tri.declarative 0.18.0

tri.declarative contains class decorators to define classes with subclass semantics in the style of django Model classes.

tri.declarative

tri.declarative contains class decorators to define classes with subclass semantics in the style of django Model classes.

Running tests

You need tox installed then just make test.

License

BSD

Documentation

http://declarative.readthedocs.org.

Usage

@declarative

In example below, the @declarative(str) decorator will ensure that all str members of class Foo will be collected and sent as members constructor keyword argument.

from tri.declarative import declarative

@declarative(str)
class Foo(object):
    bar = 'barbar'
    baz = 'bazbaz'
    boink = 17

    def __init__(self, members):
        assert members['bar'] == 'barbar'
        assert members['baz'] == 'bazbaz'
        assert 'boink' not in members

f = Foo()

The value of the members argument will also be collected from sub-classes:

from tri.declarative import declarative

@declarative(str)
class Foo(object):

    def __init__(self, members):
        assert members['bar'] == 'barbar'
        assert members['baz'] == 'bazbaz'

class MyFoo(Foo):
    bar = 'barbar'
    baz = 'bazbaz'

    def __init__(self):
        super(MyFoo, self).__init__()

f = MyFoo()

The members argument can be given another name (things in the example below).

from tri.declarative.declarative import declarative

@declarative(str, 'things')
class Foo(object):

    bar = 'barbar'

    def __init__(self, **kwargs):
        assert 'things' in kwargs
        assert kwargs['things']['bar'] == 'barbar'

f = Foo()

Note that the collected dict is an OrderedDict and will be ordered by class inheritance and by using sorted of the values within each class. (In the ‘str’ example, sorted yields in alphabetical order).

Also note that the collection of class members based on their class does not interfere with instance constructor argument of the same type.

from tri.declarative import declarative

@declarative(str)
class Foo(object):
    charlie = '3'
    alice = '1'

    def __init__(self, members):
        assert members == OrderedDict([('alice', '1'), ('charlie', '3'),
                                       ('bob, '2'), ('dave', '4'),
                                       ('eric', '5')])
        assert 'animal' not in members


class MyFoo(Foo):
    dave = '4'
    bob = '2'

class MyOtherFoo(MyFoo):
    eric = '5'

    def __init__(self, animal)
        assert animal == 'elephant'

f = MyOtherFoo('elephant')

@creation_ordered

Class decorator that ensures that instances will be ordered after creation order when sorted.

This is useful for classes intended to be used as members of a @declarative class when member order matters.

from tri.declarative import creation_ordered

@creation_ordered
class Thing(object):
    pass

t1 = Thing()
t2 = Thing()
t3 = Thing()

assert sorted([t2, t3, t1]) == [t1, t2, t3]

Real world use-case

Below is a more complete example of using @declarative:

from tri.declarative import declarative, creation_ordered


@creation_ordered
class Field(object):
    pass


class IntField(Field):
    def render(self, value):
        return '%s' % value


class StringField(Field):
    def render(self, value):
        return "'%s'" % value


@declarative(Field, 'table_fields')
class SimpleSQLModel(object):

    def __init__(self, **kwargs):
        self.table_fields = kwargs.pop('table_fields')

        for name in kwargs:
            assert name in self.table_fields
            setattr(self, name, kwargs[name])

    def insert_statement(self):
        return 'INSERT INTO %s(%s) VALUES (%s)' % (self.__class__.__name__,
                                                 ', '.join(self.table_fields.keys()),
                                                 ', '.join([field.render(getattr(self, name))
                                                            for name, field in self.table_fields.items()]))


class User(SimpleSQLModel):
    username = StringField()
    password = StringField()
    age = IntField()


my_user = User(username='Bruce_Wayne', password='Batman', age=42)
assert my_user.username == 'Bruce_Wayne'
assert my_user.password == 'Batman'
assert my_user.insert_statement() == "INSERT INTO User(username, password, age) VALUES ('Bruce_Wayne', 'Batman', 42)"

# Fields are ordered by creation time (due to having used the @creation_ordered decorator)
assert my_user.get_meta().table_fields.keys() == ['username', 'password', 'age']

@with_meta

Class decorator to enable a class (and it’s sub-classes) to have a ‘Meta’ class attribute.

The members of the Meta class will be injected as arguments to constructor calls. e.g.:

from tri.declarative import with_meta

@with_meta
class Foo(object):

    class Meta:
        foo = 'bar'

    def __init__(self, foo, buz):
        assert foo == 'bar'
        assert buz == 'buz'

foo = Foo(buz='buz')

# Members of the 'Meta' class can be accessed thru the get_meta() class method.
assert foo.get_meta() == {'foo': 'bar'}
assert Foo.get_meta() == {'foo': 'bar'}

Foo()  # Crashes, has 'foo' parameter, but no has no 'buz' parameter.

The passing of the merged name space to the constructor is optional. It can be disabled by passing add_init_kwargs=False to the decorator.

from tri.declarative import with_meta

@with_meta(add_init_kwargs=False)
class Foo(object):
    class Meta:
        foo = 'bar'

Foo()  # No longer crashes
assert Foo().get_meta() == {'foo': 'bar'}

Another example:

from tri.declarative import with_meta

class Foo(object):

    class Meta:
        foo = 'bar'
        bar = 'bar'

@with_meta
class Bar(Foo):

    class Meta:
        foo = 'foo'
        buz = 'buz'

    def __init__(self, *args, **kwargs):
        assert kwargs['foo'] == 'foo'  # from Bar (overrides Foo)
        assert kwargs['bar'] == 'bar'  # from Foo
        assert kwargs['buz'] == 'buz'  # from Bar

This can be used e.g to enable sub-classes to modify constructor default arguments.

Changelog