wtform validator

def length(min=-1, max=-1):
    message = 'Must be between %d and %d characters long.' % (min, max)

    def _length(form, field):
        l = field.data and len(field.data) or 0
        if l < min or max != -1 and l > max:
            raise ValidationError(message)

    return _length

class MyForm(Form):
    name = TextField('Name', [Required(), length(max=50)])