How to use list in Pydgraph

Hi,I have this problem.I want to query with Pydgraph,and have to query for every element in the list.I can do this with Retal, but it won’t work in client.
In the retal:

 {
  var(func: eq(fruits,["苹果","西瓜","香蕉"])){
     uid
    B AS fruits
   ~sall_fruits{
      uid
      A AS seller_name
    }
  }
  seller_info(func:uid(A)){
    uid
    seller_name
    sall_fruits@filter(uid(B)){
      uid
      fruits
     }
  }   
}

Code in Pydgraph :

     def query_test(self,fruits_list):
        query = """query all($a: string) {
            var(func: eq(fruits,$a)){
                uid
                B AS fruits
                ~sall_fruits{
                     uid
                    A AS seller_name
                    }
            }
            seller_info(func:uid(A)){
               uid
               seller_name
               sall_fruits@filter(uid(B)){
                     uid
                     fruits
               }
            }   
        }"""
        variables = {'$a': fruits_list}
        res = self.client.txn(read_only=True).query(query)
        all_node = json.loads(res.json)
        if len(all_node["seller_info"]) == 0:
            return 0
        else:
            return all_node["seller_info"]

An error will be reported using res = self.client.txn(read_only=True).query(query,variables=variables)
The results are not available using res = self.client.txn(read_only=True).query(query)

I’m using version 20.03.

Trying to replicate this. Please hold

Just like this