title: Setting defaults for empty arguments (Python)
body: <p>Let's say we have the function <code>f</code> and I need the argument <code>b</code> to default to an empty list, but can't set b=[] because of the issue around mutable default args.</p>
  
  <p>Which of these is the most Pythonic, or is there a better way?</p>
  
  <pre><code>def f(a, b=None):
     if not b:
       b = []
     pass
  
  def f(a, b=None):
     b = b or []
     pass
  </code></pre>