It seems that current API does not support it (which returns python dictionary, instead), or I’ve not carefully read the documentation.
Thanks for all your generous helps!
The findOne method returns a document or null. In PyMongo it is the same as specified at find_one.
To get the value of a field you have to the print(doc.key) # => "value". But, you can use projection in the query to retrieve limited fields from the document.
There are ORM like tools which are listed at Tools - PyMongo - this might be something you are looking for.