title: PyMongo: ObjectId() prints string instead of Object
body: <p>I am trying to convert an <code>id</code> string to an <code>ObjectId()</code></p>
  
  <p>I have the following imports:</p>
  
  <pre><code>from pymongo import MongoClient
  import pymongo
  from bson.objectid import ObjectId
  </code></pre>
  
  <p>If i print:</p>
  
  <pre><code>print(ObjectId(session['id']))
  print(ObjectId())
  </code></pre>
  
  <p>I get the following:</p>
  
  <pre><code>58a09f4255c205690833f9dd
  58f7d1606cb710c54a14ae82
  </code></pre>
  
  <p>Expected:</p>
  
  <pre><code>ObjectId("58a09f4255c205690833f9dd")
  ObjectId("58f7d1606cb710c54a14ae82")
  </code></pre>
  
  <p>FYI:</p>
  
  <pre><code>pymongo==3.4.0
  bson==0.4.7
  </code></pre>
  
  <p>I have tried (with no luck):</p>
  
  <pre><code>import bson
  print(bson.ObjectId(session['id']))
  print(bson.ObjectId())
  </code></pre>