
**********************
Dumping Python classes
**********************

Only ``yaml = YAML(typ='unsafe')`` loads and dumps Python objects out-of-the-box. And
since it loads **any** Python object, this can be unsafe.

If you have instances of some class(es) that you want to dump or load, it is
easy to allow the YAML instance to do that explicitly. You can either register the
class with the ``YAML`` instance or decorate the class.

Registering is done with ``YAML.register_class()``::

    import sys
    import ruyaml


    class User:
        def __init__(self, name, age):
            self.name = name
            self.age = age


    yaml = ruyaml.YAML()
    yaml.register_class(User)
    yaml.dump([User('Anthon', 18)], sys.stdout)

which gives as output::

    - !User
      name: Anthon
      age: 18

The tag ``!User`` originates from the name of the class.

You can specify a different tag by adding the attribute ``yaml_tag``, and
explicitly specify dump and/or load *classmethods* which have to be called
``from_yaml`` resp. ``from_yaml``::

    import sys
    import ruyaml


    class User:
        yaml_tag = u'!user'

        def __init__(self, name, age):
            self.name = name
            self.age = age

        @classmethod
        def to_yaml(cls, representer, node):
            return representer.represent_scalar(cls.yaml_tag,
                                                u'{.name}-{.age}'.format(node, node))

        @classmethod
        def from_yaml(cls, constructor, node):
            return cls(*node.value.split('-'))


    yaml = ruyaml.YAML()
    yaml.register_class(User)
    yaml.dump([User('Anthon', 18)], sys.stdout)

which gives as output::

    - !user Anthon-18


When using the decorator, which takes the ``YAML()`` instance as a parameter,
the ``yaml = YAML()`` line needs to be moved up in the file::

    import sys
    from ruyaml import YAML, yaml_object

    yaml = YAML()


    @yaml_object(yaml)
    class User:
        yaml_tag = u'!user'

        def __init__(self, name, age):
            self.name = name
            self.age = age

        @classmethod
        def to_yaml(cls, representer, node):
            return representer.represent_scalar(cls.yaml_tag,
                                                u'{.name}-{.age}'.format(node, node))

        @classmethod
        def from_yaml(cls, constructor, node):
            return cls(*node.value.split('-'))


    yaml.dump([User('Anthon', 18)], sys.stdout)

The ``yaml_tag``, ``from_yaml`` and ``to_yaml`` work in the same way as when using
``.register_class()``.
