readpref.py

class Model(AttrDict):
    read_preference = read_preferences.PRIMARY
    ...


@contextmanager
def read_preference(pref):
    try:
        prev_pref = Model.read_preference
        Model.read_preference = pref
        yield
    finally:
        Model.read_preference = prev_pref


with read_preference(read_preferences.SECONDARY_PREFERRED):
    User.collection.find()  # secondary
    with read_preference(read_preferences.PRIMARY):
        User.collection.find()  # primary
    User.collection.find()  # secondary