Validating list elements are unique in marshmallow

Question

I would like to understand if I can use marshmallow validate function to check whether all elements in a list are unique.

I have the following schema:

from marshmallow import Schema, fields, validate

class PaymentSchema(Schema):
    ...
    currencies = fields.List(
        fields.String(
            required=True,
            validate=[
                validate.OneOf(["USD", "EUR", "AUS", "GBP"]),
                validate.Length(min=1, max=4)
            ]
        )
    )

Is there any possibility to guarantee that given a list of currencies with duplicated currencies (["USD", "EUR", "EUR"]) raises a ValidationError?

Thanks!

Answer 1

The validates decorator allows custom validation:

from marshmallow import Schema, ValidationError, fields, validate, validates

class PaymentSchema(Schema):
    currencies = fields.List(
        fields.String(
            required=True,
            validate=[
                validate.OneOf(["USD", "EUR", "AUS", "GBP"]),
                validate.Length(min=1, max=4)
            ]
        )
    )

    @validates('currencies')
    def no_duplicate_currencies(self, value):
        if len(value) != len(set(value)):
            raise ValidationError('currencies must not contain duplicate elements')