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I created memory files by chaining multiple processes (dissolve, multiparttosingleparts, pointsalonglines) and in the last step I add a new field to the memory layer.

My code looks this:

from qgis.core import *
import os
import processing

pathInput = "path/to/input/file.shp"


DISSOLVE


dissolve = processing.run("native:dissolve",{'INPUT':pathInput, 
                                    'FIELD':['highway', 'maxspeed', 'surface'],
                                    'OUTPUT':'memory:'})


Get dissolve layer


dissolve_layer = dissolve['OUTPUT']


Create in-memory spatial index


QgsSpatialIndex(dissolve_layer.getFeatures())


MULTIPART TO SINGLEPARTS


multiparttosingleparts=processing.run("qgis:multiparttosingleparts",{'INPUT':dissolve_layer, 
                                              'OUTPUT':'memory:'})


layer_singleparts = multiparttosingleparts['OUTPUT']


Create in-memory spatial index


QgsSpatialIndex(layer_singleparts.getFeatures())


POINTS ALONG GEOMETRY


pointsAlongGeom=processing.run("native:pointsalonglines", {'INPUT':layer_singleparts,
                                            'DISTANCE':10,
                                            'START_OFFSET':5,
                                            'END_OFFSET':5,
                                            'OUTPUT':'memory:'})


layer_PntsAlngGeom = pointsAlongGeom['OUTPUT']
QgsSpatialIndex(layer_PntsAlngGeom.getFeatures())


ADD UNIQUE ID TO POINT MEMORY LAYER


layer_PntsAlngGeom.dataProvider().addAttributes([QgsField('point_id', QVariant.Int)])
layer_PntsAlngGeom.updateFields()
fld_idx = layer_PntsAlngGeom.fields().lookupField('point_id')
atts_map = {ft.id(): {fld_idx: ft.id()} for ft in layer_PntsAlngGeom.getFeatures()}
layer_PntsAlngGeom.dataProvider().changeAttributeValues(atts_map)
print('Done')

If I then add the in-memory layer to the map as a temporary layer like :

QgsProject.instance().addMapLayer(layer_PntsAlngGeom)

I can see, that everything worked correctly, so that there was an ID created but also that the CRS is correct. So I assume, that the in-memory layer has all it needs to become a file. However, how can I now create a shapefile from it? If this makes a difference: I do NOT want to make a file from the QgsMapLayer added to the map, but from the memory layer.

How can I do this? I couldn't find a section about in-memory layer in the PyQGIS Cookbook, therefore I hope to get some help here.

System Windows 10

QGIS 3.28 Firenze

Matt
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i.i.k.
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  • In the pointsalonglines algorithm, instead of 'OUTPUT':'memory' you can specify a path to where you want to save your shapefile. E.g. 'OUTPUT':'C:/users/my_name/shapefiles/my_file.shp'. – Matt Apr 20 '23 at 14:19
  • @Matt Yes, I know, but I would like to add the ID to the memory layer and only then store the layer as file, which should include the ID. – i.i.k. Apr 20 '23 at 14:29
  • The last section of your code # ADD UNIQUE ID TO POINT MEMORY LAYER will affect the file on disk, so the ID will be added to the saved shapefile. – Matt Apr 20 '23 at 14:41
  • @Matt Yes, I know that too (actually, this is how I do this currently), but I would still rather not save and load the point file to then add the id to the file but would like to first add the id to the memory layer and then save the layer to a file. – i.i.k. Apr 20 '23 at 14:50
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    See this post: Saving layer as shapefile using PyQGIS – Matt Apr 20 '23 at 15:03
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    Since you are already chaining processing algorithms, why don't you add 'native:fieldcalculator' as a final step. Use it to add the point_id field, fill it with $id and set your file destination as 'OUTPUT'. – Ben W Apr 20 '23 at 22:55
  • @Ben W I simply didn't know that this method exists :) Thank you for the tip! – i.i.k. Apr 24 '23 at 09:07

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