Implementation of Value Object Pattern for Python

From Wikipedia:

A value object is a small object that represents a simple entity whose equality isn't based on identity: i.e. two value objects are equal when they have the same value, not necessarily being the same object.

By default (if you subclass from object) Python follows "reference semantics", i.e. two objects are equal if they are the same instance. Value class implements "value semantics", i.e. if you subclass it your objects will be equall if they hold the same data.

This implementation will also inspect your __init__ signature to automatically assign instance variables and produce a nice __repr__ for your objects, dogether with a suitable __hash__ implementation.

Instead of asigning each instance variable manually:

>>> class Date(object):
...
...     def __init__(self, year, month=1, day=1):
...         self.year = year
...         self.month = month
...         self.day = day

Value defines __new__ that will look at your __init__ signature and assign instance variables based on it:

>>> from value import Value
...
>>> class Date(Value):
...
...     def __init__(self, year, month=1, day=1):
...         pass
...
>>> Date(2013, 3).year == 2013
True
>>> Date(2013, 3).month == 3
True
>>> Date(2013, 3).day == 1
True

Value defines __eq__ and __ne__ to implement value object semantics, i.e. objects holding the same data are compared equal:

>>> Date(2013, 3, 18) == Date(2013, 3, 18)
True
>>> Date(2013, 3, 18) != Date(1988)
True

Value also defines __repr__ for you based on __init__ signature:

>>> repr(Date(2013, 3, 18))
'Date(2013, 3, 18)'
>>> repr(Date(1988, 1, 1))
'Date(1988)'

Value also defines __hash__ for you, so that instances could be used in sets and as dictionary keys.

Installation

Use pip or easy_install:

pip install value==0.1.0

Alternatively, you can just drop value.py file into your project—it is self-contained.

• value is tested with Python 2.6, 2.7, 3.2, 3.3 and PyPy.
• value follows semantic versioning.