It would be very useful to have possibility to define field level validators especially for those fields which do not have resolvers. My proposal is to add validators parameter to field to strawberry.field and validator decorator. This idea came to my mind when I was designing API for strawberry-graphql-django package (strawberry-graphql/strawberry-django#10). Soon we realized that this could be core feature of strawberry library.

I implemented prototype code and made draft pull request #809.

Validators could be used for following purposes

• field data validation
• data pre processing (input types)
• data post processing (output types)

Option 1

I'd propose following syntax for validators

def permission_validator(value, info):
    if not info.context.request.user.has_permission():
        raise Exception('Permission denied')
    return value

@strawberry.type
class User:
    name: str = strawberry.field(validators=[permission_validator])

Option 2

I'd also propose decorator syntax, which is quite similar with existing field resolver syntax. Strawberry field would provide validator decorator which could be then used to tie the field together with validator function.

@strawberry.input
class UserInput:
    name: str = strawberry.field()

    @name.validator
    def name_validator(value, info):
        if 'bad' in value:
            raise ValueError('name contains bad word')
      return value.title()

Option 3 (not doable)

def permission_validator(value, info):
    if not info.context.request.user.has_permission():
        raise Exception('Permission denied')
    return value

@strawberry.type
class User:
    # this does not work
    @strawberry.validator(permission_validator)
    name: str = strawberry.field()