title: Python Dictonary KeyError when key exist
body: <p>I'm new to python, and I'm doing a homework with it.</p>
  
  <p>I'm using PyEDA with Python3 on Debian 8.</p>
  
  <p>I created a variable named:</p>
  
  <pre><code>R = exprvars('r', n, n)
  </code></pre>
  
  <p>with <code>n = 4</code>, this gives me:</p>
  
  <pre><code>farray([[r[0,0], r[0,1], r[0,2], r[0,3]],
      [r[1,0], r[1,1], r[1,2], r[1,3]],
      [r[2,0], r[2,1], r[2,2], r[2,3]],
      [r[3,0], r[3,1], r[3,2], r[3,3]]])
  </code></pre>
  
  <p>Then, after some logic, I create an CNF boolean function f and a BDD with it using:</p>
  
  <pre><code>f = expr2bdd(f)
  </code></pre>
  
  <p>Then, the expression:</p>
  
  <pre><code>U = f.satisfy_one()
  </code></pre>
  
  <p>gives me:</p>
  
  <pre><code>{r[2,1]: 0, r[3,2]: 1, r[1,1]: 1, r[0,2]: 0, r[0,3]: 1, r[2,2]: 0, r[2,3]: 0, r[3,3]: 0, r[3,1]: 0, r[1,2]: 0, r[0,1]: 0, r[1,0]: 0, r[2,0]: 1, r[3,0]: 0, r[0,0]: 0, r[1,3]: 0}
  </code></pre>
  
  <p>But here is what I can't understand:
  I was expecting </p>
  
  <pre><code>U[R[0,0]]
  </code></pre>
  
  <p>To return a 0, but instead it gives me</p>
  
  <pre><code>KeyError: r[0,0]
  </code></pre>
  
  <p>What is the problem? <code>R[0,0]</code> gives me <code>r[0,0]</code> and the dictionary do have it as a key.</p>
  
  <p>[edit]</p>
  
  <p>When I said <code>R[0,0]</code> gives me <code>r[0,0]</code>, this means that I printed it using pdb, placing a breakpoint right after <code>U = f.satisfy_one()</code>:</p>
  
  <pre><code>(Pdb) p R[0,0]
  r[0,0]
  </code></pre>