This defines a new enum type using the sqlalchemy type decorator. Data is stored with the sqlalchemy SmallInteger type while strings are used with the model. Preferable to the varchar (too large) or native enum (pain on postgres).

enum_int_type.py

import sqlalchemy as sa

class EnumIntType(sa.types.TypeDecorator):
    impl = sa.types.SmallInteger

    values = None

    def __init__(self, values=None):
        sa.types.TypeDecorator.__init__(self)
        self.values = values

    def process_bind_param(self, value, dialect):
        return None if value == None else self.values.index(value)

    def process_result_value(self, value, dialect):
        return None if value == None else self.values[value]

Use is similar to the sqlalchemy Enum type:

sa.Column("type", EnumIntType(("one", "two", "three")), nullable=False)