python - SQLAlchemy custom field with default value for file uploads in Flask -

i'm trying create custom field in sqlalchemy managing uploaded files in flask application. i'm trying achieve similar django's filefield.

so want able is, given model:

class testmodel(base):     __tablename__ = "testmodel"     id = column(integer(), primary_key=true)     profile_photo = column(filefield(upload_set, 100)) 

i want able like:

m = testmodel() m.profile_photo.save(request.files["profile_photo"]) models.db.session.add(m) models.db.session.commit() 

what have based on built in string type , works this:

class filefield(types.typedecorator):     impl = string      def __init__(self, upload_set, *args, **kwargs):         super(filefield, self).__init__(*args, **kwargs)         self.upload_set = upload_set      def process_bind_param(self, value, dialect):         if value not none:             return value.filename         return value      def process_result_value(self, value, dialect):         return filenamestring(self.upload_set, value)   class filenamestring(object):      def __init__(self, upload_set, filename):         self.upload_set = upload_set         self.filename = filename      def save(self, req_file):         self.filename = self.upload_set.save(req_file)         return self.filename      @property     def url(self):         return self.upload_set.url(self.filename) 

this works when model exists in database not when model created scratch profile_photo attribute none. ideally want default value of type blank filefield, although haven't figured out like. problem can't figure out how create default value attribute without having in constructor of class uses filefield.

so, idea how create default value profile_photo attribute?