django admin search bar add placeholder variable and how to call it in modelAdmin

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I want to add placeholder to search bar .First,I override the template change_list.html and find the search bar <input>,I know add attribute placeholder ,and I defined a variable placeholder={{'''search_tip'}}to placeholder, as my app has three searchbar in different modelAdmin, I want to change the context of the placeholder by changing the variable..but I don't konw how to call the variable 'search_tip''  in the modelAdmin!