title: Check Field Exist in Pymongo
body: <p>I have a collection like this : </p>
  
  <pre><code>{ 
    "_id":"1321464"
    "Sex":"Male"
    "Age":"20" 
    "City":"Toronto" #Maybe this field are not present.
  }
  </code></pre>
  
  <p>I want to find all my document with the field "City" not exist.
  I try :</p>
  
  <pre><code>collection.find({"sex":"Male"},{"City":{"$exists": False}},{'Age': 1, '_id':0})
  </code></pre>
  
  <p>And i have this error message : </p>
  
  <pre><code>File "/usr/local/lib/python2.7/dist-packages/pymongo/collection.py", line 1239, in find
      return Cursor(self, *args, **kwargs)
    File "/usr/local/lib/python2.7/dist-packages/pymongo/cursor.py", line 125, in __init__
      raise TypeError("skip must be an instance of int")
  TypeError: skip must be an instance of int
  </code></pre>