Marshmallow: How to create a schema that has no

[sivabudh]

Given the following schema:

class CalendarSchema(Schema):
    days_of_month = fields.List(fields.Integer(), default=[], validate=validate.ContainsOnly(range(1, 31 + 1)))

The above schema works fine as long as the array passed contains the value between [1, 31]. However, if the array was an empty array ([]), the validation fails. How can I tweak fields.List such that it accepts an empty array?

[maximkulkin]

Looks like it's a bug in ContainsOnly validator which I guess treats None values and empty arrays the same (see these lines ). Also, I just found out that this validator is a bit tricky, because values can not be mentioned twice (which is not obvious from docstring).

[maximkulkin]

Solution for you would be either to write your own custom validator or to wait till bug in ContainsOnly is fixed.

class CalendarSchema(Schema):
    days_of_month = fields.List(fields.Integer(), default=[])

    @validates('days_of_month')
    def validate_days_of_month(self, data):
        for item in data:
            if item not in range(1, 31 + 1):
                raise ValidationError('Contains invalid value %s' % item)

[alexpantyukhin]

It seems that not allow empty values is put-up behavior. See test:
https://github.com/marshmallow-code/marshmallow/blob/dev/tests/test_validate.py#L607-L608

[YuriHeupa]

Would be nice to have ContainsOnly accepting a param like allow_empty to make empty lists allowed

[sloria]

I'm going to hold off on moving forward with this. You can use a custom validator if you want to accept empty collections as valid input.

from marshmallow import validate, Schema, fields
from marshmallow.utils import is_iterable_but_not_string


class LenientContainsOnly(validate.ContainsOnly):

    def __call__(self, value):
        if is_iterable_but_not_string(value) and not len(value):
            return True
        return super().__call__(value)

class MySchema(Schema):
    foo = fields.List(fields.Int(), validate=[LenientContainsOnly([1, 2])])

    class Meta:
        strict = True

sch = MySchema()
assert sch.load({'foo': []}).data == {'foo': []}

[lafrech]

As pointed out by @maximkulkin

validate.ContainsOnly([1, 2, 3])([1, 2, 3, 3])

raises ValidationError.

It seems intended (explicit del choices[index] in the code) but is not explicit in the docs, and in fact, I don't get the rationale.

The docs says

succeeds if value is a sequence and each element in the sequence is also in the sequence passed as choices

[sloria]

Good point, @lafrech and @maximkulkin . That specific behavior seems incorrect; I'll open a new issue.

[sloria]

After giving this some thought, I think ContainsOnly should accept empty values as valid inputs. Validating against empty vs. non-empty is outside the responsibility of ContainsOnly. If you need to validate that input is non-empty, use Length(min=1).