Optional but not empty field in WTForms

I took a look at the source of the Optional validator:

class Optional(object):
    ...
    def __call__(self, form, field):
        if not field.raw_data or isinstance(field.raw_data[0], string_types) and not self.string_check(field.raw_data[0]):
            field.errors[:] = []
            raise StopValidation()

As you can see in and not self.string_check(field.raw_data[0]), empty strings are explicitly considered here. I wonder what would happen if I sent two values like a=foo&a=&b=bar.

Anyway, the quick solution for me was to implement a new validator:

class OptionalButNotEmpty(object):
    """
    Allows missing but not empty input and stops the validation chain from continuing.
    """
    # Code is a modified version of `Optional` (https://github.com/wtforms/wtforms/blob/master/wtforms/validators.py#L148)
    field_flags = ('optional', )

    def __call__(self, form, field):
        if not field.raw_data:
            raise wtforms.validators.StopValidation()