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I am currently using the following code to merge two vector layers together: 

def append_layers(layer1, layer2, output):
    from processing.core.Processing import Processing
    Processing.initialize()
    import processing.tools
    processing.tools.general.runalg("qgis:mergevectorlayers", layer1, layer2, output)

append_layers(layer1.shp, layer2.shp, output.shp)

This seems to work fine in QGIS 2.14, however it fails in 2.18.1 with the following message:

Error: Wrong number of parameters
ALGORITHM: Merge vector layers

Is there something that has changed between 2.14 and 2.18? Or am i just missing something really obvious?

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2 Answers 2

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Yes, if you type in the following code in the Python Console, the input layer parameter now incorporates multiple layers:

>>>import processing
>>>processing.alghelp("qgis:mergevectorlayers")
ALGORITHM: Merge vector layers
    LAYERS <ParameterMultipleInput>
    OUTPUT <OutputVector>

So one way of resolving this is to add both layers into the input parameter and split them:

processing.tools.general.runalg("qgis:mergevectorlayers", layer1 + ";" + layer2, output)
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4
  • Thank you, that worked fine. Has the procedure for 'runalg("gdalogr:convertformat")' changed too? On 2.14 it works using 'processing.tools.general.runalg('gdalogr:convertformat', layer, filetype, "", save_location)' but on 2.18 no errors are flagged up but new files are not created.
    – JamieTasker
    Commented Jan 6, 2017 at 14:19
  • @JamieTasker - Most welcome! Nope, should be the same parameters. Just tested it and it works fine. If you have problems, you should ask a new question and include all the details such as the input layer type, the output layer type, your code etc :)
    – Joseph
    Commented Jan 6, 2017 at 14:43
  • @Joseph I get the same error Jochen has described in his answer. I'm using version 2.18.16 Python console. It would be interesting to know since which version only a Python list works as vector layer input.
    – Stefan
    Commented Mar 7, 2018 at 6:34
  • @Stefan - I believe the developers changed how the inputs should be used. In the early 2.18 version, layers needed to be separated by a semi-colon which was eventually changed to allow lists. There was a post asking about this.
    – Joseph
    Commented Mar 7, 2018 at 10:14
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Since trying above answer causes an exception (l1 and l2 are both of type QgsVectorLayer, QGIS 2.18.17 Python console) for me

import processing
[...]
processing.runalg('qgis:mergevectorlayers', l1 + ";" + l2, None)
Traceback (most recent call last):
    File "<input>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'QgsVectorLayer' and 'str'

I've been trying 

>>> merged = processing.runalg('qgis:mergevectorlayers', [l1, l2], None)
>>> merged['OUTPUT']
u'C:\\Users\\Jochen\\AppData\\Local\\Temp\\processing30eee19448ef42a497403dd3660cafcf\\82e112158f194f59a53e18d5020a9a9d\\OUTPUT.shp'

successfully.

So passing <ParameterMultipleInput> as a python list is a solution to this.

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