificationMethod(myClassificationMethod)
31myRenderer.setClassAttribute(myTargetField)
32
33myVectorLayer.setRenderer(myRenderer)
6.8.4. Working with Symbols
For representation of symbols, there is QgsSymbol
base class with
three derived classes:
QgsMarkerSymbol
— for point featuresQgsLineSymbol
— for line featuresQgsFillSymbol
— 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:
marker_symbol = QgsMarkerSymbol()
for i in range(marker_symbol.symbolLayerCount()):
lyr = marker_symbol.symbolLayer(i)
print("{}: {}".format(i, lyr.layerType()))
0: SimpleMarker
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.
6.8.4.1. 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:
1from qgis.core import QgsSymbolLayerRegistry
2myRegistry = QgsApplication.symbolLayerRegistry()
3myMetadata = myRegistry.symbolLayerMetadata("SimpleFill")
4for item in myRegistry.symbolLayersForType(QgsSymbol.Marker):
5 print(item)
1AnimatedMarker
2EllipseMarker
3FilledMarker
4FontMarker
5GeometryGenerator
6MaskMarker
7RasterMarker
8SimpleMarker
9SvgMarker
10VectorField
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.
6.8.4.2. 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
1from qgis.core import QgsMarkerSymbolLayer
2from qgis.PyQt.QtGui import QColor
3
4class FooSymbolLayer(QgsMarkerSymbolLayer):
5
6 def __init__(self, radius=4.0):
7 QgsMarkerSymbolLayer.__init__(self)
8 self.radius = radius
9 self.color = QColor(255,0,0)
10
11 def layerType(self):
12 return "FooMarker"
13
14 def properties(self):
15 return { "radius" : str(self.radius) }
16
17 def startRender(self, context):
18 pass
19
20 def stopRender(self, context):
21 pass
22
23 def renderPoint(self, point, context):
24 # Rendering depends on whether the symbol is selected (QGIS >= 1.5)
25 color = context.selectionColor() if context.selected() else self.color
26 p = context.renderContext().painter()
27 p.setPen(color)
28 p.drawEllipse(point, self.radius, self.radius)
29
30 def clone(self):
31 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
1from qgis.gui import QgsSymbolLayerWidget
2
3class FooSymbolLayerWidget(QgsSymbolLayerWidget):
4 def __init__(self, parent=None):
5 QgsSymbolLayerWidget.__init__(self, parent)
6
7 self.layer = None
8
9 # setup a simple UI
10 self.label = QLabel("Radius:")
11 self.spinRadius = QDoubleSpinBox()
12 self.hbox = QHBoxLayout()
13 self.hbox.addWidget(self.label)
14 self.hbox.addWidget(self.spinRadius)
15 self.setLayout(self.hbox)
16 self.connect(self.spinRadius, SIGNAL("valueChanged(double)"), \
17 self.radiusChanged)
18
19 def setSymbolLayer(self, layer):
20 if layer.layerType() != "FooMarker":
21 return
22 self.layer = layer
23 self.spinRadius.setValue(layer.radius)
24
25 def symbolLayer(self):
26 return self.layer
27
28 def radiusChanged(self, value):
29 self.layer.radius = value
30 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
1from qgis.core import QgsSymbol, QgsSymbolLayerAbstractMetadata, QgsSymbolLayerRegistry
2
3class FooSymbolLayerMetadata(QgsSymbolLayerAbstractMetadata):
4
5 def __init__(self):
6 super().__init__("FooMarker", "My new Foo marker", QgsSymbol.Marker)
7
8 def createSymbolLayer(self, props):
9 radius = float(props["radius"]) if "radius" in props else 4.0
10 return FooSymbolLayer(radius)
11
12fslmetadata = FooSymbolLayerMetadata()
QgsApplication.symbolLayerRegistry().addSymbolLayerType(fslmetadata)
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.
6.8.5. 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
1import random
2from qgis.core import QgsWkbTypes, QgsSymbol, QgsFeatureRenderer
3
4
5class RandomRenderer(QgsFeatureRenderer):
6 def __init__(self, syms=None):
7 super().__init__("RandomRenderer")
8 self.syms = syms if syms else [
9 QgsSymbol.defaultSymbol(QgsWkbTypes.geometryType(QgsWkbTypes.Point)),
10 QgsSymbol.defaultSymbol(QgsWkbTypes.geometryType(QgsWkbTypes.Point))
11 ]
12
13 def symbolForFeature(self, feature, context):
14 return random.choice(self.syms)
15
16 def startRender(self, context, fields):
17 super().startRender(context, fields)
18 for s in self.syms:
19 s.startRender(context, fields)
20
21 def stopRender(self, context):
22 super().stopRender(context)
23 for s in self.syms:
24 s.stopRender(context)
25
26 def usedAttributes(self, context):
27 return []
28
29 def clone(self):
30 return RandomRenderer(self.syms)
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
1from qgis.gui import QgsRendererWidget, QgsColorButton
2
3
4class RandomRendererWidget(QgsRendererWidget):
5 def __init__(self, layer, style, renderer):
6 super().__init__(layer, style)
7 if renderer is None or renderer.type() != "RandomRenderer":
8 self.r = RandomRenderer()
9 else:
10 self.r = renderer
11 # setup UI
12 self.btn1 = QgsColorButton()
13 self.btn1.setColor(self.r.syms[0].color())
14 self.vbox = QVBoxLayout()
15 self.vbox.addWidget(self.btn1)
16 self.setLayout(self.vbox)
17 self.btn1.colorChanged.connect(self.setColor1)
18
19 def setColor1(self):
20 color = self.btn1.color()
21 if not color.isValid(): return
22 self.r.syms[0].setColor(color)
23
24 def renderer(self):
25 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
1from qgis.core import (
2 QgsRendererAbstractMetadata,
3 QgsRendererRegistry,
4 QgsApplication
5)
6
7class RandomRendererMetadata(QgsRendererAbstractMetadata):
8
9 def __init__(self):
10 super().__init__("RandomRenderer", "Random renderer")
11
12 def createRenderer(self, element):
13 return RandomRenderer()
14
15 def createRendererWidget(self, layer, style, renderer):
16 return RandomRendererWidget(layer, style, renderer)
17
18rrmetadata = RandomRendererMetadata()
QgsApplication.rendererRegistry().addRenderer(rrmetadata)
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).
6.9. Further Topics
TODO:
creating/modifying symbols
working with style (
QgsStyle
)working with color ramps (
QgsColorRamp
)exploring symbol layer and renderer registries