21. Cheat sheet for PyQGIS

Hint

The code snippets on this page need the following imports if you're outside the pyqgis console:

 1from qgis.PyQt.QtCore import (
 2    QRectF,
 3)
 4
 5from qgis.core import (
 6    QgsProject,
 7    QgsLayerTreeModel,
 8)
 9
10from qgis.gui import (
11    QgsLayerTreeView,
12)

21.1. User Interface

Change Look & Feel

1from qgis.PyQt.QtWidgets import QApplication
2
3app = QApplication.instance()
4app.setStyleSheet(".QWidget {color: blue; background-color: yellow;}")
5# You can even read the stylesheet from a file
6with  open("testdata/file.qss") as qss_file_content:
7    app.setStyleSheet(qss_file_content.read())

Change icon and title

1from qgis.PyQt.QtGui import QIcon
2
3icon = QIcon("/path/to/logo/file.png")
4iface.mainWindow().setWindowIcon(icon)
5iface.mainWindow().setWindowTitle("My QGIS")

21.2. Settings

Get QgsSettings list

1from qgis.core import QgsSettings
2
3qs = QgsSettings()
4
5for k in sorted(qs.allKeys()):
6    print (k)

21.3. Toolbars

Remove toolbar

1toolbar = iface.helpToolBar()
2parent = toolbar.parentWidget()
3parent.removeToolBar(toolbar)
4
5# and add again
6parent.addToolBar(toolbar)

Remove actions toolbar

actions = iface.attributesToolBar().actions()
iface.attributesToolBar().clear()
iface.attributesToolBar().addAction(actions[4])
iface.attributesToolBar().addAction(actions[3])

21.5. Canvas

Access canvas

canvas = iface.mapCanvas()

Change canvas color

from qgis.PyQt.QtCore import Qt

iface.mapCanvas().setCanvasColor(Qt.black)
iface.mapCanvas().refresh()

Map Update interval

from qgis.core import QgsSettings
# Set milliseconds (150 milliseconds)
QgsSettings().setValue("/qgis/map_update_interval", 150)

21.6. Layers

Add vector layer

layer = iface.addVectorLayer("testdata/airports.shp", "layer name you like", "ogr")
if not layer or not layer.isValid():
    print("Layer failed to load!")

Get active layer

layer = iface.activeLayer()

List all layers

from qgis.core import QgsProject

QgsProject.instance().mapLayers().values()

Obtain layers name

1from qgis.core import QgsVectorLayer
2layer = QgsVectorLayer("Point?crs=EPSG:4326", "layer name you like", "memory")
3QgsProject.instance().addMapLayer(layer)
4
5layers_names = []
6for layer in QgsProject.instance().mapLayers().values():
7    layers_names.append(layer.name())
8
9print("layers TOC = {}".format(layers_names))
layers TOC = ['layer name you like']

Otherwise

layers_names = [layer.name() for layer in QgsProject.instance().mapLayers().values()]
print("layers TOC = {}".format(layers_names))
layers TOC = ['layer name you like']

Find layer by name

from qgis.core import QgsProject

layer = QgsProject.instance().mapLayersByName("layer name you like")[0]
print(layer.name())
layer name you like

Set active layer

from qgis.core import QgsProject

layer = QgsProject.instance().mapLayersByName("layer name you like")[0]
iface.setActiveLayer(layer)

Refresh layer at interval

1from qgis.core import QgsProject
2
3layer = QgsProject.instance().mapLayersByName("layer name you like")[0]
4# Set seconds (5 seconds)
5layer.setAutoRefreshInterval(5000)
6# Enable auto refresh
7layer.setAutoRefreshEnabled(True)

Show methods

dir(layer)

Adding new feature with feature form

1from qgis.core import QgsFeature, QgsGeometry
2
3feat = QgsFeature()
4geom = QgsGeometry()
5feat.setGeometry(geom)
6feat.setFields(layer.fields())
7
8iface.openFeatureForm(layer, feat, False)

Adding new feature without feature form

1from qgis.core import QgsGeometry, QgsPointXY, QgsFeature
2
3pr = layer.dataProvider()
4feat = QgsFeature()
5feat.setGeometry(QgsGeometry.fromPointXY(QgsPointXY(10,10)))
6pr.addFeatures([feat])

Get features

for f in layer.getFeatures():
    print (f)
<qgis._core.QgsFeature object at 0x7f45cc64b678>

Get selected features

for f in layer.selectedFeatures():
    print (f)

Get selected features Ids

selected_ids = layer.selectedFeatureIds()
print(selected_ids)

Create a memory layer from selected features Ids

from qgis.core import QgsFeatureRequest

memory_layer = layer.materialize(QgsFeatureRequest().setFilterFids(layer.selectedFeatureIds()))
QgsProject.instance().addMapLayer(memory_layer)

Get geometry

# Point layer
for f in layer.getFeatures():
    geom = f.geometry()
    print ('%f, %f' % (geom.asPoint().y(), geom.asPoint().x()))
10.000000, 10.000000

Move geometry

1from qgis.core import QgsFeature, QgsGeometry
2poly = QgsFeature()
3geom = QgsGeometry.fromWkt("POINT(7 45)")
4geom.translate(1, 1)
5poly.setGeometry(geom)
6print(poly.geometry())
<QgsGeometry: Point (8 46)>

Set the CRS

from qgis.core import QgsProject, QgsCoordinateReferenceSystem

for layer in QgsProject.instance().mapLayers().values():
    layer.setCrs(QgsCoordinateReferenceSystem('EPSG:4326'))

See the CRS

1from qgis.core import QgsProject
2
3for layer in QgsProject.instance().mapLayers().values():
4    crs = layer.crs().authid()
5    layer.setName('{} ({})'.format(layer.name(), crs))

Hide a field column

 1from qgis.core import QgsEditorWidgetSetup
 2
 3def fieldVisibility (layer,fname):
 4    setup = QgsEditorWidgetSetup('Hidden', {})
 5    for i, column in enumerate(layer.fields()):
 6        if column.name()==fname:
 7            layer.setEditorWidgetSetup(idx, setup)
 8            break
 9        else:
10            continue

Layer from WKT

 1from qgis.core import QgsVectorLayer, QgsFeature, QgsGeometry, QgsProject
 2
 3layer = QgsVectorLayer('Polygon?crs=epsg:4326', 'Mississippi', 'memory')
 4pr = layer.dataProvider()
 5poly = QgsFeature()
 6geom = QgsGeometry.fromWkt("POLYGON ((-88.82 34.99,-88.0934.89,-88.39 30.34,-89.57 30.18,-89.73 31,-91.63 30.99,-90.8732.37,-91.23 33.44,-90.93 34.23,-90.30 34.99,-88.82 34.99))")
 7poly.setGeometry(geom)
 8pr.addFeatures([poly])
 9layer.updateExtents()
10QgsProject.instance().addMapLayers([layer])

Load all vector layers from GeoPackage

 1from qgis.core import QgsDataProvider
 2
 3fileName = "testdata/sublayers.gpkg"
 4layer = QgsVectorLayer(fileName, "test", "ogr")
 5subLayers = layer.dataProvider().subLayers()
 6
 7for subLayer in subLayers:
 8    name = subLayer.split(QgsDataProvider.SUBLAYER_SEPARATOR)[1]
 9    uri = "%s|layername=%s" % (fileName, name,)
10    # Create layer
11    sub_vlayer = QgsVectorLayer(uri, name, 'ogr')
12    # Add layer to map
13    QgsProject.instance().addMapLayer(sub_vlayer)

Load tile layer (XYZ-Layer)

1from qgis.core import QgsRasterLayer, QgsProject
2
3def loadXYZ(url, name):
4    rasterLyr = QgsRasterLayer("type=xyz&url=" + url, name, "wms")
5    QgsProject.instance().addMapLayer(rasterLyr)
6
7urlWithParams = 'https://tile.openstreetmap.org/%7Bz%7D/%7Bx%7D/%7By%7D.png&zmax=19&zmin=0&crs=EPSG3857'
8loadXYZ(urlWithParams, 'OpenStreetMap')

Remove all layers

QgsProject.instance().removeAllMapLayers()

Remove all

QgsProject.instance().clear()

21.7. Table of contents

Access checked layers

iface.mapCanvas().layers()

Remove contextual menu

1ltv = iface.layerTreeView()
2mp = ltv.menuProvider()
3ltv.setMenuProvider(None)
4# Restore
5ltv.setMenuProvider(mp)

21.8. Advanced TOC

Root node

 1from qgis.core import QgsVectorLayer, QgsProject, QgsLayerTreeLayer
 2
 3root = QgsProject.instance().layerTreeRoot()
 4node_group = root.addGroup("My Group")
 5
 6layer = QgsVectorLayer("Point?crs=EPSG:4326", "layer name you like", "memory")
 7QgsProject.instance().addMapLayer(layer, False)
 8
 9node_group.addLayer(layer)
10
11print(root)
12print(root.children())

Access the first child node

1from qgis.core import QgsLayerTreeGroup, QgsLayerTreeLayer, QgsLayerTree
2
3child0 = root.children()[0]
4print (child0.name())
5print (type(child0))
6print (isinstance(child0, QgsLayerTreeLayer))
7print (isinstance(child0.parent(), QgsLayerTree))
My Group
<class 'qgis._core.QgsLayerTreeGroup'>
False
True

Find groups and nodes

 1from qgis.core import QgsLayerTreeGroup, QgsLayerTreeLayer
 2
 3def get_group_layers(group):
 4   print('- group: ' + group.name())
 5   for child in group.children():
 6      if isinstance(child, QgsLayerTreeGroup):
 7         # Recursive call to get nested groups
 8         get_group_layers(child)
 9      else:
10         print('  - layer: ' + child.name())
11
12
13root = QgsProject.instance().layerTreeRoot()
14for child in root.children():
15   if isinstance(child, QgsLayerTreeGroup):
16      get_group_layers(child)
17   elif isinstance(child, QgsLayerTreeLayer):
18      print ('- layer: ' + child.name())
- group: My Group
  - layer: layer name you like

Find group by name

print (root.findGroup("My Group"))
<QgsLayerTreeGroup: My Group>

Find layer by id

print(root.findLayer(layer.id()))
<QgsLayerTreeLayer: layer name you like>

Add layer

1from qgis.core import QgsVectorLayer, QgsProject
2
3layer1 = QgsVectorLayer("Point?crs=EPSG:4326", "layer name you like 2", "memory")
4QgsProject.instance().addMapLayer(layer1, False)
5node_layer1 = root.addLayer(layer1)
6# Remove it
7QgsProject.instance().removeMapLayer(layer1)

Add group

1from qgis.core import QgsLayerTreeGroup
2
3node_group2 = QgsLayerTreeGroup("Group 2")
4root.addChildNode(node_group2)
5QgsProject.instance().mapLayersByName("layer name you like")[0]

Move loaded layer

 1layer = QgsProject.instance().mapLayersByName("layer name you like")[0]
 2root = QgsProject.instance().layerTreeRoot()
 3
 4myLayer = root.findLayer(layer.id())
 5myClone = myLayer.clone()
 6parent = myLayer.parent()
 7
 8myGroup = root.findGroup("My Group")
 9# Insert in first position
10myGroup.insertChildNode(0, myClone)
11
12parent.removeChildNode(myLayer)

Move loaded layer to a specific group

1QgsProject.instance().addMapLayer(layer, False)
2
3root = QgsProject.instance().layerTreeRoot()
4myGroup = root.findGroup("My Group")
5myOriginalLayer = root.findLayer(layer.id())
6myLayer = myOriginalLayer.clone()
7myGroup.insertChildNode(0, myLayer)
8parent.removeChildNode(myOriginalLayer)

Toggling active layer visibility

root = QgsProject.instance().layerTreeRoot()
node = root.findLayer(layer.id())
new_state = Qt.Checked if node.isVisible() == Qt.Unchecked else Qt.Unchecked
node.setItemVisibilityChecked(new_state)

Is group selected

1def isMyGroupSelected( groupName ):
2    myGroup = QgsProject.instance().layerTreeRoot().findGroup( groupName )
3    return myGroup in iface.layerTreeView().selectedNodes()
4
5print(isMyGroupSelected( 'my group name' ))
False

Expand node

print(myGroup.isExpanded())
myGroup.setExpanded(False)

Hidden node trick

 1from qgis.core import QgsProject
 2
 3model = iface.layerTreeView().layerTreeModel()
 4ltv = iface.layerTreeView()
 5root = QgsProject.instance().layerTreeRoot()
 6
 7layer = QgsProject.instance().mapLayersByName('layer name you like')[0]
 8node = root.findLayer(layer.id())
 9
10index = model.node2index( node )
11ltv.setRowHidden( index.row(), index.parent(), True )
12node.setCustomProperty( 'nodeHidden', 'true')
13ltv.setCurrentIndex(model.node2index(root))

Node signals

1def onWillAddChildren(node, indexFrom, indexTo):
2    print ("WILL ADD", node, indexFrom, indexTo)
3
4def onAddedChildren(node, indexFrom, indexTo):
5    print ("ADDED", node, indexFrom, indexTo)
6
7root.willAddChildren.connect(onWillAddChildren)
8root.addedChildren.connect(onAddedChildren)

Remove layer

root.removeLayer(layer)

Remove group

root.removeChildNode(node_group2)

Create new table of contents (TOC)

1root = QgsProject.instance().layerTreeRoot()
2model = QgsLayerTreeModel(root)
3view = QgsLayerTreeView()
4view.setModel(model)
5view.show()

Move node

cloned_group1 = node_group.clone()
root.insertChildNode(0, cloned_group1)
root.removeChildNode(node_group)

Rename node

cloned_group1.setName("Group X")
node_layer1.setName("Layer X")

21.9. Processing algorithms

Get algorithms list

1from qgis.core import QgsApplication
2
3for alg in QgsApplication.processingRegistry().algorithms():
4    if 'buffer' == alg.name():
5        print("{}:{} --> {}".format(alg.provider().name(), alg.name(), alg.displayName()))
QGIS (native c++):buffer --> Buffer

Get algorithms help

Random selection

from qgis import processing
processing.algorithmHelp("native:buffer")
...

Run the algorithm

For this example, the result is stored in a temporary memory layer which is added to the project.

from qgis import processing
result = processing.run("native:buffer", {'INPUT': layer, 'OUTPUT': 'memory:'})
QgsProject.instance().addMapLayer(result['OUTPUT'])
Processing(0): Results: {'OUTPUT': 'output_d27a2008_970c_4687_b025_f057abbd7319'}

How many algorithms are there?

len(QgsApplication.processingRegistry().algorithms())

How many providers are there?

from qgis.core import QgsApplication

len(QgsApplication.processingRegistry().providers())

How many expressions are there?

from qgis.core import QgsExpression

len(QgsExpression.Functions())

21.10. Decorators

CopyRight

 1from qgis.PyQt.Qt import QTextDocument
 2from qgis.PyQt.QtGui import QFont
 3
 4mQFont = "Sans Serif"
 5mQFontsize = 9
 6mLabelQString = "© QGIS 2019"
 7mMarginHorizontal = 0
 8mMarginVertical = 0
 9mLabelQColor = "#FF0000"
10
11INCHES_TO_MM = 0.0393700787402 # 1 millimeter = 0.0393700787402 inches
12case = 2
13
14def add_copyright(p, text, xOffset, yOffset):
15    p.translate( xOffset , yOffset  )
16    text.drawContents(p)
17    p.setWorldTransform( p.worldTransform() )
18
19def _on_render_complete(p):
20    deviceHeight = p.device().height() # Get paint device height on which this painter is currently painting
21    deviceWidth  = p.device().width() # Get paint device width on which this painter is currently painting
22    # Create new container for structured rich text
23    text = QTextDocument()
24    font = QFont()
25    font.setFamily(mQFont)
26    font.setPointSize(int(mQFontsize))
27    text.setDefaultFont(font)
28    style = "<style type=\"text/css\"> p {color: " + mLabelQColor + "}</style>"
29    text.setHtml( style + "<p>" + mLabelQString + "</p>" )
30    # Text Size
31    size = text.size()
32
33    # RenderMillimeters
34    pixelsInchX  = p.device().logicalDpiX()
35    pixelsInchY  = p.device().logicalDpiY()
36    xOffset  = pixelsInchX  * INCHES_TO_MM * int(mMarginHorizontal)
37    yOffset  = pixelsInchY  * INCHES_TO_MM * int(mMarginVertical)
38
39    # Calculate positions
40    if case == 0:
41        # Top Left
42        add_copyright(p, text, xOffset, yOffset)
43
44    elif case == 1:
45        # Bottom Left
46        yOffset = deviceHeight - yOffset - size.height()
47        add_copyright(p, text, xOffset, yOffset)
48
49    elif case == 2:
50        # Top Right
51        xOffset  = deviceWidth  - xOffset - size.width()
52        add_copyright(p, text, xOffset, yOffset)
53
54    elif case == 3:
55        # Bottom Right
56        yOffset  = deviceHeight - yOffset - size.height()
57        xOffset  = deviceWidth  - xOffset - size.width()
58        add_copyright(p, text, xOffset, yOffset)
59
60    elif case == 4:
61        # Top Center
62        xOffset = deviceWidth / 2
63        add_copyright(p, text, xOffset, yOffset)
64
65    else:
66        # Bottom Center
67        yOffset = deviceHeight - yOffset - size.height()
68        xOffset = deviceWidth / 2
69        add_copyright(p, text, xOffset, yOffset)
70
71# Emitted when the canvas has rendered
72iface.mapCanvas().renderComplete.connect(_on_render_complete)
73# Repaint the canvas map
74iface.mapCanvas().refresh()

21.11. Composer

Get print layout by name

1composerTitle = 'MyComposer' # Name of the composer
2
3project = QgsProject.instance()
4projectLayoutManager = project.layoutManager()
5layout = projectLayoutManager.layoutByName(composerTitle)

21.12. Sources