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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def serialize(self, attr, obj, accessor=None, **kwargs):
 """Pulls the value for the given key from the object, applies the field's formatting and returns the result. :param str attr: The attribute or key to get from the object. :param str obj: The object to pull the key from. :param callable accessor: Function used to pull values from ``obj``. :param dict kwargs': Field-specific keyword arguments. :raise ValidationError: In case of formatting problem """

        if self._CHECK_ATTRIBUTE:
            value = self.get_value(obj, attr, accessor=accessor)
            if value is missing_ and hasattr(self, 'default'):
                default = self.default
                value = default() if callable(default) else default
            if value is missing_:
                return value
        else:
            value = None
        return self._serialize(value, attr, obj, **kwargs)