title: Pass different argument names to a function depending on condition
body: <p>My code needs a way to determine which one of two argument <em>names</em> (not values) it should pass to an API function. There are two possible variable names, and only one of them will be used in the call. Something like this metacode:</p>
  
  <pre><code>user_input = 'tgw-xxxx'
  
  if user_input == 'tgw-xxxx':
      &lt;gateway = (TransitGateway=user_input)&gt;
  else:
      &lt;gateway = (VpnGateway=user_input)&gt;
  
  some_api_call(
     CustomerGatewayId='blah',
     BgpAsn=65000,
     &lt;gateway&gt;      # pass either `TransitGateway` argument
                    # or `VpnGateway` argument
    )
  </code></pre>
  
  <p>So in other words the API will <em>only</em> accept either the TransitGateway or VpnGateway variable here, but not both.</p>
  
  <p>Do you know the best way to address this? Thanks!</p>