COM objects (arcobjects) in Python

Question

I'm new to OOP and trying to use COM objects (arcobjects) in Python. Program is GIS related, but I did not get any answers on GIS.SE, so I am asking here. Below is piece of my code. I am stuck at the end where I receive iFrameElement. ESRI describe it as member/interface of Abstract Class, which can not create objects itself. I need to pass information contained in it to object in its CoClass (MapFrame).

Any suggestions how to do this?

Also where can I find name conventions for objects in Python? There are p, i as prefix and I am not sure where they come from.

from comtypes.client import CreateObject, GetModule
import arcpy

def CType(obj, interface):
   """Casts obj to interface and returns comtypes POINTER or None"""
   try:
       newobj = obj.QueryInterface(interface)
       return newobj
   except:
       return None

def NewObj(MyClass, MyInterface):
   """Creates a new comtypes POINTER object where\n\
   MyClass is the class to be instantiated,\n\
   MyInterface is the interface to be assigned"""
   from comtypes.client import CreateObject
   try:
       ptr = CreateObject(MyClass, interface=MyInterface)
       return ptr
   except:
       return None

esriCarto = GetModule(r"C:\Program Files (x86)\ArcGIS\Desktop10.0\com\esriCarto.olb")
esriCartoUI = GetModule(r"C:\Program Files (x86)\ArcGIS\Desktop10.0\com\esriCartoUI.olb")
esriMapUI = GetModule(r"C:\Program Files (x86)\ArcGIS\Desktop10.0\com\esriArcMapUI.olb")
esriFrame = GetModule(r"C:\Program Files (x86)\ArcGIS\Desktop10.0\com\esriFramework.olb")

arcpy.SetProduct('Arcinfo')

pApp = NewObj(esriFrame.AppROT, esriFrame.IAppROT).Item(0)
pDoc = pApp.Document
pMxDoc = CType(pDoc, esriMapUI.IMxDocument)
pLayout = pMxDoc.PageLayout
pGraphContLayout = CType(pLayout, esriCarto.IGraphicsContainer)
iFrameElement = pGraphContLayout.FindFrame(pMxDoc.ActiveView.FocusMap)

As far as I understand, iFrameElement is an interface of an abstract class from which I need to inherit attributes (pointer) to MapFrame object. How do I do that? How do it get to object with IMapGrids interface? Any suggestions?

Answer 1

IFrameElement is an interface, so you can't create an instance of it per se. This interface is implemented by various classes, including MapFrame, which means (in basic terms) that an instance of any of those objects 'behaves' like an IFrameElement. So if you get an IFrameElement from IGraphicsContainer.FindFrame(), you can pass it to something else that expects an IFrameElement without having to find out what the actual type of the object is.

I would suggest reading up on what Interfaces mean in OOP, because ESRI's code uses them a lot.

On naming convetions - there is no hard & fast rule on what to name your variables.

By the looks of your code, the p refers to an object with a distinct type, and i refers to an object defined only by an interface. But on that note, calling a variable by the same name as the interface it's referencing (except with a lower-case 'i') is a bad way to do things, and will lead to confusion. (IMO)

Edit: To answer your final question (sorry, I missed it originally):

If pGraphContLayout.FindFrame() returns an object of type MapFrame (and there is no guarantee that it does) then you should be able to simply cast it across to IMapGrids:

pGraphContLayout = CType(pLayout, esriCarto.IGraphicsContainer)
pFrame = pGraphContLayout.FindFrame(pMxDoc.ActiveView.FocusMap)
pGrids = CType(pFrame, IMapGrids)

It sounds like you may be getting confused by Python's abstract base classes, which seem to serve the purpose of interfaces...? This thread is useful: Difference between abstract class and interface in Python