..
    This file is part of lazr.delegates.

    lazr.delegates is free software: you can redistribute it and/or modify it
    under the terms of the GNU Lesser General Public License as published by
    the Free Software Foundation, either version 3 of the License, or (at your
    option) any later version.

    lazr.delegates is distributed in the hope that it will be useful, but
    WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
    or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
    License for more details.

    You should have received a copy of the GNU Lesser General Public License
    along with lazr.delegates.  If not, see <http://www.gnu.org/licenses/>.

The ``lazr.delegates`` Package
******************************

The ``lazr.delegates`` package makes it easy to write objects that delegate
behavior to another object. The new object adds some property or behavior on
to the other object, while still providing the underlying interface, and
delegating behavior.

=====
Usage
=====

The ``delegates`` function makes a class implement an interface by
delegating the implementation to another object. In the case of a class
providing an adapter, that object will be the 'context', but it can
really be any object stored in an attribute. So while the interfaces use
an inheritance mechanism, the classes use a composition mechanism.

For example we can define two interfaces IFoo0 <- IFoo...

    >>> from lazr.delegates import delegates
    >>> from zope.interface import Interface, Attribute
    >>> class IFoo0(Interface):
    ...     spoo = Attribute('attribute in IFoo0')

    >>> class IFoo(IFoo0):
    ...     def bar():
    ...         "some method"
    ...     baz = Attribute("some attribute")

And two classes (BaseFoo0 <- BaseFoo) that do something interesting.

    >>> class BaseFoo0:
    ...     spoo = 'some spoo'

    >>> class BaseFoo(BaseFoo0):
    ...     def bar(self):
    ...         return 'bar'
    ...     baz = 'hi baz!'

SomeClass can implement IFoo by delegating to an instance of BaseFoo
stored in the 'context' attribute. Note that ``delegates`` takes the
interface as the argument. By default, 'context' is the attribute
containing the object to which the interface implementation is
delegated.

    >>> class SomeClass(object):
    ...     delegates(IFoo)
    ...     def __init__(self, context):
    ...         self.context = context

    >>> f = BaseFoo()
    >>> s = SomeClass(f)
    >>> s.bar()
    'bar'

    >>> s.baz
    'hi baz!'

    >>> s.spoo
    'some spoo'

    >>> IFoo.providedBy(s)
    True

The ``delegates()`` function takes an optional keyword argument to change
attribute containing the object to delegate to. So an existing class,
such as SomeOtherClass, can declare the name of the attribute to which to
delegate.

    >>> class SomeOtherClass(object):
    ...     delegates(IFoo, context='myfoo')
    ...     def __init__(self, foo):
    ...         self.myfoo = foo
    ...     spoo = 'spoo from SomeOtherClass'

    >>> f = BaseFoo()
    >>> s = SomeOtherClass(f)
    >>> s.bar()
    'bar'

    >>> s.baz
    'hi baz!'

    >>> s.spoo
    'spoo from SomeOtherClass'

    >>> s.baz = 'fish'
    >>> s.baz
    'fish'

    >>> f.baz
    'fish'

The ``delegates()`` function can only be used in new-style classes. An
error is raised when a classic-style class is modified to implement an
interface.

    >>> class SomeClassicClass:
    ...     delegates(IFoo)
    Traceback (most recent call last):
    ...
    TypeError: Cannot use delegates() on a classic
        class: __builtin__.SomeClassicClass.

The ``delegates()`` function cannot be used out side of a class definition,
such as in a module or in a function.

    >>> delegates(IFoo)
    Traceback (most recent call last):
    ...
    TypeError: delegates() can be used only from a class definition.

==============
Implementation
==============

The Passthrough class is the implementation machinery of ``delegates()``. It
uses the descriptor protocol to implement the delegation behaviour provided by
``delegates()``. It takes two arguments: the name of the attribute that is
delegated, and the name of the attribute containing the object to which to
delegate.

To illustrate, p and p2 are two Passthrough instances that use the
instance assigned to 'mycontext' to call the 'foo' attribute and
the 'clsmethod' method.

    >>> from lazr.delegates import Passthrough
    >>> p = Passthrough('foo', 'mycontext')
    >>> p2 = Passthrough('clsmethod', 'mycontext')

Base is a class the implements both 'foo' and 'clsmethod'.

    >>> class Base:
    ...     foo = 'foo from Base'
    ...     def clsmethod(cls):
    ...         return str(cls)
    ...     clsmethod = classmethod(clsmethod)

Adapter is a class that has an instance of Base assigned to the
attribute 'mycontext'.

    >>> base = Base()

    >>> class Adapter:
    ...     mycontext = base

    >>> adapter = Adapter()

The Passthrough instances can get and set their prescribed attributes
when passed an instance of adapter.

    >>> p.__get__(adapter)
    'foo from Base'

    >>> p.__get__(None, Adapter) is p
    True

    >>> p2.__get__(adapter)()
    '__builtin__.Base'

    >>> p.__set__(adapter, 'new value')
    >>> base.foo
    'new value'

Passthrough does not implement __delete__. An error is raised if
it is called.

    >>> p.__delete__(adapter)
    Traceback (most recent call last):
    ...
    NotImplementedError
