An object system for building declarative configurations in Python
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An object system for building declarative configurations in Python.

A class that has rich(ish) keyword-parameter handling, including explicit handling of required parameters, declaration of parameter dependencies and basic type-checking.

Keyword parameter processing will be performed for any method on the Deconfigurable that is decorated with the @parameter decorator. Each of these decorated methods performs the processing specific to that parameter. The returned value is then assigned on the appropriately named attribute.

>>> @parameter('foo')
>>> def handle_foo(self, value):
...    return value

Pass-Through Parameters:

If your parameter doesn't require any special treatment and should simply be stored on the Deconfigurable a shorthand can be used. Simply decorate a parameter method that passes, and value will be used as is.

>>> @parameter('foo')
>>> def handle_foo(self, value):
...    pass

Required Parameters:

Parameters are required by default. This means that a RequiredParameterError will be raised if the Deconfigurable.__init__ doesn't recieve the designated named argument.

Passing default= to @parameter will prevent the parameter from raising RequiredParameterError if the parameter isn't provided a value.

>>> @parameter('foo', default='bar')
>>> def handle_foo(self, value):
...    pass

Type-Checked Parameters:

A basic mechanism is supplied for ensuring that the passed parameter value is of a specific type. By supplying a type value to the @parameter's ensure_type keyword arguement any value passed in will be validated with an isinstance check.

>>> @parameter('bars', ensure_type=list)
>>> def handle_bars(self, value):
...     pass

Dependency Parameters:

Sometimes we will want to defer processing of a parameter until some other parameter has been processed first. To do so, simply pass a list of dependencies, depends_on, to @parameter.

>>> @parameter('foo')
>>> def handle_foo(self, foo):
...    pass

>>> @parameter('bar', depends_on=('foo', ))
>>> def handle_bar(self, bar):
...     # offset bar by foo
...     return bar + 

Contrived Example:

class Person(Deconfigurable):
    def format_name(cls, name):
        return name.strip().lower().capitalize()

    @parameter('first_name', ensure_type=str)
    def handle_first(self, name): 
        return Person.format_name(name)

    @parameter('last_name', ensure_type=str)
    def handle_last(self, name):
        return Person.format_name(name)

    @parameter('age', ensure_type=int)
    def handle_age(self, value): pass

    @parameter('registered_to_vote', ensure_type=bool, depends_on=('age',))
    def handle_vote(self, value):
        if self.age < 18:
            value = False
        return value

>>> me = Person(
...    first_name='Dustin',
...    last_name='Lacewell',
...    age=25, registered_to_vote=True,
... )
>>> print me.first_name, ' can vote: ', me.registered_to_vote
Dustin can vote: True

>>> kid = Person(
...    first_name='Some Kid',
...    last_name='Smith',
...    age=12, registered_to_vote=True,
... )
>>> print kid.first_name, ' can vote: ', kid.registered_to_vote
SomeKid can vote: False

Example from Spidersilk (twisted.web config lib):

from twisted.web.static import File
from twisted.web.proxy import ReverseProxyResource

from spidersilk import Domain, Httpd

application = Httpd(
    port = 80,
    default = '',
    domains = [
            resource=ReverseProxyResource('localhost', 8088, ''),

Where Domain is implemented as:

class Domain(Deconfigurable):
    @parameter('hostname', ensure_type=str)
    def _arg_hostname(self, hostname):

    @parameter('resource', ensure_type=twisted.web.resource.Resource)
    def _arg_resource(self, resource):