title: Assigning input markers to function output in python
body: <p>Sorry for the seemingly elementary question. What I'm trying to implement is summarized in the following steps:</p>
  
  <ol>
  <li>Generate input variables: x, y.</li>
  <li>Let z = F(x,y).</li>
  <li>Plot z's for particular combinations of x and y.</li>
  </ol>
  
  <p>For example:</p>
  
  <pre><code>zlist = []
  for _ in range(100):
      x = np.random.random()*1.
      y = np.random.random()*.5
      if x &lt; .5:
          z = y / 2
      else:
          z = y * 2
      zlist.append(z)
  </code></pre>
  
  <p>Now if I want to plot z for all the x between (0, 0.3), I presumably would need some marker on each element in zlist indicating its inputs variables. How would I attach such marker and then access it from the list when plotting?</p>