"while" loops in Chameleon?

Question

I'm working on a Pyramid project and am considering something which will come later. My particular use case is ascending the resource tree to construct a breadcrumb, but I'm curious about the general implementation, too.

In Python code, I'd be wanting this:

while resource is not None:
    pass  # do something with resource
    resource = resource.__parent__

But in Chameleon, you only get tal:repeat, which is a for loop. I can perceive that you could write a function which would be provided to the template:

def ascend(resource):
    while resource is not None:
        yield resource
        resource = resource.__parent__

This could then be used as tal:repeat="item ascend(resource)".

This might be generalised to a while loop thusly:

def while_(value, condition, suite):
    while condition(value):
        yield value
        value = suite(value)

Usage: tal:repeat="item while_(resource, lambda resource: resource is not None, lambda resource: resource.__parent__)". Ugly. I think you'd fairly quickly go to specialising!

Similarly, if break were supported, an infinite for loop would do. But it isn't. (Nor would it be wise to support it, I think.

Is there any nicer or easier way of achieving this? (The general case or my specific case.)

Answer 1

No, there isn't.

Chameleon templates are not meant to implement complex logic; the original Zope Page Template philosophy is to leave all the business logic up to Python code, and the template has just enough power to turn the information generated into a presentation, and nothing more.

This is why Chameleon doesn't give you anything more than a for loop; it's not the responsibility of the template to traverse complex structures, its responsibility is limited to creating a representation of already-processed data.