title: Python Math - TypeError: 'NoneType' object is not subscriptable
body: <p>I'm making a small program for math (no particular reason, just kind of wanted to) and I ran into the error "TypeError: 'NoneType' object is not subscriptable.</p>
  
  <p>I have never before seen this error, so I have no idea what it means.</p>
  
  <pre><code>import math
  
  print("The format you should consider:")
  print str("value 1a")+str(" + ")+str("value 2")+str(" = ")+str("value 3a ")+str("value 4")+str("\n")
  
  print("Do not include the letters in the input, it automatically adds them")
  
  v1 = input("Value 1: ")
  v2 = input("Value 2: ")
  v3 = input("Value 3: ")
  v4 = input("Value 4: ")
  
  lista = [v1, v3]
  lista = list.sort(lista)
  
  a = lista[1] - lista[0]
  
  list = [v2, v4]
  list = list.sort(list)
  
  b = list[1] = list[0]
  
  print str(a)+str("a")+str(" = ")+str(b)
  </code></pre>
  
  <p>The error:</p>
  
  <pre><code>Traceback (most recent call last):
    File "C:/Users/Nathan/Documents/Python/New thing", line 16, in &lt;module&gt;
      a = lista[1] - lista[0]
  TypeError: 'NoneType' object is not subscriptable
  </code></pre>