cached-property

A decorator for caching properties in classes.

Why?

• Makes caching of time or computational expensive properties quick and easy.
• Because I got tired of copy/pasting this code from non-web project to non-web project.
• I needed something really simple that worked in Python 2 and 3.

How to use it

Let's define a class with an expensive property. Every time you stay there the price goes up by $50!

class Monopoly(object):

    def __init__(self):
        self.boardwalk_price = 500

    @property
    def boardwalk(self):
        # In reality, this might represent a database call or time
        # intensive task like calling a third-party API.
        self.boardwalk_price += 50
        return self.boardwalk_price

Now run it:

>>> monopoly = Monopoly()
>>> monopoly.boardwalk
550
>>> monopoly.boardwalk
600

Let's convert the boardwalk property into a cached_property.

from cached_property import cached_property

class Monopoly(object):

    def __init__(self):
        self.boardwalk_price = 500

    @cached_property
    def boardwalk(self):
        # Again, this is a silly example. Don't worry about it, this is
        #   just an example for clarity.
        self.boardwalk_price += 50
        return self.boardwalk_price

Now when we run it the price stays at $550.

>>> monopoly = Monopoly()
>>> monopoly.boardwalk
550
>>> monopoly.boardwalk
550
>>> monopoly.boardwalk
550

Why doesn't the value of monopoly.boardwalk change? Because it's a cached property!

Invalidating the Cache

Results of cached functions can be invalidated by outside forces. Let's demonstrate how to force the cache to invalidate:

>>> monopoly = Monopoly()
>>> monopoly.boardwalk
550
>>> monopoly.boardwalk
550
>>> # invalidate the cache
>>> del monopoly.__dict__['boardwalk']
>>> # request the boardwalk property again
>>> monopoly.boardwalk
600
>>> monopoly.boardwalk
600

Working with Threads

What if a whole bunch of people want to stay at Boardwalk all at once? This means using threads, which unfortunately causes problems with the standard cached_property. In this case, switch to using the threaded_cached_property:

from cached_property import threaded_cached_property

class Monopoly(object):

    def __init__(self):
        self.boardwalk_price = 500

    @threaded_cached_property
    def boardwalk(self):
        """threaded_cached_property is really nice for when no one waits
            for other people to finish their turn and rudely start rolling
            dice and moving their pieces."""

        sleep(1)
        self.boardwalk_price += 50
        return self.boardwalk_price

Now use it:

>>> from threading import Thread
>>> from monopoly import Monopoly
>>> monopoly = Monopoly()
>>> threads = []
>>> for x in range(10):
>>>     thread = Thread(target=lambda: monopoly.boardwalk)
>>>     thread.start()
>>>     threads.append(thread)

>>> for thread in threads:
>>>     thread.join()

>>> self.assertEqual(m.boardwalk, 550)

Timing out the cache

Sometimes you want the price of things to reset after a time. Use the ttl versions of cached_property and threaded_cached_property.

import random
from cached_property import cached_property_with_ttl

class Monopoly(object):

    @cached_property_with_ttl(ttl=5) # cache invalidates after 5 seconds
    def dice(self):
        # I dare the reader to implement a game using this method of 'rolling dice'.
        return random.randint(2,12)

Now use it:

>>> monopoly = Monopoly()
>>> monopoly.dice
10
>>> monopoly.dice
10
>>> from time import sleep
>>> sleep(6) # Sleeps long enough to expire the cache
>>> monopoly.dice
3
>>> monopoly.dice
3

Note: The ttl tools do not reliably allow the clearing of the cache. This is why they are broken out into seperate tools. See pydanny#16.

Working with a Class Property

What if you want to cache a property accross different instances, you can use cached_classproperty. Note that cached_classproperty cannot be invalidated.

from cached_property import cached_classproperty

class Monopoly(object):

    boardwalk_price = 500

    @cached_classproperty
    def boardwalk(cls):
        cls.boardwalk_price += 50
        return cls.boardwalk_price

Now use it:

>>> Monopoly().boardwalk
550
>>> Monopoly().boardwalk
550

Credits

• Pip, Django, Werkzueg, Bottle, Pyramid, and Zope for having their own implementations. This package originally used an implementation that matched the Bottle version.
• Reinout Van Rees for pointing out the cached_property decorator to me.
• My awesome wife @audreyr who created cookiecutter, which meant rolling this out took me just 15 minutes.
• @tinche for pointing out the threading issue and providing a solution.
• @bcho for providing the time-to-expire feature

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