A decorator for caching properties in classes.
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 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:
550 monopoly.boardwalk 600monopoly = Monopoly() monopoly.boardwalk
Let's convert the boardwalk property into a
from cached_property import cached_property class Monopoly(object): def __init__(self): self.boardwalk_price = 500 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.
550 monopoly.boardwalk 550 monopoly.boardwalk 550monopoly = Monopoly() monopoly.boardwalk
Why doesn't the value of
monopoly.boardwalk change? Because it's a cached property!
Results of cached functions can be invalidated by outside forces. Let's demonstrate how to force the cache to invalidate:
550 monopoly.boardwalk 550 # invalidate the cache del monopoly.__dict__['boardwalk'] # request the boardwalk property again monopoly.boardwalk 600 monopoly.boardwalk 600monopoly = Monopoly() monopoly.boardwalk
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
from cached_property import threaded_cached_property class Monopoly(object): def __init__(self): self.boardwalk_price = 500 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)
The cached property can be async, in which case you have to use await as usual to get the value. Because of the caching, the value is only computed once and then cached:
from cached_property import cached_property class Monopoly(object): def __init__(self): self.boardwalk_price = 500 async def boardwalk(self): self.boardwalk_price += 50 return self.boardwalk_price
Now use it:
async def print_boardwalk(): monopoly = Monopoly() print(await monopoly.boardwalk) print(await monopoly.boardwalk) print(await monopoly.boardwalk) import asyncio asyncio.get_event_loop().run_until_complete(print_boardwalk()) 550 550 550
Note that this does not work with threading either, most asyncio objects are not thread-safe. And if you run separate event loops in each thread, the cached version will most likely have the wrong event loop. To summarize, either use cooperative multitasking (event loop) or threading, but not both at the same time.
Sometimes you want the price of things to reset after a time. Use the
import random from cached_property import cached_property_with_ttl class Monopoly(object): 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:
10 monopoly.dice 10 from time import sleep sleep(6) # Sleeps long enough to expire the cache monopoly.dice 3 monopoly.dice 3monopoly = Monopoly() monopoly.dice
ttl tools do not reliably allow the clearing of the cache. This
is why they are broken out into seperate tools. See https://github.com/pydanny/cached-property/issues/16.
cached_propertydecorator to me.
cookiecutter, which meant rolling this out took me just 15 minutes.