title: What is `lambda` in Python code? How does it work with `key` arguments to `sorted`, `sum` etc.?
body: <p>I saw some examples using built-in functions like <code>sorted</code>, <code>sum</code> etc. that use <code>key=lambda</code>.</p>
  <p>What does <code>lambda</code> mean here? How does it work?</p>
  <hr />
  <p><sub>For the general computer science concept of a lambda, see <a href="https://stackoverflow.com/questions/16501">What is a lambda (function)?</a>.</sub></p>