Moving a geometry using PyQGIS3

Question

I have a layer which has one feature. I try to move the feature(multipolygon) using the script below:

layer = iface.activeLayer()

for feat in layer.getFeatures():
    prev_pos = feat.geometry().centroid().asPoint()
    success  = feat.geometry().translate(100, 100) # translation
    next_pos = feat.geometry().centroid().asPoint()

    print(success)        
    print(prev_pos)
    print(next_pos)

But I get different results in QGIS 2 and QGIS 3. (The multipolygon has many points, therefore, I printed out only its centroid)

Output in QGIS 2:

0 # means operation succeeded
(552084,433685)
(552184,433785) # changed coordinates

Output in QGIS 3:

0 # means operation succeeded
<QgsPointXY: POINT(552084 433685)>
<QgsPointXY: POINT(552084 433685)> # same coordinates

When I try to move a point feature, I get same results.

Why does not translate method move the feature in QGIS 3? What am I missing?

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EDIT: If I create a QgsGeometry object manually, translate method works correctly in QGIS 3.

points = [QgsPointXY(0,0), QgsPointXY(2,0), QgsPointXY(2,2), QgsPointXY(0,2)]
geom = QgsGeometry.fromMultiPolygonXY( [ [points] ] )
print(geom)

geom.translate(100, 100)
print(geom)

Output in QGIS 3:

<QgsGeometry: MultiPolygon (((0 0, 2 0, 2 2, 0 2, 0 0)))>
<QgsGeometry: MultiPolygon (((100 100, 102 100, 102 102, 100 102, 100 100)))>

Answer 1

To change QgsFeature geometry in QGIS 3 you have to call setGeometry() explicitly.

geom = feat.geometry()
geom.translate(100, 100)
feat.setGeometry(geom)

Explanation:

In QGIS 2 geometry() returns a pointer, so it can be modified in place.

QgsGeometry* QgsFeature::geometry() //QGIS 2

In QGIS 3 geometry() returns value instead of a pointer. Additionally it is const now, so it is not allowed to modify the QgsFeature.

QgsGeometry QgsFeature::geometry() const //QGIS 3

(see https://qgis.org/api/2.18/classQgsFeature.html#ab0a934a1b173ce5ad8d13363c20ef3c8)

Answer 2

In your examples the manipulated geometry is not written back to data source. The Python Cookbook for PyQGIS offers two possibilities:

(1) using te methods changeAttributeValues(), changeGeometryValues() of the dataProvider (2) do that within an editing buffer, to have the possibility to explicte save in an commit and so on.

a solution for (1) based on your script:

layer = iface.activeLayer()

for feat in layer.getFeatures():
    geom = feat.geometry()
    print(geom.asWkt())
    success  = geom.translate(100, 100) # translation

    fid = feat.id()
    print('id:',fid)
    layer.dataProvider().changeGeometryValues({ fid : geom })

# reread geoms to see if they changed
for feat in layer.getFeatures():
    geom = feat.geometry()
    print(geom.asWkt())