Change MultiLineString direction in QGIS/PyQGIS

Question

I am looking for a method in PyQGIS (QGIS3.x) of reversing the direction of a MultiLineString feature. If there is more than one part, the order of the parts should be reversed.

I have found a similar question - How can I switch line direction in QGIS? - for LineString/Polyline. - which only works for QGIS2 - and modified it slightly for use in QGIS3. However, I can't work out how to adapt the approach for MultiLineStrings.

layer = qgis.utils.iface.mapCanvas().currentLayer()
fid = 1 # feature id
feature = layer.getFeature(1)
geom = feature.geometry()
nodes = geom.asPolyline()
nodes.reverse() 
newgeom = QgsGeometry.fromPolylineXY(nodes)
layer.startEditing()
layer.changeGeometry(feature.id(),newgeom)
layer.commitChanges()

In theory the above approach could be used with newgeom = QgsGeometry.fromMultiPolylineXY() but I can't see how to generate the necessary QgsMultiPolylineXY object.

I have also noted the reversed() function on the QGSMultiLineCurve class, but I couldn't attach the generated QGSMultiLineCurve to the original feature.

I'm not really interested in UI-based approaches, as the features to be reversed are determined programatically.

Any approaches are welcome - if there's a better general approach than above, I'm happy to hear.

Answer 1

I will focus in the geometry creation part, because the feature editing part is fully explained in many places, including the example inside the question.

I will use some python list comprehension, but the key here is the use of the QgsAbstractGeometry type QgsMultiLineString, which is iterable and converts easly from/to list type.

from the Api, you will see that each QgsGeometry is a container for the "real" QgsAbstractGeometry:

A geometry is the spatial representation of a feature. Since QGIS 2.10, QgsGeometry acts as a generic container for geometry objects. QgsGeometry is implicitly shared, so making copies of geometries is inexpensive. The geometry container class can also be stored inside a QVariant object.

The actual geometry representation is stored as a QgsAbstractGeometry within the container, and can be accessed via the get() method or set using the set() method.

#First, will create an example multilinestring geometry
lines = [[[0,0],[1,1],[2,2]],[[0,1],[1,2],[2,3]],[[0,2],[1,3],[2,4]]]
mls = QgsMultiLineString()
for i in [QgsLineString([QgsPoint(x,y) for x,y in i]) for i in lines]:
    _ = mls.addGeometry(i)

old_geometry = QgsGeometry(mls)

# QgsGeometry can be converted to QgsAbstractGeometry with the
# .get() method, so real example begins HERE:

mls1 = old_geometry.get()
mls2 = QgsMultiLineString()
# For each reversed linestring, visited in reverse order
for i in [QgsLineString([*i][::-1]) for i in [*mls1][::-1]]:
    _ = mls2.addGeometry(i) # add it to new geometry

new_geometry = QgsGeometry(mls2)
old_geometry
#[out]: <QgsGeometry: MultiLineString ((0 0, 1 1, 2 2),(0 1, 1 2, 2 3),(0 2, 1 3, 2 4))>
new_geometry
#[out]: <QgsGeometry: MultiLineString ((2 4, 1 3, 0 2),(2 3, 1 2, 0 1),(2 2, 1 1, 0 0))>

Note here, QgsMultiLineString objects are iterables so, you can expand it to a list of Linestrings using [*list] python syntax.

In the developer forum they say it is recommended to use QgsMultiLineString, QgsLineString, QgsPoint,... etc when creating geometries, one advantage is the easy conversion from/to list conversion.

One thing to be aware of is that if you are using existing features, rather than a newly created one as per the example above, don't try to shortcut the "cloning" process. Given a feature f, use, for example:

geom = f.geometry()
mls1 = geom.get()

rather than

mls1 = f.geometry().get()

Otherwise you will get a crash due to python garbage collection