arcpy.mapping.AddLayer not working properly with Python toolbox

Discussion created by tmacdonaldwinick on May 23, 2014
Latest reply on May 23, 2014 by tmacdonaldwinick
I have ArcMap 10.1 SP1 (Build 3143) for Desktop.  I'm trying to execute the following simple code.
import arcpy

class Toolbox(object):
    def __init__(self):
        self.label = 'Tools'
        self.alias = 'Tools' = [Tool]

class Tool(object):
    def __init__(self):
        self.label = 'Get it to work'
        self.canRunInBackground = False

    def getParameterInfo(self):
        parameters = []
        return parameters

    def isLicensed(self):
        return True

    def updateParameters(self, parameters):

    def updateMessages(self, parameters):

    def execute(self, parameters, messages):
        mxd = arcpy.mapping.MapDocument('CURRENT')
        df = arcpy.mapping.ListDataFrames(mxd)[0]
        layer = arcpy.mapping.Layer(r'O:\shp\mn.lyr')
        arcpy.mapping.AddLayer(df, layer)

It does not raise any exceptions and the layers are added to the dataframe but they disappear as soon as they're added.  I'm trying to automate map production and I need to add these layers.