I am Esri-trained, trying to move into QGIS using Python. Simply trying to add a vector layer in Qgis returns the below error:

TypeError: QgisInterface.addVectorLayer(QString, QString, QString): first argument of unbound method must have type 'QgisInterface'

Here's my code:

layer = qgis.core.QgsVectorLayer("C:\mypath\mypath\GTM_adm1.shp", "GTM_adm1", "ogr")
qgis.gui.QgisInterface.addVectorLayer(layer, "GTM_adm1", "ogr")

Not sure what I'm missing here but the Vector layer object is in the registry.


Try simply:

layer = iface.addVectorLayer("/path/to/shapefile/file.shp", "layer_name_you_like", "ogr")

Here you have a nice guide for PyQGIS: http://docs.qgis.org/testing/en/docs/pyqgis_developer_cookbook/intro.html

  • thanks, this worked fine. But might you offer a quick explanation as to why my code returned an error? The logic is not obvious to me. – mikeLdub Jul 6 '15 at 16:09
  • Your method wants as first argument an instance of class. Here I found a nice explanation: julien.danjou.info/blog/2013/… – dmh126 Jul 6 '15 at 16:31
  • thanks, for the link. It's a very useful explanation and has helped clarify some concepts for me. – mikeLdub Jul 6 '15 at 18:27

Just as a remark: QgisInterface is an abstract class and has virtual functions so it may not be instantiated, i.e. it defines an interface and not a complete implementation. But you can use QgisInterface class within a standalone application where the iface class is not available. There you can reimplement and override the main functions of the class. An example very trivial is the following:

from qgis.core import QgsMapLayerRegistry, QgsVectorLayer
from qgis.gui import QgisInterface

def addVectorLayer(uri, provider, name):
    vl = QgsVectorLayer(uri, name, provider)
    return vl, name

class MyIface(QgisInterface):
    def __init__(self):

    def addVectorLayer(self, path, name, provider):
        return addVectorLayer(path, provider, name)

# usage:
# >>> myiface = MyIface()
# >>> myiface.addVectorLayer("some_path_to_layer", "some_name", "ogr")

So using

>>> myiface.addVectorLayer("some_path_to_layer", "some_name", "ogr")
(<qgis._core.QgsVectorLayer object at 0x7546440>, 'some_name')

it will work, but another method e.g

>>> myiface.activeLayer()
NotImplementedError: QgisInterface.activeLayer() is abstract and must be overridden

will raise to NotImplementedError because that method is defined into the class but it is not re-implemented, i.e. it is an abstract method.

BTW: your error (...first argument of unbound method...) is a different error though, the origin of the error is you don't have instantiated the class before, so the correct way to write was:

qgis.gui.QgisInterface().addVectorLayer(layer, "GTM_adm1", "ogr")

which would raise to TypeError: qgis._gui.QgisInterface represents a C++ abstract class and cannot be instantiated

Hope this helps as explanation.

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