There are use cases when you just want to adapt an object if some condition is true. For instance I wanted to adapt xml-rpc requests only if they were meant for my application, so I wouldn’t interfere with other packages that may use xml-rpc. I found it was rather easy (after figuring out how :) to make a conditional adapter. Here’s a simple example that can be run in a zope prompt (zopectl debug).

First we have some imports, basic interfaces and classes.

from zope.interface import Interface, implements, implementer

from zope.component import adapts, adapter, provideAdapter

class IA(Interface):

pass

class A(object):

implements(IA)

x=7

class IB(Interface):

pass

An adapter class for adapting objects providing IA to IB would typically look like this:

class AdapterAtoB(object):

implements(IB)

adapts(IA)

def __init__(self, context):

....self.context = context

After registering this adapter one would be able to adapt objects providing IA to IB:

provideAdapter(AdapterAtoB)

>>> a = A()

>>> b = IB(a)

But let’s say you want the adapter lookup to succeed only when A.x = 7, then you would need to do something like this:

class AdapterAtoB(object):

implements(IB)

#we don't need adapts anymore

def __init__(self, context):

....self.context = context

@adapter(IA)

@implementer(IB)

def a_to_b(obj):

if obj.x == 7:

....return AdapterAtoB(obj)

else:

....return None

>>> provideAdapter(a_to_b)

>>> a = A()

>>> b = IB(a)

>>> a.x = 2

b = IB(a)

TypeError: ('Could not adapt', <A object at ...>, <InterfaceClass IB>)

Mission accomplished.

(had problems with indenting, that’s why the dots are there)