title: discord.py how to accept optional arguments
body: <p>I have made a system that uses JSON files to create a currency system with a discord bot using discord.py
  I want users to be able to do +balance to show them how much they have, or +balance  in order to check someone else's balance. I tried doing a try: to grab the members' balance and excepting if there was no argument for the member, but I don't know what kind of error it is for that. How do I make it so that the bot will assume if there is no  argument that the member they want is ctx.message.author?</p>
  
  <pre><code>if exists('balances.json'):
      with open('balances.json', 'r') as file:
          balances = json.load(file)
          print(balances)
  def save():
      with open('balances.json', 'w+') as f:
          json.dump(balances, f)
  
  
  
  #Allows users to check their balance
  @bot.command(pass_context=True)
  async def balance(ctx, member: discord.Member):
      global balances
      if str(member.id) not in balances.keys():
          await ctx.channel.send(f'{member.name} not registered')
          return
      if str(member.id) in balances.keys():
          requestedUserBalance = balances[str(member.id)]
          await ctx.channel.send(f'{member.name} has {requestedUserBalance} LotusTokens')
  </code></pre>