Re: How do I use python YAMLObject to_yaml and from_yaml?

Hi Matthew,

 > I've got a subclass of yaml.YAMLObject named Scratch and I want any
 > instance of Scratch to be able to render itself as yaml.  I figured I
 > could just call from_yaml, but this is a classmethod!  Why should
 > to_yaml be a classmethod?  I want to output yaml based on the
 > variables on the instance.

to_yaml accepts the parameters cls, dumper, data.  Here data is the 
instance of the class.

 > For Scratch.from_yaml, I can't figure out how to feed it the right
 > stuff to get it to take in a string of yaml and make an instance of
 > Scratch.

from_yaml doesn't take the string, it takes a YAML node. By the time 
from_yaml function is called, the YAML document is already parsed and 
converted to a graph structure.

 > I know I can use yaml.dump and yaml.load, but I want these to be
 > methods on my class.

Note that instances of YAMLObject automatically knows how to render 
themselves into YAML so you only need to override to_yaml/from_yaml if 
you want to change the way it is rendered.

I don't quite understand what you want exactly, but it seems you just 
want a method that converts a Scratch instance to a YAML document.  Here 
is what you can do

class Scratch(yaml.YAMLObject):
     tag = u'!Scratch'
     @classmethod
     def load(cls, document):
         obj = yaml.load(document)
         assert isinstance(obj, cls)
         return obj
     def dump(self):
         return yaml.dump(self)

Thanks,
Kirill

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