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"""
Alias allows the dynamic aliasing of class attributes to a sequence of aliases
using metaclass programming to give a declarative way of expressing aliases as
an inner class on the object in question.

The main use of such a system would be to unify a disparate set of objects with
similar, but differently named attributes so that they can access be a single
piece of code.

Taking the example below, we define two types of records - an old-style and
new-style record. Note that the attributes on the class describe the same
things, but are named differently. By declaring a set of suitable aliases on
each style class we can then include both styles in our record set and treat
them as one:

>>> from alias import NewRecord, OldRecord, RecordSet
>>> rs = RecordSet(NewRecord(), OldRecord())
>>> rs.records
[<alias.NewRecord object at ...>, <alias.OldRecord object at ...>]
>>> rs.sort_by_added()
>>> rs.records
[<alias.OldRecord object at ...>, <alias.NewRecord object at ...>]
>>> [r.removed for r in rs.records]
[False, False]
>>> rs.mark_all_as_removed()
>>> [r.removed for r in rs.records]
[True, True]
>>> [r.deleted for r in rs.records]
[True, True]
>>> for r in rs.records: r.save()
Storing record: Old type record
Saving record: New style record
"""

import inspect

class AliasDescriptor(object):
   
def __init__(self, alias):
       
self.alias = alias
   
   
def __get__(self, instance, owner):
       
return getattr(instance, owner.__dict__['__aliases'][self.alias])
   
   
def __set__(self, instance, value):
        setattr
(instance,
                instance
.__class__.__dict__['__aliases'][self.alias],
                value
)
   
   
def __delete__(self, instance):
       
raise AttributeError('Deleting of aliased attribute %r is not supported' % self.alias)
       

class AliasType(type):
   
def __new__(cls, name, bases, attrs):
        new_class
= super(AliasType, cls).__new__(cls, name, bases, attrs)
       
       
# see whether we've got an Alias inner class:
       
if inspect.isclass(attrs.get('Alias', None)):
           
alias = attrs['Alias']
            setattr
(new_class, '__aliases', {})
           
for attr, value in alias.__dict__.items():
               
# don't want any "private" attrs appearing
               
if attr.startswith('_'):
                   
continue
               
# don't care if these aren't tuples or lists
               
if not isinstance(value, (tuple, list)):
                   
continue
               
# no point in setting up an alias for something which isn't a
               
# class attribute:
               
if not attr in attrs:
                   
continue
               
               
# if we've got to here put the lookups into the __aliases dict:
               
for key in value:
                    new_class
.__dict__['__aliases'][key] = attr
                    setattr
(new_class, key, AliasDescriptor(key))
           
       
return new_class

class Aliaser(object):
    __metaclass__
= AliasType
   
from datetime import datetime

class BaseRecord(object):
   
"""Dummy base class to demonstrate multiple-inheretance"""
   
pass

class OldRecord(BaseRecord, Aliaser):
    title
= 'Old type record'
    created
= datetime(2007, 1, 1, 9)
    deleted
= False
   
   
def store(self):
       
print 'Storing record: %s' % self.title
   
   
class Alias:
        created
= ('date_created', 'created_date', 'added')
        deleted
= ('removed',)
        store
= ('save',)
       
class NewRecord(BaseRecord, Aliaser):
    title
= "New style record"
    added
= datetime(2009, 1, 1, 10)
    removed
= False
   
   
def save(self):
       
print 'Saving record: %s' % self.title
   
   
class Alias:
        added
= ('date_created', 'created_date', 'created')
        removed
= ('deleted',)
        save
= ('store')
       
class RecordSet(object):
   
def __init__(self, *records):
       
self.records = list(records)
   
   
def sort_by_added(self):
       
self.records.sort(key=lambda r: r.added)
   
   
def mark_all_as_removed(self):
       
for record in self.records:
            record
.removed = True

if __name__ == "__main__":
   
import doctest
    doctest
.testmod(optionflags=doctest.ELLIPSIS)

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