>>> test = pymongo.Connection().test.test
>>> test.insert(

{'a': 1}

)
ObjectId('4e0509dce4721c2217000000')

>>> test.find_and_modify({}, {'$inc': {'a': 1}})

{u'a': 1, u'_id': ObjectId('4e0509dce4721c2217000000')}

>>> test.find_and_modify({}, {'$inc': {'a': 1}}, fields=['a'])

{u'_id': ObjectId('4e0509dce4721c2217000000')}

As soon as you pass a `fields` option only the _id field is returned.