Cache Package

LRU Module

LRU cache based on stucchio’s py-lru-cache module

original copy at https://github.com/stucchio/Python-LRU-cache licensed under MIT

class LRUCacheDict(max_size=1024, expiration=900, thread_clear=False, concurrent=True)

Bases: object

A dictionary-like object, supporting LRU caching semantics.

>>> d = LRUCacheDict(max_size=3, expiration=3)
>>> d['foo'] = 'bar'
>>> d['foo']
'bar'
>>> import time
>>> time.sleep(4) # 4 seconds > 3 second cache expiry of d
>>> d['foo']
Traceback (most recent call last):
    ...
KeyError: 'foo'
>>> d['a'] = 'A'
>>> d['b'] = 'B'
>>> d['c'] = 'C'
>>> d['d'] = 'D'
>>> d['a'] # Should return value error, since we exceeded the max cache size
Traceback (most recent call last):
    ...
KeyError: 'a'

By default, this cache will only expire items whenever you poke it - all methods on this class will result in a cleanup. If the thread_clear option is specified, a background thread will clean it up every thread_clear_min_check seconds.

If this class must be used in a multithreaded environment, the option concurrent should be set to true. Note that the cache will always be concurrent if a background cleanup thread is used.

Initialize the LRUCacheDict object.

Parameters:

  • max_size (int) – Maximum number of elements in the cache.

  • expiration (int) – Number of seconds an item can be in the cache before it expires.

  • thread_clear (bool) – True if we want to use a background thread to keep the cache clear.

  • concurrent (bool) – True to make access to the cache thread-safe.

class EmptyCacheThread(cache, peek_duration=60)

Bases: Thread

Background thread that expires elements out of the cache.

Initialize the EmptyCacheThread.

Parameters:

  • cache (LRUCacheDict) – The cache to be monitored.

  • peek_duration (int) – The delay between “sweeps” of the cache.

getName()

Return a string used for identification purposes only.

This method is deprecated, use the name attribute instead.

property ident

Thread identifier of this thread or None if it has not been started.

This is a nonzero integer. See the get_ident() function. Thread identifiers may be recycled when a thread exits and another thread is created. The identifier is available even after the thread has exited.

isDaemon()

Return whether this thread is a daemon.

This method is deprecated, use the daemon attribute instead.

is_alive()

Return whether the thread is alive.

This method returns True just before the run() method starts until just after the run() method terminates. See also the module function enumerate().

join(timeout=None)

Wait until the thread terminates.

This blocks the calling thread until the thread whose join() method is called terminates – either normally or through an unhandled exception or until the optional timeout occurs.

When the timeout argument is present and not None, it should be a floating point number specifying a timeout for the operation in seconds (or fractions thereof). As join() always returns None, you must call is_alive() after join() to decide whether a timeout happened – if the thread is still alive, the join() call timed out.

When the timeout argument is not present or None, the operation will block until the thread terminates.

A thread can be join()ed many times.

join() raises a RuntimeError if an attempt is made to join the current thread as that would cause a deadlock. It is also an error to join() a thread before it has been started and attempts to do so raises the same exception.

property name

A string used for identification purposes only.

It has no semantics. Multiple threads may be given the same name. The initial name is set by the constructor.

property native_id

Native integral thread ID of this thread, or None if it has not been started.

This is a non-negative integer. See the get_native_id() function. This represents the Thread ID as reported by the kernel.

run()

Execute the background cleanup.

setDaemon(daemonic)

Set whether this thread is a daemon.

This method is deprecated, use the .daemon property instead.

setName(name)

Set the name string for this thread.

This method is deprecated, use the name attribute instead.

start()

Start the thread’s activity.

It must be called at most once per thread object. It arranges for the object’s run() method to be invoked in a separate thread of control.

This method will raise a RuntimeError if called more than once on the same thread object.

class LRUCachedFunction(function, cache=None)

Bases: object

A memoized function, backed by an LRU cache.

>>> def f(x):
...    print "Calling f(" + str(x) + ")"
...    return x
>>> f = LRUCachedFunction(f, LRUCacheDict(max_size=3, expiration=3) )
>>> f(3)
Calling f(3)
3
>>> f(3)
3
>>> import time
>>> time.sleep(4) #Cache should now be empty, since expiration time is 3.
>>> f(3)
Calling f(3)
3
>>> f(4)
Calling f(4)
4
>>> f(5)
Calling f(5)
5
>>> f(3) #Still in cache, so no print statement. At this point, 4 is the least recently used.
3
>>> f(6)
Calling f(6)
6
>>> f(4) #No longer in cache - 4 is the least recently used, and there are at least 3 others
items in cache [3,4,5,6].
Calling f(4)
4

Initialize the LRUCachedFunction object.

Parameters:

  • function (func) – The function to be used to create new items in the cache.

  • cache (LRUCacheDict) – The internal cache structure.

lru_cache_function(max_size=1024, expiration=900)

Least recently used cache function

>>> @lru_cache_function(3, 1)
... def f(x):
...    print "Calling f(" + str(x) + ")"
...    return x
>>> f(3)
Calling f(3)
3
>>> f(3)
3