title: Python: determine number of elements in a list passed as a function argument
body: <p>I have some function <code>foo(list)</code></p>
  
  <p>and i want to </p>
  
  <pre><code>assert foo(list with one element) == that one element
  
  assert foo(list with 100 elements) == foo(list)
  </code></pre>
  
  <p>how do I do this without knowing what the list itself is and ideally with just assert statements?</p>