Memcached Backend

class cachelib.memcached.MemcachedCache(servers=None, default_timeout=300, key_prefix=None)

Bases: BaseCache

A cache that uses memcached as backend.

The first argument can either be an object that resembles the API of a memcache.Client or a tuple/list of server addresses. In the event that a tuple/list is passed, Werkzeug tries to import the best available memcache library.

This cache looks into the following packages/modules to find bindings for memcached:

  • pylibmc

  • google.appengine.api.memcached

  • memcached

  • libmc

Implementation notes: This cache backend works around some limitations in memcached to simplify the interface. For example unicode keys are encoded to utf-8 on the fly. Methods such as get_dict() return the keys in the same format as passed. Furthermore all get methods silently ignore key errors to not cause problems when untrusted user data is passed to the get methods which is often the case in web applications.

Parameters

  • servers (Any) – a list or tuple of server addresses or alternatively a memcache.Client or a compatible client.

  • default_timeout (int) – the default timeout that is used if no timeout is specified on set(). A timeout of 0 indicates that the cache never expires.

  • key_prefix (Optional[str]) – a prefix that is added before all keys. This makes it possible to use the same memcached server for different applications. Keep in mind that clear() will also clear keys with a different prefix.

add(key, value, timeout=None)

Works like set() but does not overwrite the values of already existing keys.

Parameters

  • key (str) – the key to set

  • value (Any) – the value for the key

  • timeout (Optional[int]) – the cache timeout for the key in seconds (if not specified, it uses the default timeout). A timeout of 0 indicates that the cache never expires.

Returns

Same as set(), but also False for already existing keys.

Return type

boolean

clear()

Clears the cache. Keep in mind that not all caches support completely clearing the cache.

Returns

Whether the cache has been cleared.

Return type

boolean

dec(key, delta=1)

Decrements the value of a key by delta. If the key does not yet exist it is initialized with -delta.

For supporting caches this is an atomic operation.

Parameters

  • key (str) – the key to increment.

  • delta (int) – the delta to subtract.

Returns

The new value or None for backend errors.

Return type

Optional[int]

delete(key)

Delete key from the cache.

Parameters

key (str) – the key to delete.

Returns

Whether the key existed and has been deleted.

Return type

boolean

delete_many(*keys)

Deletes multiple keys at once.

Parameters

keys (str) – The function accepts multiple keys as positional arguments.

Returns

A list containing all sucessfuly deleted keys

Return type

boolean

get(key)

Look up key in the cache and return the value for it.

Parameters

key (str) – the key to be looked up.

Returns

The value if it exists and is readable, else None.

Return type

Any

get_dict(*keys)

Like get_many() but return a dict:

d = cache.get_dict("foo", "bar")
foo = d["foo"]
bar = d["bar"]
Parameters

keys (str) – The function accepts multiple keys as positional arguments.

Return type

Dict[str, Any]

get_many(*keys)

Returns a list of values for the given keys. For each key an item in the list is created:

foo, bar = cache.get_many("foo", "bar")

Has the same error handling as get().

Parameters

keys (str) – The function accepts multiple keys as positional arguments.

Return type

List[Any]

has(key)

Checks if a key exists in the cache without returning it. This is a cheap operation that bypasses loading the actual data on the backend.

Parameters

key (str) – the key to check

Return type

bool

import_preferred_memcache_lib(servers)

Returns an initialized memcache client. Used by the constructor.

Parameters

servers (Any) –

Return type

Any

inc(key, delta=1)

Increments the value of a key by delta. If the key does not yet exist it is initialized with delta.

For supporting caches this is an atomic operation.

Parameters

  • key (str) – the key to increment.

  • delta (int) – the delta to add.

Returns

The new value or None for backend errors.

Return type

Optional[int]

set(key, value, timeout=None)

Add a new key/value to the cache (overwrites value, if key already exists in the cache).

Parameters

  • key (str) – the key to set

  • value (Any) – the value for the key

  • timeout (Optional[int]) – the cache timeout for the key in seconds (if not specified, it uses the default timeout). A timeout of 0 indicates that the cache never expires.

Returns

True if key has been updated, False for backend errors. Pickling errors, however, will raise a subclass of pickle.PickleError.

Return type

boolean

set_many(mapping, timeout=None)

Sets multiple keys and values from a mapping.

Parameters

  • mapping (Dict[str, Any]) – a mapping with the keys/values to set.

  • timeout (Optional[int]) – the cache timeout for the key in seconds (if not specified, it uses the default timeout). A timeout of 0 indicates that the cache never expires.

Returns

A list containing all keys sucessfuly set

Return type

boolean