The Python interface to the Redis key-value store.
$ sudo pip install redis
or alternatively (you really should be using pip though):
$ sudo easy_install redis
$ sudo python setup.py install
>>> import redis >>> r = redis.Redis(host='localhost', port=6379, db=0) >>> r.set('foo', 'bar') True >>> r.get('foo') 'bar'
Behind the scenes, redis-py uses a connection pool to manage connections to a Redis server. By default, each Redis instance you create will in turn create its own connection pool. You can override this behavior and use an existing connection pool by passing an already created connection pool instance to the connection_pool argument of the Redis class. You may choose to do this in order to implement client side sharding or have finer grain control of how connections are managed.
>>> pool = redis.ConnectionPool(host='localhost', port=6379, db=0) >>> r = redis.Redis(connection_pool=pool)
ConnectionPools manage a set of Connection instances. redis-py ships with two types of Connections. The default, Connection, is a normal TCP socket based connection. The UnixDomainSocketConnection allows for clients running on the same device as the server to connect via a unix domain socket. To use a UnixDomainSocketConnection connection, simply pass the class to the connection_class argument of either the Redis or ConnectionPool class. You must also specify the path argument, which is a string to the unix domain socket file. Additionally, make sure the unixsocket parameter is defined in your redis.conf file. It's commented out by default.
>>> r = redis.Redis(connection_class=redis.UnixDomainSocketConnection, >>> path='/tmp/redis.sock')
You can create your own Connection subclasses in this way as well. This may be useful if you want to control the socket behavior within an async framework.
Parser classes provide a way to control how responses from the Redis server are parsed. redis-py ships with two parser classes, the PythonParser and the HiredisParser. By default, redis-py will attempt to use the HiredisParser if you have the hiredis module installed and will fallback to the PythonParser otherwise.
Hiredis is a C library maintained by the core Redis team. Pieter Noordhuis was kind enough to create Python bindings. Using Hiredis can provide up to a 10x speed improvement in parsing responses from the Redis server. The performance increase is most noticeable when retrieving many pieces of data, such as from LRANGE or SMEMBERS operations.
Hiredis is available on Pypi, and can be installed via pip or easy_install just like redis-py.
$ pip install hiredis
$ easy_install hiredis
The client class uses a set of callbacks to cast Redis responses to the appropriate Python type. There are a number of these callbacks defined on the Redis client class in a dictionary called RESPONSE_CALLBACKS.
Custom callbacks can be added on a per-instance basis using the set_response_callback method. This method accepts two arguments: a command name and the callback. Callbacks added in this manner are only valid on the instance the callback is added to. If you want to define or override a callback globally, you should make a subclass of the Redis client and add your callback to its REDIS_CALLBACKS class dictionary.
Response callbacks take at least one parameter: the response from the Redis server. Keyword arguments may also be accepted in order to further control how to interpret the response. These keyword arguments are specified during the command's call to execute_command. The ZRANGE implementation demonstrates the use of response callback keyword arguments with its "withscores" argument.
Redis client instances can safely be shared between threads. Internally, connection instances are only retrieved from the connection pool during command execution, and returned to the pool directly after. Command execution never modifies state on the client instance.
However, there is one caveat: the Redis SELECT command. The SELECT command allows you to switch the database currently in use by the connection. That database remains selected until another is selected or until the connection is closed. This creates an issue in that connections could be returned to the pool that are connected to a different database.
As a result, redis-py does not implement the SELECT command on client instances. If you use multiple Redis databases within the same application, you should create a separate client instance (and possibly a separate connection pool) for each database.
The official Redis documentation does a great job of explaining each command in detail (http://redis.io/commands). In most cases, redis-py uses the same arguments as the official spec. There are a few exceptions noted here:
- SELECT: Not implemented. See the explanation in the Thread Safety section above.
- ZADD: Redis specifies the 'score' argument before 'value'. These were swapped accidentally when being implemented and not discovered until after people were already using it. As of Redis 2.4, ZADD will start supporting variable arguments. redis-py implements these as python keyword arguments where the name is the 'value' and the value is the 'score'.
- DEL: 'del' is a reserved keyword in the Python syntax. Therefore redis-py uses 'delete' instead.
- CONFIG GET|SET: These are implemented separately as config_get or config_set.
- MULTI/EXEC: These are implemented as part of the Pipeline class. Calling the pipeline method and specifying use_transaction=True will cause the pipeline to be wrapped with the MULTI and EXEC statements when it is executed.
- SUBSCRIBE/LISTEN: Similar to pipelines, PubSub is implemented as a separate class as it places the underlying connection in a state where it can't execute non-pubsub commands. Calling the pubsub method from the Redis client will return a PubSub instance where you can subscribe to channels and listen for messages. You can call PUBLISH from both classes.
redis-py is versioned after Redis. For example, redis-py 2.0.0 should support all the commands available in Redis 2.0.0.
Special thanks to:
- Ludovico Magnocavallo, author of the original Python Redis client, from which some of the socket code is still used.
- Alexander Solovyov for ideas on the generic response callback system.
- Paul Hubbard for initial packaging support.