Expandos - semi-persistent maps

An expando is actually a semi-persistent data structure that allows for some interesting operations. They can be used for testing, throw-away classes, or for those situations where you want an object that’s just like another object but with one tiny little difference.

Basic Expandos

First of all, expandos allow for arbitrary attributes to be defined when they are instantiated:

>>> from pysistence import Expando
>>> a = Expando(foo='bar')
>>> a.foo
'bar'

Expandos also allow you to make copies of them with certain changes. For example, you can add on attributes:

>>> from pysistence import Expando
>>> a = Expando(foo='bar')
>>> a
{'foo': 'bar'}
>>> b = a.using(asdf='jkl;')
>>> b
{'asdf': 'jkl;', 'foo': 'bar'}
>>> a is b
False

You can also remove attributes:

>>> from pysistence import Expando
>>> a = Expando(foo='bar')
>>> a
{'foo': 'bar'}
>>> b = a.without('foo')
>>> b
{}

Expando classes

You may also make a class that has all of the properties of an Expando, but derives from another class:

>>> from pysistence import make_expando_class
>>> class Foo(object):
...     def bar(self):
...             print 'bar'
...
>>> ExpandoFoo = make_expando_class(Foo, 'ExpandoFoo')
>>> f = ExpandoFoo(a = 'b')
>>> f.a
'b'
>>> f.bar()
bar

In fact, the class Expando itself is defined this way:

Expando = make_expando_class(object, 'Expando')

You may also define class attributes in the mkExpndoClass function:

>>> from pysistence import make_expando_class
>>> SomeExpando = make_expando_class(object, 'SomeExpando', a='b')
>>> SomeExpando.a
'b'

Reference

pysistence.expando.make_expando_class(base, name, **kwargs)

Make an Expando class that inherits from base. The class’s __name__ attribute will be set by name. Also, the class will have class attributes specified by kwargs.

The returned class will have all of the functions of the Expando class (listed below).

class pysistence.expando.Expando(**kwargs)

Make a new Expando class. Expando classes will accept arbitrary keyword arguments to define instance attributes.

to_dict()

Get a copy of the Expando object’s __dict__ object.

to_public_dict()

Get a copy of the Expando object’s __dict__ object without any names that begin with an underscore.

without(*args)

Get a copy of the Expando without the attributes named by args (which is a set of strings).

using(**kwargs)

Get a copy of the Expando with instance attributes defined by kwargs.