I think you should develop more on your fundamentals, and try out your own code before asking a question here.
I guess what you are trying to achieve can be done like this.
import json
with open("example.json", "r") as f:
json_content = json.load(f)
message = json_content["message"]
variable = "var"
print(message.format(variable=variable))
# prints
# this is a message with a var
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solved how to print a string who contains variables, the string is contained is .json file? [closed]