Tree views - Best design practices

Jonathan Gardner jgardner at
Thu Jan 8 21:35:36 CET 2009

On Jan 8, 8:16 am, MRAB <goo... at> wrote:
> Filip Gruszczyński wrote:
> > Hi!
> > I have certain design problem, which I cannot solve elegantly. Maybe
> > you know some good design patterns for this kind of tasks.
> > Task:
> > We have a model which has two kinds of objects: groups and elements.
> > Groups can hold other groups (subgroups) and elements. It's a simple
> > directory tree, for example. We would like to display it in a tree
> > view (which sound good for this kind of model). What is more required,
> > for groups and elements there are different sets of operations, which
> > should be available under right click. For example for group, there
> > should be operations: 'add element' and 'add group', and for element
> > there should be 'change properties'.
> > Do you know any smart way to achieve this? The simplest way is to ask
> > for the class and display operations accordingly. But from the first
> > day with OO programming I have heard, that asking for class is wrong.
> > But I can hardly see any easy and more maintainable solution for this
> > problem. Could you help me with this?
> You could ask the object what the operations are. Here's an example
> using strings:
>  >>> class Element(object):
>         operations = "Element operations"
>  >>> class Group(object):
>         operations = "Group operations"
>  >>> e = Element()
>  >>> g = Group()
>  >>>
>  >>> e.operations
> 'Element operations'
>  >>> g.operations
> 'Group operations'

When faced with this kind of scenario, I usually write boolean methods
like "is_leaf" or "is_branch".

    class Element(object):
        def is_leaf(): return True
        def is_branch(): return False

    class Group(object):
        def is_leaf(): return False
        def is_branch(): return True

Of course, you have to give priority to one or the other, in case an
object thinks it is both.

    if thing.is_branch():
        # Treat it like a branch
    elif thing.is_leaf():
        # Treat it like a leaf

I believe this is a simpler method than checking a single attribute
for a name.

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