The underlying philosophy

Suppose you are about to develop a reusable component offering a secure login service. This component might need two different sorts of features: say, a mandatory procedure to check login and password, and some optional others, e.g. a special procedure to be called when the maximal number of login attempts is reached. Your component implements a default procedure to check credentials against, say, the /etc/password file, and does not have any limit on the number of retries after failure.

However and since this is a reusable component you wish that the procedure that checks credentials, the maximum number of failures and the action to be taken can be conveniently changed at runtime.

One approach consists in making these parameters settable in your component ; this will work well, but this may not be really convenient:

for you, as the developper of the reusable component, because this make the API of the component all the more cumbersome since the number of possible external "tune-ups" grows
for the users of your component, for which it may be handier to be able to provide a single object gathering its own mechanisms to tune the reusable component ¹. A good reason for this is when the some processings involved depends on some other objects the reusable component has no way to know about, and/or if answers to some questions cannot be determined but at runtime.

The delegation offers you a general mechanism to solve these two issues. The DelegateWrapper class allows the following features:

it makes it simple to have a single place to specify and document what can be changed along with specific documentation: this is the delegate's interface.
The users of your component do not have to implement all methods of the delegate's API, they only provide whatever methods is adequate for their own use ².
It can also be used to notify a delegate when some actions are about to be done, have been processed, etc., thus making it possible to keep an external object in sync. with the reusable component's internal state, without violating encapsulation.

[1]	of course, you may run into cases where such a design is inaccurate. All this is obviously a general discussion which might be inadequate given a specific architecture or design philosophy!

[2]	except if you enforce your own rules, such as "if the delegates provides methodOne it should provide methodTwo as well", but this an other story (see DelegateWrapper.respondsTo())

How to use

We will stick on the example exposed above to explain how delegation is achieved with this module.

First, define a general "interface": a class, which defines and documents the methods a delegate for your component might implement:

class LoginComponentDelegate:

  'Delegate for the reusable LoginComponent'

  def checkCredentials(self, login, password):

    '''

    Implement this method if you want to replace the default checking

    mechanism by yours. This method should raise InvalidCredentials

    if parameters 'login' and 'password' do not match an valid user.

    Parameters: both 'login' and 'password' are strings.

    '''

  def maximumNumberOfFailures(self):

    '''

    Implement this method to ...

    A return value equal to '-1' means no limit.

    '''

  def numberOfFailuresHasReachedItsMaximum(self):

    '''

    ...

    '''

The delegation module supplies one class, DelegateWrapper, to handle delegate. Its role is to inspect a delegate's interface...

To use it, your reusable component's class will look like:

class LoginComponent:

  def __init__(self):

    '... Initialization  ...'

    self._delegate=DelegateWrapper(LoginComponentDelegate)

    self._nbOfFailures=0

  def setDelegate(self, object):

    '''Sets the login component's delegate'''

    self._delegate.setDelegateObject(object)

  def validateCredentials(self, login, password):

    '''

    delegate's 

    '''

    try:

      if self._delegate.respondsTo('checkCredentials'):

         # DelegateWrapper acts as a proxy for the wrapped object

        self._delegate.checkCredentials(login, password)

      else:

        # the default mechanism, raising InvalidCredentials if no user

        # can be found

    except:

      self._nbOfFailures+=1

      if self._nbOfFailures>=self.maximumNumberOfFailures():

        [...you get the idea...]

      raise

  def maximumNumberOfFailures(self):

    '''

    Defaults to -1 (no limit). You can change this limit by implementing

    the delegate's method 'maximumNumberOfFailures'

    '''

    if self._delegate.respondsTo('maximumNumberOfFailures'):

      return self._delegate.maximumNumberOfFailures()

    else:

      return -1

  [...]

The delegate itself, as it will be provided by the users of the reusable component, can be an instance of any class. You should have noticed in the sample code above that the DelegateWrapper acts as a proxy : it automatically forwards any function call it cannot service (see Gotchas, below) to the wrapped object --the real delegate). Note, however, that the DelegateWrapper is not an "absolute" proxy: the forwarded messages are __only__ those which are declared in the delegate's interface.

Gotchas

There are several points which you should be aware of when using this class, some of which should be indicated in your own documentation:

the DelegateWrapper works by inspecting the interface-class and registering the signatures of its methods. By signature we mean: name, parameters and their default values, if any. These signatures are then compared to those of the delegate object when it is supplied. Thus:
- if a delegate object does implement a function whose name matches one of the delegate's API but which uses different number of parameters, the same number of parameter with different names, or the same number of parameters and the same names but with different default values, this function will not be considered as an valid delegate method (hence 'respondsTo(thatMethodName)' will be false).
- The DelegateWrapper determines the mapping between the delegate's API and the delegate object when this object is supplied ('setDelegateObject()') ; this mapping determines the answer for message 'respondsTo' as well as whether that message should be forwarded to the delegate by the DelegateWrapper.
  
  Thus, if the delegate object can change at runtime, e.g. not responding to a message at some point of runtime and being able to service the same message later, that message will never be considered available. In that case, the provider for the delegate object should take care to call the component's 'setDelegate()' method each time the delegate object changes --this ensures that the mapping between the delegate's API and the object's capabilities is correctly recomputed.
The DelegateWrapper itself declares some methods: DelegateWrapper.respondsTo, DelegateWrapper.delegateObject, DelegateWrapper.setDelegateObject, DelegateWrapper.reevaluateDelegateObject. If the delegate's interface uses at least one of these methods, the automatic forwarding of method calls is automatically disabled. In that case, a warnings is issued (warning level: RuntimeWarning) when the delegate object is set. You can still send messages to the delegate object by sending the messages to the DelegateWrapper.delegateObject().

Licence

Copyright (c) 2001-2004, Sebastien Bigaret
All rights reserved.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are
met:

    * Redistributions of source code must retain the above copyright
      notice, this list of conditions and the following disclaimer.

    * Redistributions in binary form must reproduce the above copyright
      notice, this list of conditions and the following disclaimer in the
      documentation and/or other materials provided with the distribution.

    * Neither the name of the software's copyright holder, Sebastien
      Bigaret, nor the names of its contributors may be used to endorse or
      promote products derived from this software without specific prior
      written permission.

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IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
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Module Modeling.delegation

The underlying philosophy

How to use

Gotchas

Licence

CVS information