QGIS + Python Part 1

――Since I have just started studying GIS (QGIS), there are many doubts about my understanding.

Execution operation check environment

• Windows 7 64bit
• QGIS 2.8.11

Preparation

--Select "Python Console" from "Plugins" in the QGIS menu bar --Select the editor display from the Python console icon ――After that, do not use the interactive console, but write and execute the code with an editor.

Check Python version

test.py

import platform
print(platform.python_version())

--The execution result is displayed on the console → 2.7.5 ――It seems to be Python of 2.x series

Check Python execution path

test.py

import os
print(os.getcwd())

--Execution result → C: \ PROGRA ~ 1 \ QGIS2 ~ 1.18 \ bin - C:\Program Files\QGIS 2.18\bin

Add vector layer

test.py

filePath='C:/Users/xxxx/Documents/GIS DataBase/PyTest/lay01.shp'
iface.addVectorLayer(filePath,'test','ogr')

--First argument: File path --The file path can also be described using "slash" instead of "". --Be careful of escape sequences when using "" --Second argument: The name you want to give to the layer --In the case of the above code, the layer name on QGIS will be "test lay01 Polygon" --Second argument + SP + file name + SP + type - --Third argument: Unknown for now ʻogr. providerKey --Don't make it ʻorg.

Get active layer

--Select the layer with the mouse etc. in advance.

test.py

layer=iface.activeLayer()
print('\n'.join(dir(layer)))

--You can get the member list (list of available methods etc.) of xxx object with dir (xxx)

• geometryType()、getFeatures() --For details, see QgsVectorLayer Class Reference

Examine the geometry type of a layer

――It seems that points (points), lines (polylines, lines), and polygons (faces) cannot be mixed in the vector layer of QGIS ?. --Therefore, the type of vector layer can be (probably) point, line, or polygon. -

--Points, lines, and polygons are collectively called geometry.

test.py

layer=iface.activeLayer()
type = layer.geometryType()
if type == QGis.Point :
    print('Point')
elif type == QGis.Line :
    print('Line')
elif type == QGis.Polygon:
    print('Polygon')

Output the attributes of the selected feature (selected polygon)

--Assuming that the feature has an attribute column called ʻid`. -

--Suppose a feature is selected singularly or multiple times.

test.py

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

for feature in features:
    print(feature['id'])

--Use layer.getFeatures () if you want to target all features on the active layer, with or without selection.

test.py

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

for feature in features:
    print(feature['id'])

Output the vertex coordinates of a feature (polygon)

test.py

# -*- coding: utf-8 -*- 

layer=iface.activeLayer()

if layer is None :
    print(u'There is no active layer.')

if layer.geometryType() != QGis.Polygon:
    print(u'The geometry type of the active layer is not polygon.')

print(u'Active layer{0}Executes the process for.'.format(layer.name()))
print('')

for feature in layer.getFeatures():

    print(u'ID={0}'.format(feature['id']))
    if feature.geometry().isMultipart():
        polygons = feature.geometry().asMultiPolygon()
    else:
        polygons = [ feature.geometry().asPolygon() ]
                
    for polygon in polygons:
        for vertices in polygon:
            for vertex in vertices:
                print('X={0:.9f}  Y={1:.9f}'.format(vertex.x(),vertex.y()))
 
    print('')

--The above vertex is an instance of class'qgis._core.QgsPoint' --For details, see QgsPoint Class Reference

Get the layer's bounding box

--Get the bounding box for all features in the layer --The bounding box is the QgsRectangle class --For details, see QgsRectangle Class Reference

test.py

# -*- coding: utf-8 -*- 
layer=iface.activeLayer()

if layer is None :
    print(u'There is no active layer.')

box = layer.extent()
print('xMinimum={0:.9f}'.format(box.xMinimum()))
print('xMaximum={0:.9f}'.format(box.xMaximum()))
print('yMinimum={0:.9f}'.format(box.yMinimum()))
print('yMaximum={0:.9f}'.format(box.yMaximum()))
print('width={0:.9f}'.format(box.width()))
print('height={0:.9f}'.format(box.height()))

Reference material

-Introduction to QGIS Programming 2016 Osaka Edition

• GIS Stack Exchange
• QGIS API Documentation 2.18
• Getting Started With Python Programming