You just assign to the attribute...
mymodel = MyModel()
mymodel.name = 'John'
mymodel.surname = 'Smith'
If you wanted to in effect do what the initialiser does, then you can use setattr
attributes = {'name':'John', 'surname':'Smith'}
record = Model()
for k, v in attributes.iteritems():
setattr(record, k, v)
See: https://developers.google.com/appengine/docs/python/datastore/modelclass#Model
and from there:
An application creates a new data entity by instantiating a subclass
of the Model class. Properties of an entity can be assigned using
attributes of the instance, or as keyword arguments to the
constructor.
s = Story()
s.title = "The Three Little Pigs"
s = Story(title="The Three Little Pigs")