这一节中所有的代码，如果不在QGIS命令行中执行，都需要导入如下包库

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from qgis.PyQt.QtCore import (
    QRectF,
)

from qgis.core import (
    QgsProject,
    QgsLayerTreeModel,
)

from qgis.gui import (
    QgsLayerTreeView,
)

20. Cheat sheet for PyQGIS

20.1. User Interface

Change Look & Feel

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from qgis.PyQt.QtWidgets import QApplication

app = QApplication.instance()
app.setStyleSheet(".QWidget {color: blue; background-color: yellow;}")
# You can even read the stylesheet from a file
with  open("testdata/file.qss") as qss_file_content:
    app.setStyleSheet(qss_file_content.read())

Change icon and title

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from qgis.PyQt.QtGui import QIcon

icon = QIcon("/path/to/logo/file.png")
iface.mainWindow().setWindowIcon(icon)
iface.mainWindow().setWindowTitle("My QGIS")

20.2. Settings

Get QgsSettings list

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from qgis.core import QgsSettings

qs = QgsSettings()

for k in sorted(qs.allKeys()):
    print (k)

20.3. Toolbars

Remove toolbar

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toolbar = iface.helpToolBar()
parent = toolbar.parentWidget()
parent.removeToolBar(toolbar)

# and add again
parent.addToolBar(toolbar)

Remove actions toolbar

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

20.5. 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)

20.6. 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

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from qgis.core import QgsVectorLayer
layer = QgsVectorLayer("Point?crs=EPSG:4326", "layer name you like", "memory")
QgsProject.instance().addMapLayer(layer)

layers_names = []
for layer in QgsProject.instance().mapLayers().values():
    layers_names.append(layer.name())

print("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

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from qgis.core import QgsProject

layer = QgsProject.instance().mapLayersByName("layer name you like")[0]
# Set seconds (5 seconds)
layer.setAutoRefreshInterval(5000)
# Enable auto refresh
layer.setAutoRefreshEnabled(True)

Show methods

dir(layer)

Adding new feature with feature form

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from qgis.core import QgsFeature, QgsGeometry

feat = QgsFeature()
geom = QgsGeometry()
feat.setGeometry(geom)
feat.setFields(layer.fields())

iface.openFeatureForm(layer, feat, False)

Adding new feature without feature form

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from qgis.core import QgsGeometry, QgsPointXY, QgsFeature

pr = layer.dataProvider()
feat = QgsFeature()
feat.setGeometry(QgsGeometry.fromPointXY(QgsPointXY(10,10)))
pr.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

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from qgis.core import QgsFeature, QgsGeometry
poly = QgsFeature()
geom = QgsGeometry.fromWkt("POINT(7 45)")
geom.translate(1, 1)
poly.setGeometry(geom)
print(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

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from qgis.core import QgsProject

for layer in QgsProject.instance().mapLayers().values():
    crs = layer.crs().authid()
    layer.setName('{} ({})'.format(layer.name(), crs))

Hide a field column

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from qgis.core import QgsEditorWidgetSetup

def fieldVisibility (layer,fname):
    setup = QgsEditorWidgetSetup('Hidden', {})
    for i, column in enumerate(layer.fields()):
        if column.name()==fname:
            layer.setEditorWidgetSetup(idx, setup)
            break
        else:
            continue

Layer from WKT

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from qgis.core import QgsVectorLayer, QgsFeature, QgsGeometry, QgsProject

layer = QgsVectorLayer('Polygon?crs=epsg:4326', 'Mississippi', 'memory')
pr = layer.dataProvider()
poly = QgsFeature()
geom = 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))")
poly.setGeometry(geom)
pr.addFeatures([poly])
layer.updateExtents()
QgsProject.instance().addMapLayers([layer])

Load all vector layers from GeoPackage

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fileName = "testdata/sublayers.gpkg"
layer = QgsVectorLayer(fileName, "test", "ogr")
subLayers = layer.dataProvider().subLayers()

for subLayer in subLayers:
    name = subLayer.split('!!::!!')[1]
    uri = "%s|layername=%s" % (fileName, name,)
    # Create layer
    sub_vlayer = QgsVectorLayer(uri, name, 'ogr')
    # Add layer to map
    QgsProject.instance().addMapLayer(sub_vlayer)

Load tile layer (XYZ-Layer)

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from qgis.core import QgsRasterLayer, QgsProject

def loadXYZ(url, name):
    rasterLyr = QgsRasterLayer("type=xyz&url=" + url, name, "wms")
    QgsProject.instance().addMapLayer(rasterLyr)

urlWithParams = 'type=xyz&url=https://a.tile.openstreetmap.org/%7Bz%7D/%7Bx%7D/%7By%7D.png&zmax=19&zmin=0&crs=EPSG3857'
loadXYZ(urlWithParams, 'OpenStreetMap')

Remove all layers

QgsProject.instance().removeAllMapLayers()

Remove all

QgsProject.instance().clear()

20.7. Table of contents

Access checked layers

iface.mapCanvas().layers()

Remove contextual menu

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ltv = iface.layerTreeView()
mp = ltv.menuProvider()
ltv.setMenuProvider(None)
# Restore
ltv.setMenuProvider(mp)

20.8. Advanced TOC

Root node

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from qgis.core import QgsVectorLayer, QgsProject, QgsLayerTreeLayer

root = QgsProject.instance().layerTreeRoot()
node_group = root.addGroup("My Group")

layer = QgsVectorLayer("Point?crs=EPSG:4326", "layer name you like", "memory")
QgsProject.instance().addMapLayer(layer, False)

node_group.addLayer(layer)

print(root)
print(root.children())

Access the first child node

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from qgis.core import QgsLayerTreeGroup, QgsLayerTreeLayer, QgsLayerTree

child0 = root.children()[0]
print (child0.name())
print (type(child0))
print (isinstance(child0, QgsLayerTreeLayer))
print (isinstance(child0.parent(), QgsLayerTree))

My Group
<class 'qgis._core.QgsLayerTreeGroup'>
False
True

Find groups and nodes

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from qgis.core import QgsLayerTreeGroup, QgsLayerTreeLayer

def get_group_layers(group):
   print('- group: ' + group.name())
   for child in group.children():
      if isinstance(child, QgsLayerTreeGroup):
         # Recursive call to get nested groups
         get_group_layers(child)
      else:
         print('  - layer: ' + child.name())


root = QgsProject.instance().layerTreeRoot()
for child in root.children():
   if isinstance(child, QgsLayerTreeGroup):
      get_group_layers(child)
   elif isinstance(child, QgsLayerTreeLayer):
      print ('- layer: ' + child.name())

- group: My Group
  - layer: layer name you like

Find group by name

print (root.findGroup("My Group"))

<qgis._core.QgsLayerTreeGroup object at 0x7fd75560cee8>

Find layer by id

print(root.findLayer(layer.id()))

<qgis._core.QgsLayerTreeLayer object at 0x7f56087af288>

Add layer

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from qgis.core import QgsVectorLayer, QgsProject

layer1 = QgsVectorLayer("Point?crs=EPSG:4326", "layer name you like 2", "memory")
QgsProject.instance().addMapLayer(layer1, False)
node_layer1 = root.addLayer(layer1)
# Remove it
QgsProject.instance().removeMapLayer(layer1)

Add group

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from qgis.core import QgsLayerTreeGroup

node_group2 = QgsLayerTreeGroup("Group 2")
root.addChildNode(node_group2)
QgsProject.instance().mapLayersByName("layer name you like")[0]

Move loaded layer

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

myLayer = root.findLayer(layer.id())
myClone = myLayer.clone()
parent = myLayer.parent()

myGroup = root.findGroup("My Group")
# Insert in first position
myGroup.insertChildNode(0, myClone)

parent.removeChildNode(myLayer)

Move loaded layer to a specific group

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QgsProject.instance().addMapLayer(layer, False)

root = QgsProject.instance().layerTreeRoot()
myGroup = root.findGroup("My Group")
myOriginalLayer = root.findLayer(layer.id())
myLayer = myOriginalLayer.clone()
myGroup.insertChildNode(0, myLayer)
parent.removeChildNode(myOriginalLayer)

Changing visibility

myGroup.setItemVisibilityChecked(False)
myLayer.setItemVisibilityChecked(False)

Is group selected

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def isMyGroupSelected( groupName ):
    myGroup = QgsProject.instance().layerTreeRoot().findGroup( groupName )
    return myGroup in iface.layerTreeView().selectedNodes()

print(isMyGroupSelected( 'my group name' ))

False

Expand node

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

Hidden node trick

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from qgis.core import QgsProject

model = iface.layerTreeView().layerTreeModel()
ltv = iface.layerTreeView()
root = QgsProject.instance().layerTreeRoot()

layer = QgsProject.instance().mapLayersByName('layer name you like')[0]
node = root.findLayer( layer.id())

index = model.node2index( node )
ltv.setRowHidden( index.row(), index.parent(), True )
node.setCustomProperty( 'nodeHidden', 'true')
ltv.setCurrentIndex(model.node2index(root))

Node signals

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def onWillAddChildren(node, indexFrom, indexTo):
    print ("WILL ADD", node, indexFrom, indexTo)

def onAddedChildren(node, indexFrom, indexTo):
    print ("ADDED", node, indexFrom, indexTo)

root.willAddChildren.connect(onWillAddChildren)
root.addedChildren.connect(onAddedChildren)

Remove layer

root.removeLayer(layer)

Remove group

root.removeChildNode(node_group2)

Create new table of contents (TOC)

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root = QgsProject.instance().layerTreeRoot()
model = QgsLayerTreeModel(root)
view = QgsLayerTreeView()
view.setModel(model)
view.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")

20.9. Processing algorithms

Get algorithms list

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from qgis.core import QgsApplication

for alg in QgsApplication.processingRegistry().algorithms():
    if 'buffer' == alg.name():
        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())

20.10. Decorators

CopyRight

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from qgis.PyQt.Qt import QTextDocument
from qgis.PyQt.QtGui import QFont

mQFont = "Sans Serif"
mQFontsize = 9
mLabelQString = "© QGIS 2019"
mMarginHorizontal = 0
mMarginVertical = 0
mLabelQColor = "#FF0000"

INCHES_TO_MM = 0.0393700787402 # 1 millimeter = 0.0393700787402 inches
case = 2

def add_copyright(p, text, xOffset, yOffset):
    p.translate( xOffset , yOffset  )
    text.drawContents(p)
    p.setWorldTransform( p.worldTransform() )

def _on_render_complete(p):
    deviceHeight = p.device().height() # Get paint device height on which this painter is currently painting
    deviceWidth  = p.device().width() # Get paint device width on which this painter is currently painting
    # Create new container for structured rich text
    text = QTextDocument()
    font = QFont()
    font.setFamily(mQFont)
    font.setPointSize(int(mQFontsize))
    text.setDefaultFont(font)
    style = "<style type=\"text/css\"> p {color: " + mLabelQColor + "}</style>"
    text.setHtml( style + "<p>" + mLabelQString + "</p>" )
    # Text Size
    size = text.size()

    # RenderMillimeters
    pixelsInchX  = p.device().logicalDpiX()
    pixelsInchY  = p.device().logicalDpiY()
    xOffset  = pixelsInchX  * INCHES_TO_MM * int(mMarginHorizontal)
    yOffset  = pixelsInchY  * INCHES_TO_MM * int(mMarginVertical)

    # Calculate positions
    if case == 0:
        # Top Left
        add_copyright(p, text, xOffset, yOffset)

    elif case == 1:
        # Bottom Left
        yOffset = deviceHeight - yOffset - size.height()
        add_copyright(p, text, xOffset, yOffset)

    elif case == 2:
        # Top Right
        xOffset  = deviceWidth  - xOffset - size.width()
        add_copyright(p, text, xOffset, yOffset)

    elif case == 3:
        # Bottom Right
        yOffset  = deviceHeight - yOffset - size.height()
        xOffset  = deviceWidth  - xOffset - size.width()
        add_copyright(p, text, xOffset, yOffset)

    elif case == 4:
        # Top Center
        xOffset = deviceWidth / 2
        add_copyright(p, text, xOffset, yOffset)

    else:
        # Bottom Center
        yOffset = deviceHeight - yOffset - size.height()
        xOffset = deviceWidth / 2
        add_copyright(p, text, xOffset, yOffset)

# Emitted when the canvas has rendered
iface.mapCanvas().renderComplete.connect(_on_render_complete)
# Repaint the canvas map
iface.mapCanvas().refresh()

20.11. Composer

Get print layout by name

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composerTitle = 'MyComposer' # Name of the composer

project = QgsProject.instance()
projectLayoutManager = project.layoutManager()
layout = projectLayoutManager.layoutByName(composerTitle)