pass object or use self.object?

[Tim Arnold]

Hi,
I have a few classes that manipulate documents. One is really a
process that I use a class for just to bundle a bunch of functions
together (and to keep my call signatures the same for each of my
manipulator classes).

So my question is whether it's bad practice to set things up so each
method operates on self.document or should I pass document around from
one function to the next?
pseudo code:

class ManipulatorA(object):
    def process(self, document):
        document = self.do_one_thing(document)
        document = self.do_another_thing(document)
        # bunch of similar lines
        return document

or

class ManipulatorA(object):
    def process(self, document):
        self.document = document
        self.do_one_thing() # operates on self.document
        self.do_another_thing()
        # bunch of similar lines
        return self.document

I ask because I've been told that the first case is easier to
understand. I never thought of it before, so I'd appreciate any
comments.
thanks,
--Tim