How would I get access to a file I don't know the name of

Question

I might describe this very poorly but here we go. What I am doing is creating a file with the user id from discord. In a different function, I want to access the file but I don't know the name of it

import json
import discord

@client.command()
async def function1(ctx):
 test1 = 3
 author = ctx.message.author.mention
 await ctx.send(author)

 with open(author+'.txt', 'w+') as outfile:
   json.dump(test1, outfile)

@client.command()
async def function2(ctx):
 with open(author+'.txt') as infile: #The "author+'.txt'" does not work because it does not know what author is.
   test2 = json.load(infile)
 await ctx.send(test2)

How would I get around this and access this file? I have an idea, I could save the author and then call it in function two. This would not work because there are many people using the bot. Am I on the right track? Thanks for the help in advance.