title: How to convert objectid to string
body: <p>I want to get the string character from an ObjectId object. I use pymongo.
  eg: <code>ObjectId("543b591d91b9e510a06a42e2")</code>, I want to get <code>"543b591d91b9e510a06a42e2"</code>.</p>
  
  <p>I see the doc, It says <code>ObjectId.toString()</code>, <code>ObjectId.valueOf()</code>. </p>
  
  <p>So I make this code: <code>from bson.objectid import ObjectId</code>. </p>
  
  <p>But when I use <code>ObjectId.valueOf()</code>, It shows: </p>
  
  <blockquote>
    <p>'ObjectId' object has no attribute 'valueOf'.</p>
  </blockquote>
  
  <p>How can I get it? Thanks.</p>