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Kids caching library.

Project description

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kids.cache is a Python library providing a cache decorator. It’s part of ‘Kids’ (for Keep It Dead Simple) library.

It main concern is to offer a very simple default usage. Without forgetting to offer power inside when needed.

Maturity

This code is in alpha stage.

Compatibility

This code is tested for compatibility with python 2.7 and python >= 3 .

Features

using kids.cache:

  • Use one simple call to @cache, and a majority of all hidden complexity will vanish.

    • works out of the box on function, methods, property.

    • support to be called before or after @property.

  • Or go deeper into customization:

    • cache clearing functionality

    • cache stats

    • support of any cache store mecanism from cachetools package

    • support of custom key function which allows:

      • support of your exotic unhashable objects

      • fine tune what function calls should be considered identic

      • hand pick function dependencies in object (for method)

Basic Usage

Function

This cache decorator is quite straightforward to use:

>>> from kids.cache import cache

>>> @cache
... def answer_to_everything():
...     print("many insightfull calculation")
...     return 42

Then the function answer_to_everything would only do the calculation the first time called, and would save the result, and directly return it the next calls:

>>> answer_to_everything()
many insightfull calculation
42

>>> answer_to_everything()
42

The body of the function was not executed anymore and the cache value was used.

It’ll work with arguments:

>>> @cache
... def mysum(*args):
...     print("calculating...")
...     return sum(args)

>>> mysum(2, 2, 3)
calculating...
7
>>> mysum(1, 1, 1, 1)
calculating...
4
>>> mysum(2, 2, 3)
7
>>> mysum(1, 1, 1, 1)
4

And notice that by default, object are not typed, thus:

>>> mysum(1.0, 1, 1, 1)
4

Did trigger the cache, despite the first argument is a float and not an integer.

Methods

With methods:

>>> class MyObject(object):
...    def __init__(self, a, b):
...        self.a, self.b = a, b
...
...    @cache
...    def total(self):
...        print("calculating...")
...        return self.a + self.b

>>> xx = MyObject(2, 3)
>>> xx.total()
calculating...
5
>>> xx.total()
5

Cache is not shared between instances:

>>> yy = MyObject(2, 3)
>>> yy.total()
calculating...
5

Of course, if you change the inner values of the instance, this will NOT be detected by the caching method:

>>> xx.a = 5
>>> xx.total()
5

Look at advanced usages to see how to changes some of these behaviors.

Property

You can use the cache decorator with properties, and provides a good way to have lazy evaluated attributes:

>>> class WithProperty(MyObject):
...
...    @property
...    @cache
...    def total(self):
...        print("evaluating...")
...        return self.a + self.b

>>> xx = WithProperty(1, 1)
>>> xx.total
evaluating...
2
>>> xx.total
2

You can use @cache decorator before or after @property decorator:

>>> class WithProperty(MyObject):
...
...    @cache
...    @property
...    def total(self):
...        print("evaluating...")
...        return self.a + self.b

>>> xx = WithProperty(2, 2)
>>> xx.total
evaluating...
4
>>> xx.total
4

Advanced Usage

Most of the advanced usage implies to call the @cache decorator with arguments. Please notice that:

>>> @cache
... def mysum1(*args):
...     print("calculating...")
...     return sum(args)

Or:

>>> @cache()
... def mysum2(*args):
...     print("calculating...")
...     return sum(args)

is equivalent:

>>> mysum1(1,1)
calculating...
2
>>> mysum1(1,1)
2

>>> mysum2(1,1)
calculating...
2
>>> mysum2(1,1)
2

Provide a key function

Providing a key function can be extremely powerfull and will allow to fine tune when the cache should be recalculated.

hashing functions will receive exactly the same arguments than the main function called. It must return an hashable structure (combination of tuples, int, string… avoid list and dicts). This will identify uniquely the result.

For example you could:

>>> class WithKey(MyObject):
...    @cache(key=lambda s: (id(s), s.a, s.b))
...    def total(self):
...        print("calculating...")
...        return self.a + self.b

>>> xx = WithKey(2, 3)
>>> xx.total()
calculating...
5
>>> xx.total()
5

It should detect changes of the given values of the instance:

>>> xx.a = 5
>>> xx.total()
calculating...
8

Without bothering to recalculate when other values change:

>>> xx.c = 7
>>> xx.total()
8

But it should make a difference between instances:

>>> yy = WithKey(2, 3)
>>> yy.total()
calculating...
5

Typed key functions

You could ask for typed argument to NOT be treated the same:

>>> @cache(typed=True)
... def mysum(*args):
...     print("calculating...")
...     return sum(args)
>>> mysum(1, 1)
calculating...
2

>>> mysum(1.0, 1)
calculating...
2.0

Key functions

The default key function if not provided is a bold try to make list and dict also keyable despite these not being hashable.

The name of the key function is called hippie_hashing, and this is the default value for the key argument:

>>> from kids.cache import hippie_hashing

>>> @cache(key=hippie_hashing)
... def mylength(obj):
...     return len(obj)

This allows you to use the function with list, dict or combination of these:

>>> mylength([3, 2, 1])
3

But hippie_hashing will probably fail on special object:

>>> class Unhashable(object):
...    def __hash__(self):
...        raise ValueError("unhashable!")

>>> hippie_hashing(Unhashable())  ## doctest: +ELLIPSIS
Traceback (most recent call last):
...
ValueError: <Unhashable ...> can not be hashed. Try providing a custom key function.

If you are not a hippie, you should consider using strict=True and a much more limited by sober method will be used to make a key from your arguments:

>>> @cache(strict=True)
... def mylength(obj):
...     return len(obj)

>>> mylength("hello")
5

But then, don’t be surprised if it fails with dict, list, or set arguments:

>>> mylength([3, 2, 1])
Traceback (most recent call last):
...
TypeError: unhashable type: 'list'

And typed=True can be used in combination with strict=True:

>>> @cache(strict=True, typed=True)
... def mysum(*args):
...     print("calculating...")
...     return sum(args)
>>> mysum(1, 1)
calculating...
2

>>> mysum(1.0, 1)
calculating...
2.0

A good key function can:

  • make some cache timeout (but you should then look at cache store section to limit the size of the cache)

  • finely select which argument are pertinent to the method.

  • allow you to cache callables that have very special arguments that can’t be hashed properly.

Cleaning Cache

kids.cache uses some cachetools ideas, and each function cached received a cache_clear method:

>>> @cache
... def mysum(*args):
...     print("calculate...")
...     return sum(args)

>>> mysum(1,1)
calculate...
2
>>> mysum(1,1)
2

By calling cache_clear method, we flush all previous cached value:

>>> mysum.cache_clear()
>>> mysum(1,1)
calculate...
2

Cache stats

kids.cache uses some cachetools ideas, and each function cached received a cache_info method:

>>> @cache
... def mysum(*args):
...     print("calculate...")
...     return sum(args)

>>> mysum(1,1)
calculate...
2
>>> mysum(1,1)
2

>>> mysum.cache_info()
CacheInfo(hits=1, misses=1, maxsize=None, currsize=None)

Cache Store

kids.cache can use any dict-like structure as a cache store. This means you can provide some more clever cache stores. For example, you can use cachetools caches under the hood to manage the caching store.

Keep in mind that the default cache store is… a dict ! which is not a good idea if your program will run for a long time.

So if you need any caching store from cachetools you can provide it:

>>> from cachetools import LRUCache

LRU stands for Least Recent Used…

>>> @cache(use=LRUCache(maxsize=2))
... def mysum(*args):
...     print("calculate...")
...     return sum(args)

>>> mysum(1, 1)
calculate...
2
>>> mysum(1, 2)
calculate...
3
>>> mysum(1, 3)
calculate...
4

We have exceeded the cache memory and the least recent used have been tossed away:

>>> mysum(1, 1)
calculate...
2

But we still have this one in memory:

>>> mysum(1, 3)
4

Contributing

Any suggestion or issue is welcome. Push request are very welcome, please check out the guidelines.

Push Request Guidelines

You can send any code. I’ll look at it and will integrate it myself in the code base and leave you as the author. This process can take time and it’ll take less time if you follow the following guidelines:

  • check your code with PEP8 or pylint. Try to stick to 80 columns wide.

  • separate your commits per smallest concern.

  • each commit should pass the tests (to allow easy bisect)

  • each functionality/bugfix commit should contain the code, tests, and doc.

  • prior minor commit with typographic or code cosmetic changes are very welcome. These should be tagged in their commit summary with !minor.

  • the commit message should follow gitchangelog rules (check the git log to get examples)

  • if the commit fixes an issue or finished the implementation of a feature, please mention it in the summary.

If you have some questions about guidelines which is not answered here, please check the current git log, you might find previous commit that would show you how to deal with your issue.

Changelog

0.0.1 (2014-05-23)

  • First import. [Valentin Lab]

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