MongoDB 3.6 was released December 5. Today I’ve uploaded a release candidate for version 1.2 of Motor, the async Python driver for MongoDB. This will be a big release so I hope you try the release candidate and tell me if it works for you or if you find bugs.

Install the release candidate with pip:

python -m pip install motor==1.2rc0

Compatibility Changes

• MongoDB: drop 2.4, add 3.6
• Python: drop 2.6, continue to support 2.7 and 3.3+
• Tornado: drop 3, continue to support Tornado 4.5+
• aiohttp: support 2.0 and later, drop older version

See the Compatibility Matrix for the relationships among Motor, Python, Tornado, and MongoDB versions.

MongoDB 3.6 Features

Change Streams

There’s a new method MotorCollection.watch to acquire a Change Stream on a collection:

async for change in collection.watch():
    print(change)

I’ve written you a little sample app that watches a collection for changes and posts each notification over a websocket using the Tornado webserver:

Causal Consistency

There’s a new Session API to support causal consistency, meaning you can read your writes and perform monotonic reads, even reading from secondaries in a replica set or a sharded cluster. Create a session with MotorClient.start_session and use it for a sequence of related operations:

collection = client.db.collection

with (await client.start_session()) as s:
    doc = {'_id': ObjectId(), 'x': 1}
    await collection.insert_one(doc, session=s)

    secondary = collection.with_options(
        read_preference=ReadPreference.SECONDARY)

    # Sessions are causally consistent by default, we can read the doc
    # we just inserted, even reading from a secondary.
    async for doc in secondary.find(session=s):
        print(doc)

Array Filters

You can now update subdocuments in arrays within documents, and use “array filters” to choose which subdocuments to change. Pass the array_filters argument to MotorCollection.update_one, MotorCollection.update_many, MotorCollection.find_one_and_update, or MotorCollection.bulk_write.

For example if your document looks like this:

{
    _id: 1,
    points: [
        {x: 0, y: 0},
        {x: 1, y: 0},
        {x: 2, y: 0}
    ]
}

You can update all subdocuments where x is greater than zero:

await collection.update_one(
    {'_id': 1},
    {'$set': {'points.$[i].y': 5}},
    array_filters=[{'i.x': {'$gt': 0}}])

“mongodb+srv://” URIs

This new URI scheme is convenient for short, maintainable URIs that represent large clusters, especially in Atlas. See PyMongo’s MongoClient for details.

Retryable Writes

Now, with a MongoDB 3.6 replica set or sharded cluster, Motor will safely retry any write that’s interrupted by a network glitch or a primary failover, without any risk of a double write. Just add retryWrites to the URI:

mongodb://host1,host2,host3/?replicaSet=rs&retryWrites=true

See PyMongo’s MongoClient for details about retryable writes.

Thanks to everyone who contributed to this version:

• A. Jesse Jiryu Davis
• Bernie Hackett
• Jakub Wilk
• Karol Horowski

Peace,
A. Jesse Jiryu Davis