PyQGIS – How to Get the ID from a Selected Feature

pyqgisqgsvectorlayer

using self.iface.mapCanvas().selectionChanged.connect(self.getselectionid) how could I get the id of a manually selected feature on the Qgis map specifically on a vectorlayer? so it can be used to program a function.

I tried a couple of things to see if I could figure what selectionChanged is returning, these are my attempts and its results:

print("I selected {}".format(str(selection.feature.id())))
>>I selected <qgis._core.QgsVectorLayer object at 0x7f9d9f189c18>

print("I selected {}".format(str(selection[0].id())))
>>I selected <qgis._core.QgsVectorLayer object at 0x7f9d9f189c18>
>>I selected project_s_shapefile_f403eb35_14b9_4e61_b327_263dce57a8cb

print("I selected {}".format(str(selection)))
>>I selected <qgis._core.QgsVectorLayer object at 0x7f9d9f189c18>

Best Answer

You need to iterate over the vector layer after your signal.

def get_selection_id(layer):
    for feature in layer.selectedFeatures():
        print(feature.id())

iface.mapCanvas().selectionChanged.connect(get_selection_id)

Related Solutions

[GIS] pyQGIS changing Paletted Raster Layers to Grayscale

Next script works for me.

layer=iface.activeLayer()

myGrayRenderer = QgsSingleBandGrayRenderer(layer.dataProvider(), 
                                           1)

layer.setRenderer(myGrayRenderer)

renderer = layer.renderer()
provider = layer.dataProvider()

layer_extent = layer.extent()
uses_band = renderer.usesBands()

myType = renderer.dataType(uses_band[0])

stats = provider.bandStatistics(uses_band[0], 
                                QgsRasterBandStats.All, 
                                layer_extent, 
                                0)

myEnhancement = QgsContrastEnhancement(myType)

contrast_enhancement = QgsContrastEnhancement.StretchToMinimumMaximum

myEnhancement.setContrastEnhancementAlgorithm(contrast_enhancement,True)
myEnhancement.setMinimumValue(stats.minimumValue)
myEnhancement.setMaximumValue(stats.maximumValue)

layer.renderer().setContrastEnhancement(myEnhancement)
layer.triggerRepaint()

I tested it for changing an object of QgsSingleBandPseudoColorRenderer class (in your case it would be a QgsPalettedRasterRenderer object) in a QgsSingleBandGrayRenderer class object.

Before running the script at the Python Console:

After running the script:

Editing Note:

I tested the script with the same raster and a paletted render type (see next image) and it worked too.

[GIS] Cutting line feature by the nearest point in QGIS

You can use closestSegmentWithContext from QgsGeometry. The argument is the cut nearest QgsPoint and, it produces a tuple with the squared Cartesian distance from closest segment to polyline, the nearest point on the segment and, finally, the index of the vertex (at polyline) after the closest segment.

To illustrate the approach, I used next shapefiles for only cutting in two features the polyline layer.

PyQGIS 2 code is:

registry = QgsMapLayerRegistry.instance()

my_border = registry.mapLayersByName('my_border')
my_point = registry.mapLayersByName('my_point')

feats_my_border = [ feat for feat in my_border[0].getFeatures() ]

feats_my_point = [ feat for feat in my_point[0].getFeatures() ]

my_tupla = feats_my_border[0].geometry().closestSegmentWithContext(feats_my_point[0].geometry().asPoint())

my_line = [ feat.geometry().asPolyline() for feat in feats_my_border ]

first_segment = []
second_segment = [my_tupla[1]]

for line in my_line:
    for i, point in enumerate(line):
        if i < my_tupla[2]:
            first_segment.append(point)
        else:
            second_segment.append(point)

first_segment.append(my_tupla[1])

epsg = my_border[0].crs().postgisSrid()

uri = "LineString?crs=epsg:" + str(epsg) + "&field=id:integer""&index=yes"

mem_layer = QgsVectorLayer(uri,
                           'split_line',
                           'memory')

prov = mem_layer.dataProvider()

feat = QgsFeature()

feat.setAttributes([0])

feat.setGeometry(QgsGeometry.fromPolyline(first_segment))

prov.addFeatures([feat])

feat.setAttributes([1])

feat.setGeometry(QgsGeometry.fromPolyline(second_segment))

prov.addFeatures([feat])

QgsMapLayerRegistry.instance().addMapLayer(mem_layer)

After running it, at the Python Console of QGIS, I got:

It can be observed the first selected feature at the created memory layer.

PyQGIS 3 code is:

registry = QgsProject.instance()

my_border = registry.mapLayersByName('my_border')
my_point = registry.mapLayersByName('my_point')

feats_my_border = [ feat for feat in my_border[0].getFeatures() ]

feats_my_point = [ feat for feat in my_point[0].getFeatures() ]

my_tupla = feats_my_border[0].geometry().closestSegmentWithContext(feats_my_point[0].geometry().asPoint())

my_line = [ feat.geometry().asMultiPolyline() for feat in feats_my_border ]

first_segment = []
second_segment = [my_tupla[1]]

for line in my_line[0]:
    for i, point in enumerate(line):
        if i < my_tupla[2]:
            first_segment.append(point)
        else:
            second_segment.append(point)

first_segment.append(my_tupla[1])

epsg = my_border[0].crs().postgisSrid()

uri = "LineString?crs=epsg:" + str(epsg) + "&field=id:integer""&index=yes"

mem_layer = QgsVectorLayer(uri,
                           'split_line',
                           'memory')

prov = mem_layer.dataProvider()

feat = QgsFeature()

feat.setAttributes([0])

feat.setGeometry(QgsGeometry.fromPolylineXY(first_segment))

prov.addFeatures([feat])

feat.setAttributes([1])

feat.setGeometry(QgsGeometry.fromPolylineXY(second_segment))

prov.addFeatures([feat])

QgsProject.instance().addMapLayer(mem_layer)

Editing Note:

When there are more than one cut point, next code (PyQGIS 2) works well:

registry = QgsMapLayerRegistry.instance()

my_border = registry.mapLayersByName('my_border')
my_point = registry.mapLayersByName('my_point')

feats_my_border = [ feat for feat in my_border[0].getFeatures() ]
feats_my_points = [ feat for feat in my_point[0].getFeatures() ]

points = [ feat.geometry().asPoint() for feat in feats_my_points ]

my_tuplas = []

for point in points:
    tupla = feats_my_border[0].geometry().closestSegmentWithContext(point)
    my_tuplas.append(tupla)

my_line = [ feat.geometry().asPolyline() for feat in feats_my_border ]

new_line = []

k = 0

try:
    for i, line in enumerate(my_line):
        for j, point in enumerate(line):
            new_line.append(point)
            if j == my_tuplas[k][2] - 1:
                new_line.append(my_tuplas[k][1])
                k += 1

except IndexError:
    pass

k = 0

lines = [ [] for i in range(len(points) + 1) ]

epsg = my_border[0].crs().postgisSrid()

uri = "LineString?crs=epsg:" + str(epsg) + "&field=id:integer""&index=yes"

mem_layer = QgsVectorLayer(uri,
                           'split_line',
                           'memory')

prov = mem_layer.dataProvider()

try:
    for point in new_line:
        lines[k].append(point)
        if point == my_tuplas[k][1]:
            k += 1
            lines[k].append(point)

except IndexError:
    pass
    
for i, line in enumerate(lines):

    feat = QgsFeature()
    feat.setAttributes([i])
    feat.setGeometry(QgsGeometry.fromPolyline(line))
    prov.addFeatures([feat])

QgsMapLayerRegistry.instance().addMapLayer(mem_layer)

After running above code at the Python Console of QGIS, I got:

At the image, it was selected only the second feature of polyline memory layer.

With QGIS 3 following code also works:

registry = QgsProject.instance()

my_border = registry.mapLayersByName('my_border')
my_point = registry.mapLayersByName('my_points')

feats_my_border = [ feat for feat in my_border[0].getFeatures() ]
feats_my_points = [ feat for feat in my_point[0].getFeatures() ]

points = [ feat.geometry().asPoint() for feat in feats_my_points ]

my_tuplas = []

for point in points:
    tupla = feats_my_border[0].geometry().closestSegmentWithContext(point)
    my_tuplas.append(tupla)

my_line = [ feat.geometry().asMultiPolyline()[0] for feat in feats_my_border ]

new_line = []

k = 0

try:
    for i, line in enumerate(my_line):
        for j, point in enumerate(line):
            new_line.append(point)
            if j == my_tuplas[k][2] - 1:
                new_line.append(my_tuplas[k][1])
                k += 1

except IndexError:
    pass

k = 0

lines = [ [] for i in range(len(points) + 1) ]

epsg = my_border[0].crs().postgisSrid()

uri = "LineString?crs=epsg:" + str(epsg) + "&field=id:integer""&index=yes"

mem_layer = QgsVectorLayer(uri,
                           'split_line',
                           'memory')

prov = mem_layer.dataProvider()

try:
    for point in new_line:
        lines[k].append(point)
        if point == my_tuplas[k][1]:
            k += 1
            lines[k].append(point)

except IndexError:
    pass

for i, line in enumerate(lines):

    feat = QgsFeature()
    feat.setAttributes([i])
    feat.setGeometry(QgsGeometry.fromPolylineXY(line))
    prov.addFeatures([feat])

QgsProject.instance().addMapLayer(mem_layer)