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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 target_for_product(self, product):
 """Looks up the target key for a product. :API: public :param product: The product to search for :return: None if there is no target for the product """

    for target, products in self._products_by_target.items():
      if product in products:
        return target
    return None