Convex hull for selected features - PyQGIS

Question

I changed the code to create a convexhull for selected features from this question (Convexhull for selected features and performing buffer for the resultant convexhull using python) to adapt it to QGIS3, but something does not work. Where is the problem?

layer = iface.activeLayer()
feat = layer.selectedFeatures()[0]
geom = feat.geometry()
convexhull = geom.convexHull()
CRS = layer.crs().postgisSrid()
URI = "Polygon?crs=epsg:"+str(CRS)+"&field=id:integer""&index=yes"
mem_layer = QgsVectorLayer(URI,"convexhull","memory")
mem_layer.startEditing()
feat2 = QgsFeature()
feat2.setGeometry(convexhull)
feat2.setAttributes([1])
mem_layer.addFeature(feat2, True)
mem_layer.commitChanges()
QgsProject.instance().addMapLayers([mem_layer])
iface.mapCanvas().refresh()

EDIT: To be more clear, using this code this is what I get. It's a convexhull for just one feature.

And this is what I want. It's for all selected features. I reached this using Minimum bounding geometry from processing toolbox.

Answer 1

I've tested your code and made modifications to merge geometry of selected features using Merge selected feature pyqgis post. You will find below a working correction. After this first solution, you will find an alternate solution using processing from pyqgis

layer = iface.activeLayer()

selectFeatures = layer.selectedFeatures()

geom = None
for feat in selectFeatures:
    if geom == None:
       geom = feat.geometry()
    else:
       geom = geom.combine(feat.geometry())

convexhull = geom.convexHull()
CRS = layer.crs().postgisSrid()
URI = "Polygon?crs=epsg:"+str(CRS)+"&field=id:integer""&index=yes"
mem_layer = QgsVectorLayer(URI,"convexhull","memory")

mem_layer.startEditing()
feat2 = QgsFeature()
feat2.setGeometry(convexhull)
feat2.setAttributes([1])
mem_layer.addFeature(feat2)
mem_layer.commitChanges()
QgsProject.instance().addMapLayer(mem_layer)
iface.mapCanvas().refresh()

Processing alternative from PyQGIS

from qgis import processing

layer = iface.activeLayer()
newconvexhulllayer = processing.run("qgis:minimumboundinggeometry", {
              'INPUT': QgsProcessingFeatureSourceDefinition(layer.id(), True),
              'TYPE': 3,
              'OUTPUT': 'memory:'})

QgsProject.instance().addMapLayer(newconvexhulllayer['OUTPUT'])