Let’s say I insert the document.

post = { some dictionary }
mongo_id = mycollection.insert(post)

Now, let’s say I want to add a field and update it. How do I do that? This doesn’t seem to work…..

post = mycollection.find_one({"_id":mongo_id}) 
post['newfield'] = "abc"
mycollection.save(post)

In pymongo you can update with:

mycollection.update({'_id':mongo_id}, {"$set": post}, upsert=False)

Upsert parameter will insert instead of updating if the post is not found in the database.
Documentation is available at mongodb site.

UPDATE For version > 3 use update_one instead of update:

mycollection.update_one({'_id':mongo_id}, {"$set": post}, upsert=False)

mycollection.find_one_and_update({"_id": mongo_id}, 
                                 {"$set": {"newfield": "abc"}})

should work splendidly for you. If there is no document of id mongo_id, it will fail, unless you also use upsert=True. This returns the old document by default. To get the new one, pass return_document=ReturnDocument.AFTER. All parameters are described in the API.

The method was introduced for MongoDB 3.0. It was extended for 3.2, 3.4, and 3.6.

I will use collection.save(the_changed_dict) this way. I’ve just tested this, and it still works for me. The following is quoted directly from pymongo doc.:

save(to_save[, manipulate=True[, safe=False[, **kwargs]]])

Save a document in this collection.

If to_save already has an “_id” then
an update() (upsert) operation is performed and
any existing document with that “_id” is
overwritten. Otherwise an insert() operation is performed. In this
case if manipulate is True an “_id” will be added to to_save and this
method returns the “_id” of the saved document. If manipulate is False
the “_id” will be added by the server but this method will return
None.

According to the latest documentation about PyMongo titled Insert a Document (insert is deprecated) and following defensive approach, you should insert and update as follows:

result = mycollection.insert_one(post)
post = mycollection.find_one({'_id': result.inserted_id})

if post is not None:
    post['newfield'] = "abc"
    mycollection.save(post)

This is an old question, but I stumbled onto this when looking for the answer so I wanted to give the update to the answer for reference.

The methods save and update are deprecated.

save(to_save, manipulate=True, check_keys=True, **kwargs)¶ Save a
document in this collection.

DEPRECATED – Use insert_one() or replace_one() instead.

Changed in version 3.0: Removed the safe parameter. Pass w=0 for
unacknowledged write operations.

update(spec, document, upsert=False, manipulate=False, multi=False,
check_keys=True, **kwargs) Update a document(s) in this collection.

DEPRECATED – Use replace_one(), update_one(), or update_many()
instead.

Changed in version 3.0: Removed the safe parameter. Pass w=0 for
unacknowledged write operations.

in the OPs particular case, it’s better to use replace_one.