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QGIS + Python Notes

QGIS Startup Python Script

Must be located at: C:\Users\Lucas Ritzdorf\AppData\Roaming\QGIS\QGIS3\profiles\default\python\startup.py

Layers

Loading Delimited Text

To load a vector layer from a delimited text file, use: uri = "/path/to/file.csv?delimiter={}&xField={}&yField={}".format(delimiter, lonFieldName, latFieldName)

And either: layer = QgsVectorLayer(uri, layerName, "delimitedtext")

Or: layer = iface.addVectorLayer(uri, layerName, "delimitedtext")

Loading Shapefiles

To load a Shapefile layer, use: layer = QgsVectorLayer("/path/to/shapefile.shp", "layerName", "ogr")

Checking for Successful Loading

Check that the layer was loaded correctly with:

if not layer.isValid():
	print("Layer failed to load. Please try again.")

Or, if the iface.addVectorLayer or addRasterLayer function was used:

if not layer:
	print("Layer failed to load. Please try again.")

Iterating Over Layers

To iterate over the contents of a vector layer, use the following format:

for feature in layer.getFeatures():
	# Do something, such as:
	# Printing a list of the feature's attributes and their values
	print(feature.attributes())

Updating Layers

To redraw a layer, use: layer.triggerRepaint() Or, to update the entire map canvas, use: iface.mapCanvas().refresh()

To update the symbology shown in the legend entry for a layer, use: iface.legendInterface().refreshLayerSymbology(layer)

Removing Layers

To delete a layer: QgsProject.instance().removeMapLayer(layer_id)

More Information

For more information on using vector layers, go here.

Editing Features

Direct Modification

To edit a layer's features and immediately push changes to the data source (probably a file, but possibly a database, web server, etc.), use something like this (for the method DeleteFeatures):

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

Here, res is a boolean value corresponding to the result of the operation (success or failure). For more information, go here.

Using an Editing Buffer

To determine whether a layer is in editing mode, use layer.isEditable().

Must wrap editing calls (see link at end of section) in editing commands:

layer.beginEditCommand("Description of Actions")
# Editing commands
if problem_occurred:
  layer.destroyEditCommand()
  return
# More editing commands
layer.endEditCommand()

Or, for a more Pythonic alternative, use:

with edit(layer):
	# Editing commands
	# More editing commands

Here, an error will cause the edit to be canceled, and successful execution will cause the action to complete.

For more information, go here.

In Practice

Successfully added a line (from this file) with the following:

layer = iface.activeLayer()
headers = fin.readline().strip().split(",")

line = fin.readline().strip().split(",")
feat = QgsFeature(layer.fields())
feat.setAttribute("name", line[headers.index("WaterbodyName")])
feat.setAttribute("id", line[headers.index(r"OBJECTID *")])
feat.setGeometry(QgsGeometry.fromPolyline([QgsPoint(float(line[headers.index("BEGLON")]), float(line[headers.index("BEGLAT")])), QgsPoint(float(line[headers.index("ENDLON")]), float(line[headers.index("ENDLAT")]))]))
(res, outFeats) = layer.dataProvider().addFeatures([feat])

Here, res is the boolean result of the operation, and outFeats is a list of all QgsFeature objects that were added.

Note that the second section of this code block could be wrapped in a for line in file.readlines()-type loop (although readlines() might be problematic for very large files).

Or, as in the second example above:

layer = iface.activeLayer()
headers = fin.readline().strip().split(",")

line = fin.readline().strip().split(",")
with edit(layer):
	feat = QgsFeature(layer.fields())
	feat.setAttribute("name", line[headers.index("WaterbodyName")])
	feat.setAttribute("id", line[headers.index(r"OBJECTID *")])
	feat.setGeometry(QgsGeometry.fromPolyline([QgsPoint(float(line[headers.index("BEGLON")]), float(line[headers.index("BEGLAT")])), QgsPoint(float(line[headers.index("ENDLON")]), float(line[headers.index("ENDLAT")]))]))
	(res, outFeats) = layer.dataProvider().addFeatures([feat])

Optimization

Spatial Indexes

Spatial indexes can dramatically improve the performance of code that makes frequent queries to a vector layer.

To create a spatial index containing all of layer's features: index = QgsSpatialIndex(layer.getFeatures())

Then, use something like nearest = index.nearestNeighbor(QgsPoint(25.4, 12.7), 5) or intersect = index.intersects(QgsRectangle(22.5, 15.3, 23.1, 17.2)).

Route Finding

OpenRouteService

There is a Python library for OpenRouteService. It facilitates easy access to ORS functions, and provides the results as native Python data structures (lists, dicts, etc). ORS limits API queries to 2500 per day, but users can contact them to request quota increases as necessary for their use case(s).

The API requires that input coordinates be in the format [LON, LAT]. This is the opposite of the normal order, and can be achieved with revCoords = [ item[::-1] for item in coords ] (where coords is a tuple of 2-tuples, each representing a point).

Example (Kalispell to Ashley Lake)

import openrouteservice
end = (-114.63478,48.19400)
start = (-114.31506,48.20218)
# Add actual ORS API key below
client = openrouteservice.Client(key='')
routes = client.directions((start,end))
encoded = routes['routes'][0]['geometry']
decoded = openrouteservice.convert.decode_polyline(encoded)
layer = QgsVectorLayer("LineString", "Kalispell to Ashley Lake", "memory")
provider = layer.dataProvider()
feat = QgsFeature()
feat.setGeometry(QgsGeometry.fromPolyline([QgsPoint(pt[0],pt[1]) for pt in decoded['coordinates']]))
provider.addFeature(feat)
QgsProject.instance().addMapLayer(layer)
import processing
result = processing.run('qgis:densifygeometriesgivenaninterval', {
	'INPUT':layer,
	'INTERVAL':0.001,
	'OUTPUT':'memory:'
	})
result2 = processing.runAndLoadResults('native:extractvertices', {
	'INPUT':result['OUTPUT'],
	'OUTPUT':'memory:'
	})
for layer in QgsProject.instance().mapLayers().values():
	if layer.id() == result2['OUTPUT']:
		addedLayer = layer
addedLayer.setRenderer(QgsHeatmapRenderer)
addedLayer.renderer().setColorRamp(QgsGradientColorRamp(QColor('transparent'),QColor(227,26,28)))
addedLayer.renderer().setRadiusUnit(1)
addedLayer.renderer().setRadius(500)

Use addedLayer.renderer().setWeightExpression(exp) to set the weighting expression for each feature.

The routes object is: {'routes': [{'summary': {'distance': 41161.1, 'duration': 5248.4}, 'geometry_format': 'encodedpolyline', 'geometry': 'sneeHddvxTY?D|@BZ~Cy@X@p@rHp@dHn@hHn@|Gp@jHn@|GJrAJhB?fA?f@ArH?zI?hG@@BnCJzBJvAZjCJp@Jj@ZxAZjABLd@xApBjEvBhELv@tBvDh@pAj@vAVbALh@Hb@Hb@RzABZBV@R@HFlB@nBD|AAlCAb[BnQCdI?MDzU@H@dHCnKGvM?bJKlSErCChC@@FzBPvBF@BXZrB@tBdFjV@lBz@zDf@xBn@~Bf@nBd@bBfBrG^vA\\lARv@z@|CCzIjAhEbBlGnE|O~@tDrCnKXbAxCjKz@bDt@Cd@AhAfEdBnGl@|BAlD|A|FhAbELd@jAjEpAzEtChKj@lBvBdGBxDArBdAtBhBfDhEIj@bA|AtCfCdEp@AhBpBr@p@fMtKrCbCpAjArPnNhIbHrE|DzGxFdCvBtCbCx@p@vLhKtDDnCtBpCnBrBlAAz@vJFbGhDb@X@XtDxBrCbBhHdExA~@~BjBvBzBlMjOtABtKlMp@v@d@j@d@h@lArAXZjXz[dL|MdS|UjApAnAjAtAAvAt@l@Tl@R|AZ|ANlGPlADZ@hFNl@B~DJlHRnDHpGTd@@p@BzBL|@DT@XB~APpAPz@J|ATbEz@JBhD|@|Af@jC~@vD|AbAf@dAf@rDnBH|EnAbAv@p@VRf@@rAlAtBtBj@d@bGzFrMM|HrHnCpCz@dA|AtBhAfBdAjBbApB|@tBjGhPfEnLjAdEfCLl@dCHTPn@v@C^|@@tBdApBhAhBnAA@t@dGnIhBlC@n@|@hBx@pBn@zBhBjIrLbl@z@rE\rCHjALvCBlA@hTGn@BtHFjHJxELnCPfCRzC^dExDr^|@fJVnCFdAHpC?lCGdAShCMbAe@CgAxDaFvOm@zBS~@c@fC_CrPcBbL{@nEsAdGqDfO}AnGsElRqAzFcA|FmAdJMAy@dG_@vC{CVOjAc@lDe@lDa@LoBr@yBAkAd@aC~@{@@e@\a@d@@l@i@lA{AjEyBbGyBjFaBfDo@fAmCbEqAhCqApDsAzEo@nBwEjMyBnE}@tAaAfAw@j@e@T}DfAoAf@k@JcA@Dc@oBSm@Mk@Si@@QSq@cA]s@i@qAe@wAWw@Ma@[w@@g@SO{@g@uCqA}@MSIeCS_JeAo@Wi@Yg@o@sAo@y@e@{@YsAMiBD_A?iBI{@@oAHe@Pc@TkCbBmC~@g@ViCbBwAj@o@NKDy@\gAl@m@d@kDhDoC|BmAp@u@Vu@Nw@FkIXSIa@Um@e@q@u@w@q@sA_Ai@[kBa@aAIeCFcAP_AZiDAy@Pg@BSDM?S?e@FmE@aANaA?iBOe@?y@PyEfBi@Fi@ASGc@[@@QKQCQ?OD]XaB|BEJe@^kDvD{AfA]XY@e@v@c@z@GJc@j@YV]N_@FE?I?mASs@U_@WMMYc@]}@g@gBUc@_@i@q@q@[Qq@Ka@B_@J]TcBB[b@qA|B[^@Za@TmAZ{@JyBLu@b@Wb@Sh@aAEe@|A_@AWh@e@x@[\a@LO?QEOIOS{@w@c@WSE{@CsALi@JoAf@{@h@DdCiAlAa@jAYrBg@vE[BIVUh@_@\\q@Xy@Jy@@_AKi@M{@]e@YcA{@}@cAw@qAsCmGeEyDOSYo@i@eBa@aAk@w@QKc@MQAg@Fe@TQPa@h@gA~Bw@rA_BpB_@@c@ZSFg@Fi@@uBKwCUq@?q@FaJbASHa@Vo@t@y@xAKR@h@o@l@w@\c@Lw@@g@KqAa@}BeAcA]o@Ms@AiAFg@XQNOTYl@Or@EZExAB~BRvH?^IxA@tEY|EYpCu@lF_@rCk@lFcAtFQhAUxAm@DIv@CvADhDC|@G~@MpAAH{@fGe@jCKz@Ez@?z@LxICfAId@s@dB[f@WVi@VuCbA{@b@kBlAe@p@a@x@_@tAQ|@GfAGzECr@A^Fv@Pr@r@tA~CnFjEHxAfEr@pBp@hFxDhXB\FX|@lGFV@\PlAFXZCHTTlBLtAHlB?t@Er@OdA_@nAERGTe@zAQl@I\\c@lA]t@u@tAg@n@i@l@cDzCcBxB]h@{@~ABJWNc@l@}DlEU^KZ{@pAITONo@nAu@A_@X}CbAwBrA{@t@wAbBiEhGmA|AcDxCWXu@jAm@v@mEfFk@j@qBdBoAtAmBfB_At@s@V}@NoA?eAEk@?yGA', 'segments': [{'distance': 41161.1, 'duration': 5248.4, 'steps': [{'distance': 106.2, 'duration': 7.6, 'type': 11, 'instruction': 'Head north on North Main Street, US 93', 'name': 'North Main Street, US 93', 'way_points': [0, 2]}, {'distance': 120.4, 'duration': 10.5, 'type': 6, 'instruction': 'Continue straight onto North Main Street, US 93', 'name': 'North Main Street, US 93', 'way_points': [2, 5]}, {'distance': 25094.8, 'duration': 1428.8, 'type': 3, 'instruction': 'Turn sharp right onto West Idaho Street, US 2', 'name': 'West Idaho Street, US 2', 'way_points': [5, 273]}, {'distance': 5611.2, 'duration': 1346.7, 'type': 1, 'instruction': 'Turn right onto Ashley Lake Road', 'name': 'Ashley Lake Road', 'way_points': [273, 387]}, {'distance': 1472.3, 'duration': 353.4, 'type': 12, 'instruction': 'Keep left onto Ashley Lake Road', 'name': 'Ashley Lake Road', 'way_points': [387, 437]}, {'distance': 2456.5, 'duration': 589.6, 'type': 6, 'instruction': 'Continue straight onto Ashley Lake Road', 'name': 'Ashley Lake Road', 'way_points': [437, 503]}, {'distance': 2614.8, 'duration': 627.6, 'type': 6, 'instruction': 'Continue straight onto Ashley Lake Road', 'name': 'Ashley Lake Road', 'way_points': [503, 566]}, {'distance': 3684.9, 'duration': 884.4, 'type': 6, 'instruction': 'Continue straight onto North Ashley Lake Road', 'name': 'North Ashley Lake Road', 'way_points': [566, 640]}, {'distance': 0, 'duration': 0, 'type': 10, 'instruction': 'Arrive at North Ashley Lake Road, on the right', 'name': '', 'way_points': [640, 640]}]}], 'way_points': [0, 640], 'bbox': [-114.647381, 48.090738, -114.315071, 48.20311]}], 'bbox': [-114.647381, 48.090738, -114.315071, 48.20311], 'info': {'attribution': 'openrouteservice.org | OpenStreetMap contributors', 'engine': {'version': '4.7.0', 'build_date': '2018-12-18T13:44:23Z'}, 'service': 'routing', 'timestamp': 1550449490065, 'query': {'profile': 'driving-car', 'preference': 'fastest', 'coordinates': [[-114.31506, 48.20218], [-114.63478, 48.194]], 'language': 'en', 'units': 'm', 'geometry': True, 'geometry_format': 'encodedpolyline', 'instructions_format': 'text', 'instructions': True, 'elevation': False}}}