redis-py

The Python interface to the Redis key-value store.

Installation | Contributing | Getting Started | Connecting To Redis | Contributing

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Python Notice

redis-py 4.3.x will be the last generation of redis-py to support python 3.6 as it has been End of Life'd. Async support was introduced in redis-py 4.2.x thanks to aioredis, which necessitates this change. We will continue to maintain 3.6 support as long as possible - but the plan is for redis-py version 4.4+ to officially remove 3.6.

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Installation

To install redis-py, simply:

$ pip install redis

Looking for a high-level library to handle object mapping? See redis-om-python!

redis-py requires a running Redis server. Assuming you have docker

docker run -p 6379:6379 -it redis/redis-stack-server:latest

Getting Started

redis-py supports Python 3.7+.

>>> import redis
>>> r = redis.Redis(host='localhost', port=6379, db=0)
>>> r.set('foo', 'bar')
True
>>> r.get('foo')
b'bar'

By default, all responses are returned as bytes in Python 3.

If all string responses from a client should be decoded, the user can specify decode_responses=True in Redis.__init__. In this case, any Redis command that returns a string type will be decoded with the encoding specified.

The default encoding is utf-8, but this can be customized by specifying the encoding argument for the redis.Redis class. The encoding will be used to automatically encode any strings passed to commands, such as key names and values.

API Reference

The official Redis command documentation does a great job of explaining each command in detail. redis-py attempts to adhere to the official command syntax. There are a few exceptions:

• SELECT: Not implemented. See the explanation in the Thread Safety section below.
• DEL: del is a reserved keyword in the Python syntax. Therefore redis-py uses delete instead.
• MULTI/EXEC: These are implemented as part of the Pipeline class. The pipeline is wrapped with the MULTI and EXEC statements by default when it is executed, which can be disabled by specifying transaction=False. See more about Pipelines below.
• 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 only call PUBLISH from the Redis client (see this comment on issue #151 for details).
• SCAN/SSCAN/HSCAN/ZSCAN: The *SCAN commands are implemented as they exist in the Redis documentation. In addition, each command has an equivalent iterator method. These are purely for convenience so the user doesn't have to keep track of the cursor while iterating. Use the scan_iter/sscan_iter/hscan_iter/zscan_iter methods for this behavior.

Connecting to Redis

Client Classes: Redis and StrictRedis

redis-py 3.0 drops support for the legacy Redis client class. StrictRedis has been renamed to Redis and an alias named StrictRedis is provided so that users previously using StrictRedis can continue to run unchanged.

The 2.X Redis class provided alternative implementations of a few commands. This confused users (rightfully so) and caused a number of support issues. To make things easier going forward, it was decided to drop support for these alternate implementations and instead focus on a single client class.

2.X users that are already using StrictRedis don't have to change the class name. StrictRedis will continue to work for the foreseeable future.

2.X users that are using the Redis class will have to make changes if they use any of the following commands:

• SETEX: The argument order has changed. The new order is (name, time, value).
• LREM: The argument order has changed. The new order is (name, num, value).
• TTL and PTTL: The return value is now always an int and matches the official Redis command (>0 indicates the timeout, -1 indicates that the key exists but that it has no expire time set, -2 indicates that the key does not exist)

Connection Pools

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 fine-grain control of how connections are managed.

>>> pool = redis.ConnectionPool(host='localhost', port=6379, db=0)
>>> r = redis.Redis(connection_pool=pool)

Connections

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 unix_socket_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(unix_socket_path='/tmp/redis.sock')

You can create your own Connection subclasses as well. This may be useful if you want to control the socket behavior within an async framework. To instantiate a client class using your own connection, you need to create a connection pool, passing your class to the connection_class argument. Other keyword parameters you pass to the pool will be passed to the class specified during initialization.

>>> pool = redis.ConnectionPool(connection_class=YourConnectionClass,
                                your_arg='...', ...)

Connections maintain an open socket to the Redis server. Sometimes these sockets are interrupted or disconnected for a variety of reasons. For example, network appliances, load balancers and other services that sit between clients and servers are often configured to kill connections that remain idle for a given threshold.

When a connection becomes disconnected, the next command issued on that connection will fail and redis-py will raise a ConnectionError to the caller. This allows each application that uses redis-py to handle errors in a way that's fitting for that specific application. However, constant error handling can be verbose and cumbersome, especially when socket disconnections happen frequently in many production environments.

To combat this, redis-py can issue regular health checks to assess the liveliness of a connection just before issuing a command. Users can pass health_check_interval=N to the Redis or ConnectionPool classes or as a query argument within a Redis URL. The value of health_check_interval must be an integer. A value of 0, the default, disables health checks. Any positive integer will enable health checks. Health checks are performed just before a command is executed if the underlying connection has been idle for more than health_check_interval seconds. For example, health_check_interval=30 will ensure that a health check is run on any connection that has been idle for 30 or more seconds just before a command is executed on that connection.

If your application is running in an environment that disconnects idle connections after 30 seconds you should set the health_check_interval option to a value less than 30.

This option also works on any PubSub connection that is created from a client with health_check_interval enabled. PubSub users need to ensure that get_message() or listen() are called more frequently than health_check_interval seconds. It is assumed that most workloads already do this.

If your PubSub use case doesn't call get_message() or listen() frequently, you should call pubsub.check_health() explicitly on a regularly basis.

SSL Connections

redis-py 3.0 changes the default value of the ssl_cert_reqs option from None to 'required'. See Issue 1016. This change enforces hostname validation when accepting a cert from a remote SSL terminator. If the terminator doesn't properly set the hostname on the cert this will cause redis-py 3.0 to raise a ConnectionError.

This check can be disabled by setting ssl_cert_reqs to None. Note that doing so removes the security check. Do so at your own risk.

Example with hostname verification using a local certificate bundle (linux):

>>> import redis
>>> r = redis.Redis(host='xxxxxx.cache.amazonaws.com', port=6379, db=0,
                    ssl=True,
                    ssl_ca_certs='/etc/ssl/certs/ca-certificates.crt')
>>> r.set('foo', 'bar')
True
>>> r.get('foo')
b'bar'

Example with hostname verification using certifi:

>>> import redis, certifi
>>> r = redis.Redis(host='xxxxxx.cache.amazonaws.com', port=6379, db=0,
                    ssl=True, ssl_ca_certs=certifi.where())
>>> r.set('foo', 'bar')
True
>>> r.get('foo')
b'bar'

Example turning off hostname verification (not recommended):

>>> import redis
>>> r = redis.Redis(host='xxxxxx.cache.amazonaws.com', port=6379, db=0,
                    ssl=True, ssl_cert_reqs=None)
>>> r.set('foo', 'bar')
True
>>> r.get('foo')
b'bar'

Sentinel support

redis-py can be used together with Redis Sentinel to discover Redis nodes. You need to have at least one Sentinel daemon running in order to use redis-py's Sentinel support.

Connecting redis-py to the Sentinel instance(s) is easy. You can use a Sentinel connection to discover the master and slaves network addresses:

>>> from redis import Sentinel
>>> sentinel = Sentinel([('localhost', 26379)], socket_timeout=0.1)
>>> sentinel.discover_master('mymaster')
('127.0.0.1', 6379)
>>> sentinel.discover_slaves('mymaster')
[('127.0.0.1', 6380)]

To connect to a sentinel which uses SSL (see SSL connections for more examples of SSL configurations):

>>> from redis import Sentinel
>>> sentinel = Sentinel([('localhost', 26379)],
                        ssl=True,
                        ssl_ca_certs='/etc/ssl/certs/ca-certificates.crt')
>>> sentinel.discover_master('mymaster')
('127.0.0.1', 6379)

You can also create Redis client connections from a Sentinel instance. You can connect to either the master (for write operations) or a slave (for read-only operations).

>>> master = sentinel.master_for('mymaster', socket_timeout=0.1)
>>> slave = sentinel.slave_for('mymaster', socket_timeout=0.1)
>>> master.set('foo', 'bar')
>>> slave.get('foo')
b'bar'

The master and slave objects are normal Redis instances with their connection pool bound to the Sentinel instance. When a Sentinel backed client attempts to establish a connection, it first queries the Sentinel servers to determine an appropriate host to connect to. If no server is found, a MasterNotFoundError or SlaveNotFoundError is raised. Both exceptions are subclasses of ConnectionError.

When trying to connect to a slave client, the Sentinel connection pool will iterate over the list of slaves until it finds one that can be connected to. If no slaves can be connected to, a connection will be established with the master.

See Guidelines for Redis clients with support for Redis Sentinel to learn more about Redis Sentinel.

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Parsers

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 just like redis-py.

$ pip install hiredis

Response Callbacks

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 RESPONSE_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.

Thread Safety

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.

It is not safe to pass PubSub or Pipeline objects between threads.

Pipelines

Pipelines are a subclass of the base Redis class that provide support for buffering multiple commands to the server in a single request. They can be used to dramatically increase the performance of groups of commands by reducing the number of back-and-forth TCP packets between the client and server.

Pipelines are quite simple to use:

>>> r = redis.Redis(...)
>>> r.set('bing', 'baz')
>>> # Use the pipeline() method to create a pipeline instance
>>> pipe = r.pipeline()
>>> # The following SET commands are buffered
>>> pipe.set('foo', 'bar')
>>> pipe.get('bing')
>>> # the EXECUTE call sends all buffered commands to the server, returning
>>> # a list of responses, one for each command.
>>> pipe.execute()
[True, b'baz']

For ease of use, all commands being buffered into the pipeline return the pipeline object itself. Therefore calls can be chained like:

>>> pipe.set('foo', 'bar').sadd('faz', 'baz').incr('auto_number').execute()
[True, True, 6]

In addition, pipelines can also ensure the buffered commands are executed atomically as a group. This happens by default. If you want to disable the atomic nature of a pipeline but still want to buffer commands, you can turn off transactions.

>>> pipe = r.pipeline(transaction=False)

A common issue occurs when requiring atomic transactions but needing to retrieve values in Redis prior for use within the transaction. For instance, let's assume that the INCR command didn't exist and we need to build an atomic version of INCR in Python.

The completely naive implementation could GET the value, increment it in Python, and SET the new value back. However, this is not atomic because multiple clients could be doing this at the same time, each getting the same value from GET.

Enter the WATCH command. WATCH provides the ability to monitor one or more keys prior to starting a transaction. If any of those keys change prior the execution of that transaction, the entire transaction will be canceled and a WatchError will be raised. To implement our own client-side INCR command, we could do something like this:

>>> with r.pipeline() as pipe:
...     while True:
...         try:
...             # put a WATCH on the key that holds our sequence value
...             pipe.watch('OUR-SEQUENCE-KEY')
...             # after WATCHing, the pipeline is put into immediate execution
...             # mode until we tell it to start buffering commands again.
...             # this allows us to get the current value of our sequence
...             current_value = pipe.get('OUR-SEQUENCE-KEY')
...             next_value = int(current_value) + 1
...             # now we can put the pipeline back into buffered mode with MULTI
...             pipe.multi()
...             pipe.set('OUR-SEQUENCE-KEY', next_value)
...             # and finally, execute the pipeline (the set command)
...             pipe.execute()
...             # if a WatchError wasn't raised during execution, everything
...             # we just did happened atomically.
...             break
...        except WatchError:
...             # another client must have changed 'OUR-SEQUENCE-KEY' between
...             # the time we started WATCHing it and the pipeline's execution.
...             # our best bet is to just retry.
...             continue

Note that, because the Pipeline must bind to a single connection for the duration of a WATCH, care must be taken to ensure that the connection is returned to the connection pool by calling the reset() method. If the Pipeline is used as a context manager (as in the example above) reset() will be called automatically. Of course you can do this the manual way by explicitly calling reset():

>>> pipe = r.pipeline()
>>> while True:
...     try:
...         pipe.watch('OUR-SEQUENCE-KEY')
...         ...
...         pipe.execute()
...         break
...     except WatchError:
...         continue
...     finally:
...         pipe.reset()

A convenience method named "transaction" exists for handling all the boilerplate of handling and retrying watch errors. It takes a callable that should expect a single parameter, a pipeline object, and any number of keys to be WATCHed. Our client-side INCR command above can be written like this, which is much easier to read:

>>> def client_side_incr(pipe):
...     current_value = pipe.get('OUR-SEQUENCE-KEY')
...     next_value = int(current_value) + 1
...     pipe.multi()
...     pipe.set('OUR-SEQUENCE-KEY', next_value)
>>>
>>> r.transaction(client_side_incr, 'OUR-SEQUENCE-KEY')
[True]

Be sure to call pipe.multi() in the callable passed to Redis.transaction prior to any write commands.

Publish / Subscribe

redis-py includes a PubSub object that subscribes to channels and listens for new messages. Creating a PubSub object is easy.

>>> r = redis.Redis(...)
>>> p = r.pubsub()

Once a PubSub instance is created, channels and patterns can be subscribed to.

>>> p.subscribe('my-first-channel', 'my-second-channel', ...)
>>> p.psubscribe('my-*', ...)

The PubSub instance is now subscribed to those channels/patterns. The subscription confirmations can be seen by reading messages from the PubSub instance.

>>> p.get_message()
{'pattern': None, 'type': 'subscribe', 'channel': b'my-second-channel', 'data': 1}
>>> p.get_message()
{'pattern': None, 'type': 'subscribe', 'channel': b'my-first-channel', 'data': 2}
>>> p.get_message()
{'pattern': None, 'type': 'psubscribe', 'channel': b'my-*', 'data': 3}

Every message read from a PubSub instance will be a dictionary with the following keys.

• type: One of the following: 'subscribe', 'unsubscribe', 'psubscribe', 'punsubscribe', 'message', 'pmessage'
• channel: The channel [un]subscribed to or the channel a message was published to
• pattern: The pattern that matched a published message's channel. Will be None in all cases except for 'pmessage' types.
• data: The message data. With [un]subscribe messages, this value will be the number of channels and patterns the connection is currently subscribed to. With [p]message messages, this value will be the actual published message.

Let's send a message now.

# the publish method returns the number matching channel and pattern
# subscriptions. 'my-first-channel' matches both the 'my-first-channel'
# subscription and the 'my-*' pattern subscription, so this message will
# be delivered to 2 channels/patterns
>>> r.publish('my-first-channel', 'some data')
2
>>> p.get_message()
{'channel': b'my-first-channel', 'data': b'some data', 'pattern': None, 'type': 'message'}
>>> p.get_message()
{'channel': b'my-first-channel', 'data': b'some data', 'pattern': b'my-*', 'type': 'pmessage'}

Unsubscribing works just like subscribing. If no arguments are passed to [p]unsubscribe, all channels or patterns will be unsubscribed from.

>>> p.unsubscribe()
>>> p.punsubscribe('my-*')
>>> p.get_message()
{'channel': b'my-second-channel', 'data': 2, 'pattern': None, 'type': 'unsubscribe'}
>>> p.get_message()
{'channel': b'my-first-channel', 'data': 1, 'pattern': None, 'type': 'unsubscribe'}
>>> p.get_message()
{'channel': b'my-*', 'data': 0, 'pattern': None, 'type': 'punsubscribe'}

redis-py also allows you to register callback functions to handle published messages. Message handlers take a single argument, the message, which is a dictionary just like the examples above. To subscribe to a channel or pattern with a message handler, pass the channel or pattern name as a keyword argument with its value being the callback function.

When a message is read on a channel or pattern with a message handler, the message dictionary is created and passed to the message handler. In this case, a None value is returned from get_message() since the message was already handled.

>>> def my_handler(message):
...     print('MY HANDLER: ', message['data'])
>>> p.subscribe(**{'my-channel': my_handler})
# read the subscribe confirmation message
>>> p.get_message()
{'pattern': None, 'type': 'subscribe', 'channel': b'my-channel', 'data': 1}
>>> r.publish('my-channel', 'awesome data')
1
# for the message handler to work, we need tell the instance to read data.
# this can be done in several ways (read more below). we'll just use
# the familiar get_message() function for now
>>> message = p.get_message()
MY HANDLER:  awesome data
# note here that the my_handler callback printed the string above.
# `message` is None because the message was handled by our handler.
>>> print(message)
None

If your application is not interested in the (sometimes noisy) subscribe/unsubscribe confirmation messages, you can ignore them by passing ignore_subscribe_messages=True to r.pubsub(). This will cause all subscribe/unsubscribe messages to be read, but they won't bubble up to your application.

>>> p = r.pubsub(ignore_subscribe_messages=True)
>>> p.subscribe('my-channel')
>>> p.get_message()  # hides the subscribe message and returns None
>>> r.publish('my-channel', 'my data')
1
>>> p.get_message()
{'channel': b'my-channel', 'data': b'my data', 'pattern': None, 'type': 'message'}

There are three different strategies for reading messages.

The examples above have been using pubsub.get_message(). Behind the scenes, get_message() uses the system's 'select' module to quickly poll the connection's socket. If there's data available to be read, get_message() will read it, format the message and return it or pass it to a message handler. If there's no data to be read, get_message() will immediately return None. This makes it trivial to integrate into an existing event loop inside your application.

>>> while True:
>>>     message = p.get_message()
>>>     if message:
>>>         # do something with the message
>>>     time.sleep(0.001)  # be nice to the system :)

Older versions of redis-py only read messages with pubsub.listen(). listen() is a generator that blocks until a message is available. If your application doesn't need to do anything else but receive and act on messages received from redis, listen() is an easy way to get up an running.

>>> for message in p.listen():
...     # do something with the message

The third option runs an event loop in a separate thread. pubsub.run_in_thread() creates a new thread and starts the event loop. The thread object is returned to the caller of [un_in_thread(). The caller can use the thread.stop() method to shut down the event loop and thread. Behind the scenes, this is simply a wrapper around get_message() that runs in a separate thread, essentially creating a tiny non-blocking event loop for you. run_in_thread() takes an optional sleep_time argument. If specified, the event loop will call time.sleep() with the value in each iteration of the loop.

Note: Since we're running in a separate thread, there's no way to handle messages that aren't automatically handled with registered message handlers. Therefore, redis-py prevents you from calling run_in_thread() if you're subscribed to patterns or channels that don't have message handlers attached.

>>> p.subscribe(**{'my-channel': my_handler})
>>> thread = p.run_in_thread(sleep_time=0.001)
# the event loop is now running in the background processing messages
# when it's time to shut it down...
>>> thread.stop()

run_in_thread also supports an optional exception handler, which lets you catch exceptions that occur within the worker thread and handle them appropriately. The exception handler will take as arguments the exception itself, the pubsub object, and the worker thread returned by run_in_thread.

>>> p.subscribe(**{'my-channel': my_handler})
>>> def exception_handler(ex, pubsub, thread):
>>>     print(ex)
>>>     thread.stop()
>>>     thread.join(timeout=1.0)
>>>     pubsub.close()
>>> thread = p.run_in_thread(exception_handler=exception_handler)

A PubSub object adheres to the same encoding semantics as the client instance it was created from. Any channel or pattern that's unicode will be encoded using the charset specified on the client before being sent to Redis. If the client's decode_responses flag is set the False (the default), the 'channel', 'pattern' and 'data' values in message dictionaries will be byte strings (str on Python 2, bytes on Python 3). If the client's decode_responses is True, then the 'channel', 'pattern' and 'data' values will be automatically decoded to unicode strings using the client's charset.

PubSub objects remember what channels and patterns they are subscribed to. In the event of a disconnection such as a network error or timeout, the PubSub object will re-subscribe to all prior channels and patterns when reconnecting. Messages that were published while the client was disconnected cannot be delivered. When you're finished with a PubSub object, call its .close() method to shutdown the connection.

>>> p = r.pubsub()
>>> ...
>>> p.close()

The PUBSUB set of subcommands CHANNELS, NUMSUB and NUMPAT are also supported:

>>> r.pubsub_channels()
[b'foo', b'bar']
>>> r.pubsub_numsub('foo', 'bar')
[(b'foo', 9001), (b'bar', 42)]
>>> r.pubsub_numsub('baz')
[(b'baz', 0)]
>>> r.pubsub_numpat()
1204

Monitor

redis-py includes a Monitor object that streams every command processed by the Redis server. Use listen() on the Monitor object to block until a command is received.

>>> r = redis.Redis(...)
>>> with r.monitor() as m:
>>>     for command in m.listen():
>>>         print(command)

Lua Scripting

redis-py supports the EVAL, EVALSHA, and SCRIPT commands. However, there are a number of edge cases that make these commands tedious to use in real world scenarios. Therefore, redis-py exposes a Script object that makes scripting much easier to use. (RedisClusters have limited support for scripting.)

To create a Script instance, use the register_script function on a client instance passing the Lua code as the first argument. register_script returns a Script instance that you can use throughout your code.

The following trivial Lua script accepts two parameters: the name of a key and a multiplier value. The script fetches the value stored in the key, multiplies it with the multiplier value and returns the result.

>>> r = redis.Redis()
>>> lua = """
... local value = redis.call('GET', KEYS[1])
... value = tonumber(value)
... return value * ARGV[1]"""
>>> multiply = r.register_script(lua)

multiply is now a Script instance that is invoked by calling it like a function. Script instances accept the following optional arguments:

• keys: A list of key names that the script will access. This becomes the KEYS list in Lua.
• args: A list of argument values. This becomes the ARGV list in Lua.
• client: A redis-py Client or Pipeline instance that will invoke the script. If client isn't specified, the client that initially created the Script instance (the one that register_script was invoked from) will be used.

Continuing the example from above:

>>> r.set('foo', 2)
>>> multiply(keys=['foo'], args=[5])
10

The value of key 'foo' is set to 2. When multiply is invoked, the 'foo' key is passed to the script along with the multiplier value of 5. Lua executes the script and returns the result, 10.

Script instances can be executed using a different client instance, even one that points to a completely different Redis server.

>>> r2 = redis.Redis('redis2.example.com')
>>> r2.set('foo', 3)
>>> multiply(keys=['foo'], args=[5], client=r2)
15

The Script object ensures that the Lua script is loaded into Redis's script cache. In the event of a NOSCRIPT error, it will load the script and retry executing it.

Script objects can also be used in pipelines. The pipeline instance should be passed as the client argument when calling the script. Care is taken to ensure that the script is registered in Redis's script cache just prior to pipeline execution.

>>> pipe = r.pipeline()
>>> pipe.set('foo', 5)
>>> multiply(keys=['foo'], args=[5], client=pipe)
>>> pipe.execute()
[True, 25]

Scan Iterators

The *SCAN commands introduced in Redis 2.8 can be cumbersome to use. While these commands are fully supported, redis-py also exposes the following methods that return Python iterators for convenience: scan_iter, hscan_iter, sscan_iter and zscan_iter.

>>> for key, value in (('A', '1'), ('B', '2'), ('C', '3')):
...     r.set(key, value)
>>> for key in r.scan_iter():
...     print(key, r.get(key))
A 1
B 2
C 3

Cluster Mode

redis-py now supports cluster mode and provides a client for Redis Cluster.

The cluster client is based on Grokzen's redis-py-cluster, has added bug fixes, and now supersedes that library. Support for these changes is thanks to his contributions.

To learn more about Redis Cluster, see Redis Cluster specifications.

Create RedisCluster:

Connecting redis-py to a Redis Cluster instance(s) requires at a minimum a single node for cluster discovery. There are multiple ways in which a cluster instance can be created:

• Using 'host' and 'port' arguments:

>>> from redis.cluster import RedisCluster as Redis
>>> rc = Redis(host='localhost', port=6379)
>>> print(rc.get_nodes())
    [[host=127.0.0.1,port=6379,name=127.0.0.1:6379,server_type=primary,redis_connection=Redis<ConnectionPool<Connection<host=127.0.0.1,port=6379,db=0>>>], [host=127.0.0.1,port=6378,name=127.0.0.1:6378,server_type=primary,redis_connection=Redis<ConnectionPool<Connection<host=127.0.0.1,port=6378,db=0>>>], [host=127.0.0.1,port=6377,name=127.0.0.1:6377,server_type=replica,redis_connection=Redis<ConnectionPool<Connection<host=127.0.0.1,port=6377,db=0>>>]]

• Using the Redis URL specification:

>>> from redis.cluster import RedisCluster as Redis
>>> rc = Redis.from_url("redis://localhost:6379/0")

• Directly, via the ClusterNode class:

>>> from redis.cluster import RedisCluster as Redis
>>> from redis.cluster import ClusterNode
>>> nodes = [ClusterNode('localhost', 6379), ClusterNode('localhost', 6378)]
>>> rc = Redis(startup_nodes=nodes)

When a RedisCluster instance is being created it first attempts to establish a connection to one of the provided startup nodes. If none of the startup nodes are reachable, a 'RedisClusterException' will be thrown. After a connection to the one of the cluster's nodes is established, the RedisCluster instance will be initialized with 3 caches: a slots cache which maps each of the 16384 slots to the node/s handling them, a nodes cache that contains ClusterNode objects (name, host, port, redis connection) for all of the cluster's nodes, and a commands cache contains all the server supported commands that were retrieved using the Redis 'COMMAND' output. See RedisCluster specific options below for more.

RedisCluster instance can be directly used to execute Redis commands. When a command is being executed through the cluster instance, the target node(s) will be internally determined. When using a key-based command, the target node will be the node that holds the key's slot. Cluster management commands and other commands that are not key-based have a parameter called 'target_nodes' where you can specify which nodes to execute the command on. In the absence of target_nodes, the command will be executed on the default cluster node. As part of cluster instance initialization, the cluster's default node is randomly selected from the cluster's primaries, and will be updated upon reinitialization. Using r.get_default_node(), you can get the cluster's default node, or you can change it using the 'set_default_node' method.

The 'target_nodes' parameter is explained in the following section, 'Specifying Target Nodes'.

>>> # target-nodes: the node that holds 'foo1's key slot
>>> rc.set('foo1', 'bar1')
>>> # target-nodes: the node that holds 'foo2's key slot
>>> rc.set('foo2', 'bar2')
>>> # target-nodes: the node that holds 'foo1's key slot
>>> print(rc.get('foo1'))
b'bar'
>>> # target-node: default-node
>>> print(rc.keys())
[b'foo1']
>>> # target-node: default-node
>>> rc.ping()

Specifying Target Nodes:

As mentioned above, all non key-based RedisCluster commands accept the kwarg parameter 'target_nodes' that specifies the node/nodes that the command should be executed on. The best practice is to specify target nodes using RedisCluster class's node flags: PRIMARIES, REPLICAS, ALL_NODES, RANDOM. When a nodes flag is passed along with a command, it will be internally resolved to the relevant node/s. If the nodes topology of the cluster changes during the execution of a command, the client will be able to resolve the nodes flag again with the new topology and attempt to retry executing the command.

>>> from redis.cluster import RedisCluster as Redis
>>> # run cluster-meet command on all of the cluster's nodes
>>> rc.cluster_meet('127.0.0.1', 6379, target_nodes=Redis.ALL_NODES)
>>> # ping all replicas
>>> rc.ping(target_nodes=Redis.REPLICAS)
>>> # ping a random node
>>> rc.ping(target_nodes=Redis.RANDOM)
>>> # get the keys from all cluster nodes
>>> rc.keys(target_nodes=Redis.ALL_NODES)
[b'foo1', b'foo2']
>>> # execute bgsave in all primaries
>>> rc.bgsave(Redis.PRIMARIES)

You could also pass ClusterNodes directly if you want to execute a command on a specific node / node group that isn't addressed by the nodes flag. However, if the command execution fails due to cluster topology changes, a retry attempt will not be made, since the passed target node/s may no longer be valid, and the relevant cluster or connection error will be returned.

>>> node = rc.get_node('localhost', 6379)
>>> # Get the keys only for that specific node
>>> rc.keys(target_nodes=node)
>>> # get Redis info from a subset of primaries
>>> subset_primaries = [node for node in rc.get_primaries() if node.port > 6378]
>>> rc.info(target_nodes=subset_primaries)

In addition, the RedisCluster instance can query the Redis instance of a specific node and execute commands on that node directly. The Redis client, however, does not handle cluster failures and retries.

>>> cluster_node = rc.get_node(host='localhost', port=6379)
>>> print(cluster_node)
[host=127.0.0.1,port=6379,name=127.0.0.1:6379,server_type=primary,redis_connection=Redis<ConnectionPool<Connection<host=127.0.0.1,port=6379,db=0>>>]
>>> r = cluster_node.redis_connection
>>> r.client_list()
[{'id': '276', 'addr': '127.0.0.1:64108', 'fd': '16', 'name': '', 'age': '0', 'idle': '0', 'flags': 'N', 'db': '0', 'sub': '0', 'psub': '0', 'multi': '-1', 'qbuf': '26', 'qbuf-free': '32742', 'argv-mem': '10', 'obl': '0', 'oll': '0', 'omem': '0', 'tot-mem': '54298', 'events': 'r', 'cmd': 'client', 'user': 'default'}]
>>> # Get the keys only for that specific node
>>> r.keys()
[b'foo1']

Multi-key commands:

Redis supports multi-key commands in Cluster Mode, such as Set type unions or intersections, mset and mget, as long as the keys all hash to the same slot. By using RedisCluster client, you can use the known functions (e.g. mget, mset) to perform an atomic multi-key operation. However, you must ensure all keys are mapped to the same slot, otherwise a RedisClusterException will be thrown. Redis Cluster implements a concept called hash tags that can be used in order to force certain keys to be stored in the same hash slot, see Keys hash tag. You can also use nonatomic for some of the multikey operations, and pass keys that aren't mapped to the same slot. The client will then map the keys to the relevant slots, sending the commands to the slots' node owners. Non-atomic operations batch the keys according to their hash value, and then each batch is sent separately to the slot's owner.

# Atomic operations can be used when all keys are mapped to the same slot
>>> rc.mset({'{foo}1': 'bar1', '{foo}2': 'bar2'})
>>> rc.mget('{foo}1', '{foo}2')
[b'bar1', b'bar2']
# Non-atomic multi-key operations splits the keys into different slots
>>> rc.mset_nonatomic({'foo': 'value1', 'bar': 'value2', 'zzz': 'value3')
>>> rc.mget_nonatomic('foo', 'bar', 'zzz')
[b'value1', b'value2', b'value3']

Cluster PubSub:

When a ClusterPubSub instance is created without specifying a node, a single node will be transparently chosen for the pubsub connection on the first command execution. The node will be determined by:

1. Hashing the channel name in the request to find its keyslot
2. Selecting a node that handles the keyslot: If read_from_replicas is set to true, a replica can be selected.

Known limitations with pubsub:

Pattern subscribe and publish do not currently work properly due to key slots. If we hash a pattern like fo* we will receive a keyslot for that string but there are endless possibilities for channel names based on this pattern - unknowable in advance. This feature is not disabled but the commands are not currently recommended for use. See redis-py-cluster documentation for more.

>>> p1 = rc.pubsub()
# p1 connection will be set to the node that holds 'foo' keyslot
>>> p1.subscribe('foo')
# p2 connection will be set to node 'localhost:6379'
>>> p2 = rc.pubsub(rc.get_node('localhost', 6379))

Read Only Mode

By default, Redis Cluster always returns MOVE redirection response on accessing a replica node. You can overcome this limitation and scale read commands by triggering READONLY mode.

To enable READONLY mode pass read_from_replicas=True to RedisCluster constructor. When set to true, read commands will be assigned between the primary and its replications in a Round-Robin manner.

READONLY mode can be set at runtime by calling the readonly() method with target_nodes='replicas', and read-write access can be restored by calling the readwrite() method.

>>> from cluster import RedisCluster as Redis
# Use 'debug' log level to print the node that the command is executed on
>>> rc_readonly = Redis(startup_nodes=startup_nodes,
...                     read_from_replicas=True)
>>> rc_readonly.set('{foo}1', 'bar1')
>>> for i in range(0, 4):
...     # Assigns read command to the slot's hosts in a Round-Robin manner
...     rc_readonly.get('{foo}1')
# set command would be directed only to the slot's primary node
>>> rc_readonly.set('{foo}2', 'bar2')
# reset READONLY flag
>>> rc_readonly.readwrite(target_nodes='replicas')
# now the get command would be directed only to the slot's primary node
>>> rc_readonly.get('{foo}1')

Cluster Pipeline

ClusterPipeline is a subclass of RedisCluster that provides support for Redis pipelines in cluster mode. When calling the execute() command, all the commands are grouped by the node on which they will be executed, and are then executed by the respective nodes in parallel. The pipeline instance will wait for all the nodes to respond before returning the result to the caller. Command responses are returned as a list sorted in the same order in which they were sent. Pipelines can be used to dramatically increase the throughput of Redis Cluster by significantly reducing the the number of network round trips between the client and the server.

>>> with rc.pipeline() as pipe:
...     pipe.set('foo', 'value1')
...     pipe.set('bar', 'value2')
...     pipe.get('foo')
...     pipe.get('bar')
...     print(pipe.execute())
[True, True, b'value1', b'value2']
...     pipe.set('foo1', 'bar1').get('foo1').execute()
[True, b'bar1']

Please note:

• RedisCluster pipelines currently only support key-based commands.
• The pipeline gets its 'read_from_replicas' value from the cluster's parameter. Thus, if read from replications is enabled in the cluster instance, the pipeline will also direct read commands to replicas.
• The 'transaction' option is NOT supported in cluster-mode. In non-cluster mode, the 'transaction' option is available when executing pipelines. This wraps the pipeline commands with MULTI/EXEC commands, and effectively turns the pipeline commands into a single transaction block. This means that all commands are executed sequentially without any interruptions from other clients. However, in cluster-mode this is not possible, because commands are partitioned according to their respective destination nodes. This means that we can not turn the pipeline commands into one transaction block, because in most cases they are split up into several smaller pipelines.

Lua Scripting in Cluster Mode

Cluster mode has limited support for lua scripting.

The following commands are supported, with caveats:

• EVAL and EVALSHA: The command is sent to the relevant node, depending on the keys (i.e., in EVAL "<script>" num_keys key_1 ... key_n ...). The keys must all be on the same node. If the script requires 0 keys, the command is sent to a random (primary) node.
• SCRIPT EXISTS: The command is sent to all primaries. The result is a list of booleans corresponding to the input SHA hashes. Each boolean is an AND of "does the script exist on each node?". In other words, each boolean is True iff the script exists on all nodes.
• SCRIPT FLUSH: The command is sent to all primaries. The result is a bool AND over all nodes' responses.
• SCRIPT LOAD: The command is sent to all primaries. The result is the SHA1 digest.

The following commands are not supported:

• EVAL_RO
• EVALSHA_RO

Using scripting within pipelines in cluster mode is not supported.

RedisCluster specific options

require_full_coverage: (default=False)

When set to False (default value): the client will not require a
full coverage of the slots. However, if not all slots are covered,
and at least one node has 'cluster-require-full-coverage' set to
'yes,' the server will throw a ClusterDownError for some key-based
commands. See -
https://redis.io/topics/cluster-tutorial#redis-cluster-configuration-parameters
When set to True: all slots must be covered to construct the
cluster client. If not all slots are covered, RedisClusterException
will be thrown.

read_from_replicas: (default=False)

 Enable read from replicas in READONLY mode. You can read possibly
 stale data.
 When set to true, read commands will be assigned between the
 primary and its replications in a Round-Robin manner.

dynamic_startup_nodes: (default=True)

 Set the RedisCluster's startup nodes to all of the discovered nodes.
 If true, the cluster's discovered nodes will be used to determine the
 cluster nodes-slots mapping in the next topology refresh.
 It will remove the initial passed startup nodes if their endpoints aren't
 listed in the CLUSTER SLOTS output.
 If you use dynamic DNS endpoints for startup nodes but CLUSTER SLOTS lists
 specific IP addresses, it is best to set it to false.

cluster_error_retry_attempts: (default=3)

 Retry command execution attempts when encountering ClusterDownError
 or ConnectionError

reinitialize_steps: (default=10)

Specifies the number of MOVED errors that need to occur before
reinitializing the whole cluster topology. If a MOVED error occurs
and the cluster does not need to be reinitialized on this current
error handling, only the MOVED slot will be patched with the
redirected node.
To reinitialize the cluster on every MOVED error, set
reinitialize_steps to 1.
To avoid reinitializing the cluster on moved errors, set
reinitialize_steps to 0.

Author

redis-py is developed and maintained by Redis Inc. It can be found here, or downloaded from pypi.

Special thanks to:

• Andy McCurdy ([email protected]) the original author of redis-py.
• 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.