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ValueObject implementation in Python

Last updated at Posted at 2015-01-15

I searched for Value object implementation in Python, and found keleshev's implementation in github. But it doesn't provide the feature of immutability of Value object. So, I implemented property based immutable Value object class by myself.

  • explicit property definition
  • value based comparison, including hash

Any feedbacks are welcome.

ValueObject class

class ValueObject(object):
    """ base class for Value Objects

    please call _set_properties in constructor.

    def __new__(class_, *args, **kwargs):
        self = object.__new__(class_, *args, **kwargs)
        self.__initialized = False
        self.__params = dict()
        return self

    def _set_properties(self, mapping):
        if self.__initialized:
            raise AttributeError('callable only by constructor')

        self.__initialized = True
        self.__params = dict(mapping)
        self.__labels = [k for k, v in mapping]

        def setprop(key):
            setattr(self.__class__, key, property(lambda x: x.__params[key]))

        for k, v in mapping:
            if not hasattr(self.__class__, k):

    def get_values(self):
        return self.__params

    def __repr__(self):
        return unicode(self).encode('utf-8')

    def __unicode__(self):
        return u'%s(%s)' % (
            u', '.join(unicode(self.__params[k]) for k in self.__labels))

    def __hash__(self):
        return hash(repr(self))

    def __eq__(self, other):
        if not isinstance(other, self.__class__):
            return False
        return repr(self) == repr(other)

    def __ne__(self, other):
        if not isinstance(other, self.__class__):
            return True
        return repr(self) != repr(other)


# -*- coding: utf-8 -*-
import pytest

from . import ValueObject

class Date(ValueObject):

    def __init__(self, year, month, day):
            ('year', int(year)),
            ('month', int(month)),
            ('day', int(day)),

class Foo(ValueObject):
    def __init__(self, text='foo'):
            ('text', text),

def date():
    return Date(2012, 2, 20)

def foo_unicode():
    return Foo(u'ふー')

def test_properties(date):
    assert date.year == 2012
    assert date.month == 2
    assert date.day == 20

def test_immutable(date):
    with pytest.raises(AttributeError):
        date.year = 2015

def test_set_properties(date):
    with pytest.raises(AttributeError):
            ('year', 2015),

def test_repr(date):
    assert repr(date) == "Date(2012, 2, 20)"

def test_get_values(date):
    date.get_values == {'year': 2012, 'month': 2, 'day': 20}

def test_unicode(foo_unicode):
    assert foo_unicode.text == u'ふー'
    assert unicode(foo_unicode) == u"Foo(ふー)"
    assert repr(foo_unicode) == u"Foo(ふー)".encode('utf-8')


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