AppEngine bulkloader export Model with self-define

I want to use bulkloader to download all entities in a model with some self-defined Property.

If I define a model like this,

class MyType:
    def __init__(self, arg):
        self.name = arg['name']
        self.id = arg['id']

class MyProperty(db.Property):
    def get_value_for_datastore(self, instance):
        val = super(MyProperty, self).get_value_for_datastore(instance)
        if type(val) == dict:
            val = MyType(val)
        return pickle.dumps(val)

    def make_value_from_datastore(self, val):
        return None if val is None else pickle.loads(str(val))

class MyModel(db.Model):
    info = MyProperty()

then how can I download MyModel using the bulkloader such that there will not be un-pickled value in the file? I think I should define the export_transform for info in bulkloader.yaml, but I don't know what it should be like.

transformers:
- kind: MyModel
  connector: csv
  property_map:
  - property: __key__
    external_name: log_id
    export_transform: transform.key_id_or_name_as_string
  - property: info
    external_name: info
    export_transform: ### HERE ###

I've seen transform.py but still have no idea about how it works. Please tell my any method that can solve my problem. Thanks.

Okay I'm answering my own questions...

I still don't know why pickle doesn't work, but after changing to use simplejson instead of pickle, I can successfully export MyProperty in the designated format.

The bulkloader.yaml may look like this.

python_preamable:
- import myutils
- import django.utils.simplejson
...

transformers:
- kind: MyModel
  connector: csv
  property_map:
  ...
  - property: info
    external_name: info
    export_transform: myutils.load_info

And in myutils.py load_info may look like this.

def load_info():
    def load(x):
        if not x:
            return ''
        info = simplejson.loads(x)
        return '%s-%s' % (info['id'], info['name']) # the output format for info
    return load