How can I verify Column data types in the SQLAlchemy ORM?

Question

Using the SQLAlchemy ORM, I want to make sure values are the right type for their columns.

For example, say I have an Integer column. I try to insert the value “hell

Answer 1

Improving on the answer of @zzzeek , I suggest the following solution:

from sqlalchemy import String
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.event import listen_for

Base = declarative_base()

@listens_for(Base, 'attribute_instrument')
def configure_listener(table_cls, attr, col_inst):
    if not hasattr(col_inst.property, 'columns'):
        return
    validator = getattr(col_inst.property.columns[0].type, 'validator', None)
    if validator:
        # Only decorate columns, that need to be decorated
        @listens_for(col_inst, "set", retval=True)
        def set_(instance, value, oldvalue, initiator):
            return validator(value)

That lets you do things like:

class Name(String):
    def validator(self, name):
        if isinstance(name, str):
            return name.upper()
        raise TypeError("name must be a string")

This has two benefits: Firstly, there is only an event triggered, when there actually is a validator attached to the data field object. It does not waste precious CPU cycles on set events for objects, that have no function for validation defined. Secondly, it allows you to define your own field types and just add a validator method there, so not all things that you want to store as Integer etc run through the same checks, just the ones derived from your new field type.