path: root/README.md

# redis-py

The Python interface to the Redis key-value store.

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[Installation](#installation) | [Contributing](#contributing) |  [Getting Started](#getting-started) | [Connecting To Redis](#connecting-to-redis)

---------------------------------------------


## Installation

redis-py requires a running Redis server. See [Redis's
quickstart](https://redis.io/topics/quickstart) for installation
instructions.

redis-py can be installed using pip similar to other
Python packages. Do not use sudo with pip.
It is usually good to work in a
[virtualenv](https://virtualenv.pypa.io/en/latest/) or
[venv](https://docs.python.org/3/library/venv.html) to avoid conflicts
with other package managers and Python projects. For a quick
introduction see [Python Virtual Environments in Five
Minutes](https://bit.ly/py-env).

To install redis-py, simply:

``` bash
$ pip install redis
```

or from source:

``` bash
$ python setup.py install
```

## Contributing

Want to contribute a feature, bug fix, or report an issue? Check out
our [guide to
contributing](https://github.com/redis/redis-py/blob/master/CONTRIBUTING.md).

## Getting Started

redis-py supports Python 3.6+.

``` pycon
>>> 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 specifiying 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.


--------------------

### MSET, MSETNX and ZADD

These commands all accept a mapping of key/value pairs. In redis-py 2.X
this mapping could be specified as **args* or as `**kwargs`. Both of
these styles caused issues when Redis introduced optional flags to ZADD.
Relying on `*args` caused issues with the optional argument order,
especially in Python 2.7. Relying on `**kwargs` caused potential
collision issues of user keys with the argument names in the method
signature.

To resolve this, redis-py 3.0 has changed these three commands to all
accept a single positional argument named mapping that is expected to be
a dict. For MSET and MSETNX, the dict is a mapping of key-names -\>
values. For ZADD, the dict is a mapping of element-names -\> score.

MSET, MSETNX and ZADD now look like:

``` pycon
def mset(self, mapping):
def msetnx(self, mapping):
def zadd(self, name, mapping, nx=False, xx=False, ch=False, incr=False):
```

All 2.X users that use these commands must modify their code to supply
keys and values as a dict to these commands.

### ZINCRBY

redis-py 2.X accidentally modified the argument order of ZINCRBY,
swapping the order of value and amount. ZINCRBY now looks like:

``` python
def zincrby(self, name, amount, value):
```

All 2.X users that rely on ZINCRBY must swap the order of amount and
value for the command to continue to work as intended.

### Encoding of User Input

redis-py 3.0 only accepts user data as bytes, strings or numbers (ints,
longs and floats). Attempting to specify a key or a value as any other
type will raise a DataError exception.

redis-py 2.X attempted to coerce any type of input into a string. While
occasionally convenient, this caused all sorts of hidden errors when
users passed boolean values (which were coerced to \'True\' or
\'False\'), a None value (which was coerced to \'None\') or other
values, such as user defined types.

All 2.X users should make sure that the keys and values they pass into
redis-py are either bytes, strings or numbers.

### Locks

redis-py 3.0 drops support for the pipeline-based Lock and now only
supports the Lua-based lock. In doing so, LuaLock has been renamed to
Lock. This also means that redis-py Lock objects require Redis server
2.6 or greater.

2.X users that were explicitly referring to *LuaLock* will have to now
refer to *Lock* instead.

### Locks as Context Managers

redis-py 3.0 now raises a LockError when using a lock as a context
manager and the lock cannot be acquired within the specified timeout.
This is more of a bug fix than a backwards incompatible change. However,
given an error is now raised where none was before, this might alarm
some users.

2.X users should make sure they're wrapping their lock code in a
try/catch like this:

``` python
try:
    with r.lock('my-lock-key', blocking_timeout=5) as lock:
        # code you want executed only after the lock has been acquired
except LockError:
    # the lock wasn't acquired
```

## API Reference

The [official Redis command documentation](https://redis.io/commands)
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](https://github.com/redis/redis-py/issues/151#issuecomment-1545015)
    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.

``` pycon
>>> 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.

``` pycon
>>> 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.

``` pycon
>>> 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](https://github.com/redis/redis-py/issues/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):

``` pycon
>>> 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](https://pypi.org/project/certifi/):

``` pycon
>>> 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):

``` pycon
>>> 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](https://redis.io/topics/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:

``` pycon
>>> 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](#ssl-connections) for more examples of SSL configurations):

``` pycon
>>> 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).

``` pycon
>>> 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](https://redis.io/topics/sentinel-clients) to learn more about
Redis Sentinel.

--------------------------

### 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.

``` bash
$ 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:

``` pycon
>>> 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:

``` pycon
>>> 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.

``` pycon
>>> 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:

``` pycon
>>> 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():

``` pycon
>>> 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:

``` pycon
>>> 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.

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

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

``` pycon
>>> 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.

``` pycon
>>> 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.

``` pycon
# 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.

``` pycon
>>> 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.

``` pycon
>>> 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.

``` pycon
>>> 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.

``` pycon
>>> 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.

``` pycon
>>> 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.

``` pycon
>>> 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.

``` pycon
>>> 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.

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

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

``` pycon
>>> 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.

``` pycon
>>> 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.

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.

``` pycon
>>> 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:

``` pycon
>>> 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.

``` pycon
>>> 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.

``` pycon
>>> 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.

``` pycon
>>> 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 is now supports cluster mode and provides a client for
[Redis Cluster](<https://redis.io/topics/cluster-tutorial>).

The cluster client is based on Grokzen's
[redis-py-cluster](https://github.com/Grokzen/redis-py-cluster), has added bug
fixes, and now supersedes that library. Support for these changes is thanks to
his contributions.


**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:

``` pycon
    >>> 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:

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

- Directly, via the ClusterNode class:

``` pycon
    >>> 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.

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'.

``` pycon
    >>> # 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.

``` pycon
    >>> 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.

``` pycon
    >>> 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.

``` pycon
    >>> 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](https://redis.io/topics/cluster-spec#keys-hash-tags).
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.

``` pycon
    #  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](https://redis-py-cluster.readthedocs.io/en/stable/pubsub.html)
 for more.

``` pycon
    >>> 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.

``` pycon
    >>> 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.

``` pycon
    >>> 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 'transcation' 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.


See [Redis Cluster tutorial](https://redis.io/topics/cluster-tutorial) and
[Redis Cluster specifications](https://redis.io/topics/cluster-spec)
to learn more about Redis Cluster.

### Author

redis-py is developed and maintained by [Redis Inc](https://redis.com). It can be found [here](
https://github.com/redis/redis-py), or downloaded from [pypi](https://pypi.org/project/redis/).

Special thanks to:

-   Andy McCurdy (<sedrik@gmail.com>) 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.

[![Redis](./docs/logo-redis.png)](https://www.redis.com)