Frequently Asked Questions — PyMongo 2.7.2 documentation

# Frequently Asked Questions

Contents

- Frequently Asked Questions
  - Is PyMongo thread-safe?
  - How does connection pooling work in PyMongo?
  - Does PyMongo support Python 3?
  - Does PyMongo support asynchronous frameworks like Gevent, Tornado, or Twisted?
  - What does CursorNotFound cursor id not valid at server mean?
  - How do I change the timeout value for cursors?
  - How can I store `decimal.Decimal` instances?
  - I’m saving `9.99` but when I query my document contains `9.9900000000000002` - what’s going on here?
  - Can you add attribute style access for documents?
  - What is the correct way to handle time zones with PyMongo?
  - How can I save a `datetime.date` instance?
  - When I query for a document by ObjectId in my web application I get no result
  - How can I use PyMongo from Django?
  - Does PyMongo work with mod_wsgi?
  - How can I use something like Python’s `json` module to encode my documents to JSON?
  - Why do I get an error for dates on or after 2038?
  - Why do I get OverflowError decoding dates stored by another language’s driver?

## Is PyMongo thread-safe?

PyMongo is thread-safe and even provides built-in connection pooling for threaded applications.

## How does connection pooling work in PyMongo?

Every `MongoClient` instance has a built-in connection pool. The pool begins with one open connection. If necessary to support concurrent access to MongoDB from multiple threads in your application, the client opens new connections on demand.

By default, there is no thread-affinity for connections.

In versions before 2.6, the default `max_pool_size` was 10, and it did not actually bound the number of open connections; it only determined the number of connections that would be kept open when no longer in use.

Starting with PyMongo 2.6, the size of the connection pool is capped at `max_pool_size`, which now defaults to 100. When a thread in your application begins an operation on MongoDB, if all other connections are in use and the pool has reached its maximum, the thread pauses, waiting for a connection to be returned to the pool by another thread.

The default configuration for a `MongoClient` works for most applications:

    client = MongoClient(host, port)

Create this client once when your program starts up, and reuse it for all operations. It is a common mistake to create a new client for each request, which is very inefficient.

To support extremely high numbers of concurrent MongoDB operations within one process, increase `max_pool_size`:

    client = MongoClient(host, port, max_pool_size=200)

... or make it unbounded:

    client = MongoClient(host, port, max_pool_size=None)

By default, any number of threads are allowed to wait for connections to become available, and they can wait any length of time. Override `waitQueueMultiple` to cap the number of waiting threads. E.g., to keep the number of waiters less than or equal to 500:

    client = MongoClient(host, port, max_pool_size=50, waitQueueMultiple=10)

When 500 threads are waiting for a socket, the 501st that needs a connection raises `ExceededMaxWaiters`. Use this option to bound the amount of queueing in your application during a load spike, at the cost of additional exceptions.

Once the pool reaches its max size, additional threads are allowed to wait indefinitely for connections to become available, unless you set `waitQueueTimeoutMS`:

    client = MongoClient(host, port, waitQueueTimeoutMS=100)

A thread that waits more than 100ms (in this example) for a connection raises `ConnectionFailure`. Use this option if it is more important to bound the duration of operations during a load spike than it is to complete every operation.

When `disconnect()` is called by any thread, all sockets are closed.

`MongoReplicaSetClient` maintains one connection pool per server in your replica set.

See also

Requests

## Does PyMongo support Python 3?

Starting with version 2.2 PyMongo supports Python 3.x where x >= 1. See the Python 3 FAQ for details.

## Does PyMongo support asynchronous frameworks like Gevent, Tornado, or Twisted?

PyMongo fully supports Gevent.

To use MongoDB with Tornado see the Motor project.

For Twisted, see TxMongo. Compared to PyMongo, TxMongo is less stable, lacks features, and is less actively maintained.

## What does CursorNotFound cursor id not valid at server mean?

Cursors in MongoDB can timeout on the server if they’ve been open for a long time without any operations being performed on them. This can lead to an `CursorNotFound` exception being raised when attempting to iterate the cursor.

## How do I change the timeout value for cursors?

MongoDB doesn’t support custom timeouts for cursors, but cursor timeouts can be turned off entirely. Pass `timeout=False` to `find()`.

## How can I store `decimal.Decimal` instances?

MongoDB only supports IEEE 754 floating points - the same as the Python float type. The only way PyMongo could store Decimal instances would be to convert them to this standard, so you’d really only be storing floats anyway - we force users to do this conversion explicitly so that they are aware that it is happening.

## I’m saving `9.99` but when I query my document contains `9.9900000000000002` - what’s going on here?

The database representation is `9.99` as an IEEE floating point (which is common to MongoDB and Python as well as most other modern languages). The problem is that `9.99` cannot be represented exactly with a double precision floating point - this is true in some versions of Python as well:

    >>> 9.99
    9.9900000000000002

The result that you get when you save `9.99` with PyMongo is exactly the same as the result you’d get saving it with the JavaScript shell or any of the other languages (and as the data you’re working with when you type `9.99` into a Python program).

## Can you add attribute style access for documents?

This request has come up a number of times but we’ve decided not to implement anything like this. The relevant jira case has some information about the decision, but here is a brief summary:

1. This will pollute the attribute namespace for documents, so could lead to subtle bugs / confusing errors when using a key with the same name as a dictionary method.
2. The only reason we even use SON objects instead of regular dictionaries is to maintain key ordering, since the server requires this for certain operations. So we’re hesitant to needlessly complicate SON (at some point it’s hypothetically possible we might want to revert back to using dictionaries alone, without breaking backwards compatibility for everyone).
3. It’s easy (and Pythonic) for new users to deal with documents, since they behave just like dictionaries. If we start changing their behavior it adds a barrier to entry for new users - another class to learn.

## What is the correct way to handle time zones with PyMongo?

Prior to PyMongo version 1.7, the correct way is to only save naive `datetime` instances, and to save all dates as UTC. In versions >= 1.7, the driver will automatically convert aware datetimes to UTC before saving them. By default, datetimes retrieved from the server (no matter what version of the driver you’re using) will be naive and represent UTC. In newer versions of the driver you can set the `MongoClient` tz_aware parameter to `True`, which will cause all `datetime` instances returned from that MongoClient to be aware (UTC). This setting is recommended, as it can force application code to handle timezones properly.

Warning

Be careful not to save naive `datetime` instances that are not UTC (i.e. the result of calling `datetime.datetime.now()`).

Something like `pytz` can be used to convert dates to localtime after retrieving them from the database.

## How can I save a `datetime.date` instance?

PyMongo doesn’t support saving `datetime.date` instances, since there is no BSON type for dates without times. Rather than having the driver enforce a convention for converting `datetime.date` instances to `datetime.datetime` instances for you, any conversion should be performed in your client code.

## When I query for a document by ObjectId in my web application I get no result

It’s common in web applications to encode documents’ ObjectIds in URLs, like:

    "/posts/50b3bda58a02fb9a84d8991e"

Your web framework will pass the ObjectId portion of the URL to your request handler as a string, so it must be converted to `ObjectId` before it is passed to `find_one()`. It is a common mistake to forget to do this conversion. Here’s how to do it correctly in Flask (other web frameworks are similar):

    from pymongo import MongoClient
    from bson.objectid import ObjectId

    from flask import Flask, render_template

    client = MongoClient()
    app = Flask(__name__)

    @app.route("/posts/<_id>")
    def show_post(_id):
       # NOTE!: converting _id from string to ObjectId before passing to find_one
       post = client.db.posts.find_one({'_id': ObjectId(_id)})
       return render_template('post.html', post=post)

    if __name__ == "__main__":
        app.run()

See also

Querying By ObjectId

## How can I use PyMongo from Django?

Django is a popular Python web framework. Django includes an ORM, `django.db`. Currently, there’s no official MongoDB backend for Django.

django-mongodb-engine is an unofficial MongoDB backend that supports Django aggregations, (atomic) updates, embedded objects, Map/Reduce and GridFS. It allows you to use most of Django’s built-in features, including the ORM, admin, authentication, site and session frameworks and caching.

However, it’s easy to use MongoDB (and PyMongo) from Django without using a Django backend. Certain features of Django that require `django.db` (admin, authentication and sessions) will not work using just MongoDB, but most of what Django provides can still be used.

One project which should make working with MongoDB and Django easier is mango. Mango is a set of MongoDB backends for Django sessions and authentication (bypassing `django.db` entirely).

## Does PyMongo work with mod_wsgi?

Yes. See the configuration guide for PyMongo and mod_wsgi.

## How can I use something like Python’s `json` module to encode my documents to JSON?

The `json` module won’t work out of the box with all documents from PyMongo as PyMongo supports some special types (like `ObjectId` and `DBRef`) that are not supported in JSON. We’ve added some utilities for working with `json` and `simplejson` in the `json_util` module.

## Why do I get an error for dates on or after 2038?

On Unix systems, dates are represented as seconds from 1 January 1970 and usually stored in the C `time_t` type. On most 32-bit operating systems `time_t` is a signed 4 byte integer which means it can’t handle dates after 19 January 2038; this is known as the year 2038 problem. Neither MongoDB nor Python uses `time_t` to represent dates internally so do not suffer from this problem, but Python’s `datetime.datetime.fromtimestamp()` does, which means it is susceptible.

Previous to version 2.0, PyMongo used `datetime.datetime.fromtimestamp()` in its pure Python `bson` module. Therefore, on 32-bit systems you may get an error retrieving dates after 2038 from MongoDB using older versions of PyMongo with the pure Python version of `bson`.

This problem was fixed in the pure Python implementation of `bson` by commit `b19ab334af2a29353529` (8 June 2011 - PyMongo 2.0).

The C implementation of `bson` also used to suffer from this problem but it was fixed in commit `566bc9fb7be6f9ab2604` (10 May 2010 - PyMongo 1.7).

## Why do I get OverflowError decoding dates stored by another language’s driver?

PyMongo decodes BSON datetime values to instances of Python’s `datetime.datetime`. Instances of `datetime.datetime` are limited to years between `datetime.MINYEAR` (usually 1) and `datetime.MAXYEAR` (usually 9999). Some MongoDB drivers (e.g. the PHP driver) can store BSON datetimes with year values far outside those supported by `datetime.datetime`.

There are a few ways to work around this issue. One option is to filter out documents with values outside of the range supported by `datetime.datetime`:

    >>> from datetime import datetime
    >>> coll = client.test.dates
    >>> cur = coll.find({'dt': {'$gte': datetime.min, '$lte': datetime.max}})

Another option, assuming you don’t need the datetime field, is to filter out just that field:

    >>> cur = coll.find({}, fields={'dt': False})