How to define the schema to make a list not allow empty?

Question

In the python-eve REST API framework I define a list in a resource, the type of the list item is dict. And I don't want the list to be empty. So, how to define the schema?

{
    'parents' : {
        'type' : 'list',
        'schema' : {
            'parent' : 'string'
        }
    }
}

Answer 1

Currently the empty validation rule is only available for string types, but you can subclass the standard validator to make it capable of processing lists:

from eve.io.mongo import Validator

class MyValidator(Validator):
    def _validate_empty(self, empty, field, value):
        # let the standard validation happen
        super(Validator, self)._validate_empty(empty, field, value)
        # add your custom list validation
        if isinstance(value, list) and len(value) == 0 and not empty:
            self._error(field, "list cannot be empty")

or, if want to provide the standard empty error message instead:

from eve.io.mongo import Validator
from cerberus import errors

class MyValidator(Validator):
    def _validate_empty(self, empty, field, value):
        # let the standard validation happen
        super(Validator, self)._validate_empty(empty, field, value)
        # add your custom list validation
        if isinstance(value, list) and len(value) == 0 and not empty:
            self._error(field, errors.ERROR_EMPTY_NOT_ALLOWED)

Then you run your API like this:

app = Eve(validator=MyValidator)
app.run()

PS: I plan on adding lists and dicts to Cerberus' empty rule sometime in the future.