ConverterProperty

properties.py

class ConverterProperty(db.Property):
    """Property class for converting data

    Example:
        Given a class MyModel which an IntegerProperty that should be converted to a StringProperty:

            class MyModel(db.Model):
                value = IntegerProperty()

        In the conversion step the model would look like this:

            class MyModel(db.Model):
                value = ConverterProperty(lambda x: str(x))

        After all data has been converted, you can replace the property:

            class MyModel(db.Model):
                value = StringProperty()
    """

    def __init__(self, fnc, **kwargs):
        """Positional argument `fnc` is the converter function that is called for the property value"""

        super(ConverterProperty, self).__init__(**kwargs)
        self.convert = fnc

    def get_value_for_datastore(self, model_instance):
        """Get value from property to send to datastore."""

        value = super(ConverterProperty, self).get_value_for_datastore(model_instance)
        return self.convert(value)