Merge Shapefiles with Attributes Using PyQGIS – How to

mergepyqgisshapefile

I have some shapefiles. They are all the same kind of shape and all have the same attributes.

If I wanted to merge them manually the "Vector/Data management tools/merge shapefiles to one" works fine.

However, as per the question title, I'd like to do this using Python from a script.

I went hunting for the "merge shape files" in the hopes I could cobble it into my script but I couldn't find it.

I tried writing one myself iterating over the features and copying them into another layer as per the pyQGIS cookbook but lost the attributes along the way.

feature_list = inputlayer.getFeatures()
  for feature in feature_list:
    (res, outFeats) = outputlayer.dataProvider().addFeatures( [ feature ] )

I also tried writing a script using the iface copy features function with the same loss of attributes as a result.

qgis.utils.iface.setActiveLayer(importlayer)
importlayer.setSelectedFeatures(range(100))
#(there are exactly 100 features)
qgis.utils.iface.actionCopyFeatures().trigger()
iface.setActiveLayer( outputlayer )
outputlayer.startEditing()
qgis.utils.iface.actionPasteFeatures().trigger()

Can someone help me fix my code to retain the copied attributes or point me to a pyQGIS script to achieve this result?

Best Answer

OK here's the answer. I have discovered the processing toolbox from which there was a simple solution: As per http://qgis.org/de/docs/user_manual/processing/console.html

From the console you can get a list of all the algorithms available which contain the word "merge" by typing:

import processing
processing.alglist("merge")

Then you could find out how to use the most appropriate function with:

processing.alghelp("qgis:mergevectorlayers")

Then simply use the algorithm in your script as follows:

processing.runalg("qgis:mergevectorlayers", layer1, layer2,"outputfilename.shp")

On the other hand manually merging shapes may still be useful. For those who still want to manually copy features from an input layer into some destination layer with attributes you need to first make sure the destination layer has those fields.

old_attribute_List = input_layer.dataProvider().fields().toList()
new_attribute_List=[]
for attrib in old_attribute_List:
  if destination_layer.fieldNameIndex(attrib.name())==-1:
    new_attribute_List.append(QgsField(attrib.name(),attrib.type()))
destination_layer_data_provider.addAttributes(new_attribute_List)
destination_layer.updateFields()

Then to manually copy over the features you can do:

destination_layer.startEditing()
cfeature = QgsFeature()
cfeatures=[]
xfeatures = input_layer.getFeatures()
for xfeature in xfeatures:
    xgeometry = xfeature.geometry()
    #GEOMETRY TRANSFORM GOES HERE IF DESTINATION CRS DIFFERS FROM INPUT CRS
    cfeature_Attributes=[]
    cfeature_Attributes.extend(xfeature.attributes())
    cfeature.setGeometry(xgeometry)
    cfeature.setAttributes(cfeature_Attributes)
    cfeatures.append(cfeature)
destination_layer_data_provider.addFeatures(cfeatures)
destination_layer_data_provider.commitChanges()

TL;DR

To get all features of a layer by the layer name you do not need to activate it. Just use

name = 'counties'
layer = QgsProject.instance().mapLayersByName( name )[0]
poles = layer.getFeatures()
for pole in poles:
    if is_north(pole):
        print('it is the north pole')

Active Layer

First of all, you do not need to care about the active layer. The active layer is the layer which is currently selected in the layer tree and therefore is nice for two things

letting the user select a layer to operate on
selecting a layer for quickly testing code from the python console

The second one is handy when you are developing, the first one is what all the maptools are based on: the currently selected layer (the active one) is modified, selected, deleted...

Methods to access layers

However, you seem to want to access layers not based on user input but based on something else.

Identifying layers

All layers have

An id that needs to be unique
A name that is not necessarily (but often) unique and can be changed by the user
A reference (also often referred to as pointer but in python the two terms are interchangeable) that is unique and stable for the lifetime of a layer during the application run time and can be assigned to a variable (e.g. vlayer is a reference to the currently active layer in your example).
Properties like the URI (table name etc.)

If you already have a reference to the layer (because you created/added it in your plugin) just use that one.

If you don't, think about what property you want to use to get access to the layer. Most likely something that is stable (the layer id is very stable for the lifetime of a project but always is different for different projects), the name is user adjustable, the table may depend on the data source.

Accessing layers

There is the map layer registry which lets you access layers. It manages references to all layers. This used to be called QgsMapLayerRegistry, but that is no longer available in QGIS 3, so QgsProject can be used instead.

registry = QgsProject.instance()

If you have the layer id go with

layer = registry.mapLayer( id )

If you have the layer name

layer = registry.mapLayersByName( name )[0] # The method returns a list since it can be ambiguous, 0 picks the first entry in the list

If you have something else you have to write your own code:

for lyr in registry.mapLayers().values():
    if lyr.dataProvider().dataSourceUri().table() == 'mytable':
        layer = lyr
        break

Or a shorthand that does the same and makes you feel more hacker'ish but will make the people who will have to maintain the code later on cost some extra time to decipher:

layer = (l for l in registry.mapLayers().values() if l.dataProvider().dataSourceUri().table() == 'mytable').next()

Best Answer

Related Solutions

[GIS] Merging shapefiles with GeoTools

PyQGIS Layers – Activating Layer by Name in QGIS 2

TL;DR

Active Layer

Methods to access layers

Identifying layers

Accessing layers

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