title: How to check if a list is empty
body: <p>My definition of empty lists are:</p>
  
  <pre><code>a = []
  a = [[], []]
  a = [[], [], [[], []]]
  </code></pre>
  
  <p>The common ways are not applicable, because the lengths of last two lists are not 0, and they will be considered as True in if condition.</p>
  
  <pre><code>if a:
      print(True)    # True
  #-------------------------
  print(len(a) != 0) # True
  </code></pre>
  
  <p>Are there any good ways to check?</p>