title: Python ValueError: too many values to unpack
body: <p>I am getting that exception from this code:</p>
  
  <pre><code>class Transaction:
      def __init__ (self):
          self.materials = {}
  
      def add_material (self, m):
          self.materials[m.type + m.purity] = m
  
      def serialize (self):
          ser_str = 'transaction_start\n'
  
          for k, m in self.materials:
              ser_str += m.serialize ()
  
          sert += 'transaction_end\n'
          return ser_str
  </code></pre>
  
  <p>The <code>for</code> line is the one throwing the exception. The <code>m</code>s are <code>Material</code> objects. Anybody have any ideas why?</p>