<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:

def save_doc(self, doc, doc_id=None, following_attachments=None):
        '''Imitate sending HTTP request to CouchDB server'''

        self.log("save_document called for doc: %r", doc)

        d = defer.Deferred()

        try:
            if not isinstance(doc, (str, unicode, )):
                raise ValueError('Doc should be either str or unicode')
            doc = json.loads(doc)
            doc = self._set_id_and_revision(doc, doc_id)

            self.increase_stat('save_doc')

            self._documents[doc['_id']] = doc
            if doc['_id'] not in self._attachments:
                self._attachments[doc['_id']] = dict()
            attachments = doc.get('_attachments', dict())
            for name in attachments:
                if attachments[name].get('follows'):
                    if name not in following_attachments:
                        raise ValueError("Document id %s had attachment name"
                                         " %s marked with follows=True, but"
                                         " it was not passed to save_doc() "
                                         % (doc['_id'], name))

                    del attachments[name]['follows']
                    attachments[name]['stub'] = True
                    b = following_attachments[name].get_body()
                    self._attachments[doc['_id']][name] = b

                elif name not in self._attachments[doc['_id']]:
                    raise ValueError("Document id %s body has attachment "
                                     "named %s "
                                     "but it is not in our cache " %
                                     (doc['_id'], name))
            for name in self._attachments[doc['_id']].keys():
                if name not in attachments:
                    del self._attachments[doc['_id']][name]
                    self.log('Deleted attachment %s of the doc: %s because '
                             'its not in the _attachments key' %
                             (name, doc['_id']))

            self._expire_cache(doc['_id'])

            r = Response(ok=True, id=doc['_id'], rev=doc['_rev'])
            self._analize_changes(doc)
            d.callback(r)
        except (ConflictError, ValueError, ) as e:
            d.errback(e)

        return d