The alias metaclass allows a declarative way of creating aliases to attributes on a class. The main purpose of which is to adapt classes for duck-typing (see doc-string for me detail).
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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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