MVC with Python

[Georg Altmann]

Hello,

I need some advice on how to implement model-view-controller. I am 
trying to develop a GUI application with PyQt, yet the problem rather 
applies to mvc in general, not just GUI applications.

Let's say the data is a list of objects with a common base class. The 
views are either a graphical representation of the objects or some form 
of textual input. The views shall change the model by using command 
objects (for undo, e.g. QUndoCommand).

My current approach is to implement the model as a class with a 
list-like interface, with methods insert(), remove(), __getitem__(), 
__setitem__(),... and a signal to notify the views. The objects in the 
list have methods to change their state as well.

My problem is, how do the commands interact with the model?
Let's say I have a command that modifies an object o in the list.

1) If list[key_to_o] returns a reference to the object, the command can 
modify the object by using this reference, i.e. list[key_to_o].setX(). 
So there is no way for the list to know when the object changed - how 
can it emit a singal then?

2) If list[key_to_o] returns a deep copy of the object,
copy_of_o = list[key_to_o], the command mofifies the copy and the 
updates the list: list[key_to_o] = copy_of_o. Now the list can emit a 
signal in __setitem__().
While this may work, it seems awkward to copy around objects just to 
perform a possibly simple operation on them. Additionally it might not 
be feasible once objects get complex.

3) The interface of the classes of the objects could be reflected in the 
  list class, i.e. list.set_x_on_obj(key_to_obj,x). This would probably 
make the list class interface very bloated.

Of course the problem is not really limited to Python, my apologies if 
I'm totally off-topic here.

Regards
Georg