sqlalchemy type coercion

coercion.py

"""
Type coercion for sqlalchemy prior to committing to some database.

http://stackoverflow.com/questions/8980735/how-can-i-verify-column-data-types-in-the-sqlalchemy-orm
"""

from sqlalchemy import (
    Column,
    Integer,
    String,
    DateTime,
)

from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import event
import datetime

Base = declarative_base()

type_coercion = {
    Integer: int,
    String: str,
    DateTime: datetime.datetime,
}

# this event is called whenever an attribute
# on a class is instrumented
@event.listens_for(Base, 'attribute_instrument')
def configure_listener(class_, key, inst):
    if not hasattr(inst.property, 'columns'):
        return
    # this event is called whenever a "set"
    # occurs on that instrumented attribute
    @event.listens_for(inst, "set", retval=True)
    def set_(instance, value, oldvalue, initiator):
        desired_type = type_coercion.get(inst.property.columns[0].type.__class__)

        coerced_value = desired_type(value)

        return coerced_value

class MyObject(Base):
    __tablename__ = 'mytable'

    id = Column(Integer, primary_key=True)
    svalue = Column(String)
    ivalue = Column(Integer)
    dvalue = Column(DateTime)

x = MyObject(svalue=50)
assert isinstance(x.svalue, str)