Creating a test charge using Stripe API

I am helping develop a website in python using Stripe API to process payments. My developer has gone home and I'm pretty new to this - so apologies if I've missed something really basic. I'm in test mode and trying to create a $300 charge but it's not doing what I ask. The relevant code is:

{ "id": "card_15rQo72eZvKYlo2CGJ9wTSdD", "currency": "usd", "amount": 300, "captured": true, }

Stripe's website suggest the following

{
  "amount" => 400,
  "currency" => "usd",
  "source" => array(
    "number" => "4242424242424242",
    "exp_month" => 12,
    "exp_year" => 2016,
    "cvc" => "314"
  ),
  "description" => "Charge for test@example.com"
}

Anyone got any ideas? Thanks so much in advance.