conditional validation

[marconi]

How can I achieve conditional validation?

Say I have two fields field_a and field_b, field_b is optional by default but it becomes required when field_a has certain values.

I've been looking at the existing validators and it seems it only receives the node that it should validate, how can I access other nodes within the same schema from a validator?

[mcdonc]

http://deformdemo.repoze.org/interfield/

http://deformdemo.repoze.org/require_one_or_another/

[thruflo]

I'm over three years late to this thread but there is (now?!) another way to do this!

Use a deferred that conditionally returns colander.required:

@colander.deferred
def conditionally_required(node, kw):
    some_condition = # whatever conditional logic you want
    if some_condition:
        return colander.drop # won't be required (or return a default like `None`)
    return colander.required # will be required

This can then be used as the missing value when defining a schema node:

foo = colander.SchemaNode(
    colander.String(),
    missing=conditionally_required
)

As a concrete example, if you have the request bound to your schema as kw['request'] and you wanted to make foo required unless a field called action has the value cheat, you could use:

@colander.deferred
def required_unless_cheating(node, kw):
    request = kw['request']
    action = request.params.get('action')
    if action == 'cheat':
        return colander.drop
    return colander.required