gocept.cache 0.3

Cache descriptors for Python and Zope

gocept.cache

Method cache

Memoize with timeout

Memoize with timeout caches methods with a certain timeout:

>>> import math
>>> import gocept.cache.method
>>>
>>> class Point(object):
...
...     def __init__(self, x, y):
...         self.x, self.y = x, y
...
...     @gocept.cache.method.Memoize(0.1)
...     def distance(self, x, y):
...         print 'computing distance'
...         return math.sqrt((self.x - x)**2 + (self.y - y)**2)
...
...     @gocept.cache.method.Memoize(0.1, ignore_self=True)
...     def add_one(self, i):
...         if not isinstance(i, int):
...             print "I want an int"
...         else:
...             print 'adding one'
...             return i + 1
...
>>> point = Point(1.0, 2.0)

When we first ask for the distance it is computed:

>>> point.distance(2, 2)
computing distance
1.0

The second time the distance is not computed but returned from the cache:

>>> point.distance(2, 2)
1.0

Now, let’s wait 0.1 secondes, the value we set as cache timeout. After that the distance is computed again:

>>> import time
>>> time.sleep(0.5)
>>> point.distance(2, 2)
computing distance
1.0

When we create a new instance, the new instance gets its own cache:

>>> p2 = Point(1.0, 2.0)
>>> p2.distance(2, 2)
computing distance
1.0

It’s also possible to explicitly ignore self. We did this for the add_one method:

>>> point.add_one(3)
adding one
4

The second time it’s not computed as you would expect: >>> point.add_one(3) 4

If we ask p2 now the result is not computed as well:

>>> p2.add_one(3)
4

If we put a non hashable argument into a memoized function it will not be cached:

>>> point.add_one({'a': 1})
I want an int
>>> point.add_one({'a': 1})
I want an int

The decorated method can be introspected and yields the same results ad the original:

>>> import inspect
>>> Point.distance.func_name
'distance'
>>> inspect.getargspec(Point.distance)
(['self', 'x', 'y'], None, None, None)

Store memoizations on an attribute

If you want more control over the cache used by gocept.cache.method.Memoize (e. g. you want to associate it with a gocept.cache.property.CacheDataManager to invalidate it on transaction boundaries), you can use the @memoize_on_attribute decorator to retrieve the cache-dictionary from the instance:

>>> class Bar(object):
...     cache = {}
...
...     @gocept.cache.method.memoize_on_attribute('cache', 10)
...     def echo(self, x):
...         print 'miss'
...         return x

>>> bar = Bar()
>>> bar.echo(5)
miss
5
>>> bar.echo(5)
5
>>> bar.cache.clear()
>>> bar.echo(5)
miss
5

This decorator should be used on methods, not on plain functions, since it must be able to retrieve the cache-dictionary from the first argument of the function (which is ‘self’ for methods):

>>> @gocept.cache.method.memoize_on_attribute('cache', 10)
... def bar():
...     print 'foo'
>>> bar()
Traceback (most recent call last):
TypeError: gocept.cache.method.memoize_on_attribute could not retrieve cache attribute 'cache' for function <function bar at 0x...>

>>> @gocept.cache.method.memoize_on_attribute('cache', 10)
... def baz(x):
...     print 'foo'
>>> baz(5)
Traceback (most recent call last):
TypeError: gocept.cache.method.memoize_on_attribute could not retrieve cache attribute 'cache' for function <function baz at 0x...>

Cached Properties

Transaction Bound Cache

The transaction bound cache is invalidated on transaction boundaries.

Create a class and set some data:

>>> import gocept.cache.property
>>> class Foo(object):
...
...     cache = gocept.cache.property.TransactionBoundCache('_cache', dict)
...
>>> foo = Foo()
>>> foo.cache
{}
>>> foo.cache['A'] = 1
>>> foo.cache
{'A': 1}

If we commit the transaction the cache is empty again:

>>> import transaction
>>> transaction.commit()
>>> foo.cache
{}

The same happens on abort:

>>> foo.cache['A'] = 1
>>> foo.cache
{'A': 1}
>>> transaction.abort()
>>> foo.cache
{}

Changes

0.3 (2008-12-19)

• Added @memoize_on_attribute to retrieve the memoization cache from the instance instead of using gocept.cache.method’s built-in cache.

0.2.2 (2007-12-17)

• Fixed the bug in TransactionBoundCache where the cache was not invalidated on transaction abort.

0.2.1 (2007-10-17)

• Fixed a bug in TransactionBoundCache which yielded an error in the log: TypeError: <lambda>() takes exactly 1 argument (2 given)