title: Python lists being edited in place?
body: <p>So this is happening:</p>
  
  <pre><code>a = [1,2]
  b = a
  b += [3]
  print(a)
  
  [1, 2, 3]
  </code></pre>
  
  <p>It's extremely confusing and it's screwing up my project's test suite, which takes a list of strings and tests that it gets modified in the correct ways by each function.</p>
  
  <p>Did I just find a bug in Python, or is there some convoluted reason for this misbehavior?</p>