250 lines
6.8 KiB
ReStructuredText
250 lines
6.8 KiB
ReStructuredText
===============================
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cached-property
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===============================
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.. image:: https://img.shields.io/pypi/v/cached-property.svg
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:target: https://pypi.python.org/pypi/cached-property
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.. image:: https://img.shields.io/travis/pydanny/cached-property/master.svg
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:target: https://travis-ci.org/pydanny/cached-property
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A decorator for caching properties in classes.
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Why?
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-----
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* Makes caching of time or computational expensive properties quick and easy.
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* Because I got tired of copy/pasting this code from non-web project to non-web project.
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* I needed something really simple that worked in Python 2 and 3.
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How to use it
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--------------
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Let's define a class with an expensive property. Every time you stay there the
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price goes up by $50!
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.. code-block:: python
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class Monopoly(object):
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def __init__(self):
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self.boardwalk_price = 500
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@property
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def boardwalk(self):
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# In reality, this might represent a database call or time
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# intensive task like calling a third-party API.
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self.boardwalk_price += 50
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return self.boardwalk_price
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Now run it:
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.. code-block:: python
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>>> monopoly = Monopoly()
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>>> monopoly.boardwalk
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550
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>>> monopoly.boardwalk
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600
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Let's convert the boardwalk property into a ``cached_property``.
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.. code-block:: python
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from cached_property import cached_property
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class Monopoly(object):
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def __init__(self):
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self.boardwalk_price = 500
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@cached_property
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def boardwalk(self):
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# Again, this is a silly example. Don't worry about it, this is
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# just an example for clarity.
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self.boardwalk_price += 50
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return self.boardwalk_price
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Now when we run it the price stays at $550.
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.. code-block:: python
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>>> monopoly = Monopoly()
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>>> monopoly.boardwalk
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550
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>>> monopoly.boardwalk
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550
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>>> monopoly.boardwalk
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550
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Why doesn't the value of ``monopoly.boardwalk`` change? Because it's a **cached property**!
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Invalidating the Cache
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----------------------
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Results of cached functions can be invalidated by outside forces. Let's demonstrate how to force the cache to invalidate:
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.. code-block:: python
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>>> monopoly = Monopoly()
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>>> monopoly.boardwalk
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550
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>>> monopoly.boardwalk
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550
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>>> # invalidate the cache
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>>> del monopoly.__dict__['boardwalk']
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>>> # request the boardwalk property again
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>>> monopoly.boardwalk
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600
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>>> monopoly.boardwalk
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600
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Working with Threads
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---------------------
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What if a whole bunch of people want to stay at Boardwalk all at once? This means using threads, which
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unfortunately causes problems with the standard ``cached_property``. In this case, switch to using the
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``threaded_cached_property``:
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.. code-block:: python
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from cached_property import threaded_cached_property
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class Monopoly(object):
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def __init__(self):
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self.boardwalk_price = 500
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@threaded_cached_property
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def boardwalk(self):
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"""threaded_cached_property is really nice for when no one waits
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for other people to finish their turn and rudely start rolling
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dice and moving their pieces."""
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sleep(1)
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self.boardwalk_price += 50
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return self.boardwalk_price
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Now use it:
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.. code-block:: python
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>>> from threading import Thread
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>>> from monopoly import Monopoly
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>>> monopoly = Monopoly()
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>>> threads = []
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>>> for x in range(10):
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>>> thread = Thread(target=lambda: monopoly.boardwalk)
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>>> thread.start()
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>>> threads.append(thread)
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>>> for thread in threads:
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>>> thread.join()
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>>> self.assertEqual(m.boardwalk, 550)
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Working with async/await (Python 3.5+)
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--------------------------------------
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The cached property can be async, in which case you have to use await
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as usual to get the value. Because of the caching, the value is only
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computed once and then cached:
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.. code-block:: python
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from cached_property import cached_property
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class Monopoly(object):
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def __init__(self):
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self.boardwalk_price = 500
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@cached_property
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async def boardwalk(self):
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self.boardwalk_price += 50
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return self.boardwalk_price
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Now use it:
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.. code-block:: python
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>>> async def print_boardwalk():
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... monopoly = Monopoly()
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... print(await monopoly.boardwalk)
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... print(await monopoly.boardwalk)
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... print(await monopoly.boardwalk)
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>>> import asyncio
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>>> asyncio.get_event_loop().run_until_complete(print_boardwalk())
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550
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550
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550
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Note that this does not work with threading either, most asyncio
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objects are not thread-safe. And if you run separate event loops in
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each thread, the cached version will most likely have the wrong event
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loop. To summarize, either use cooperative multitasking (event loop)
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or threading, but not both at the same time.
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Timing out the cache
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--------------------
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Sometimes you want the price of things to reset after a time. Use the ``ttl``
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versions of ``cached_property`` and ``threaded_cached_property``.
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.. code-block:: python
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import random
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from cached_property import cached_property_with_ttl
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class Monopoly(object):
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@cached_property_with_ttl(ttl=5) # cache invalidates after 5 seconds
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def dice(self):
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# I dare the reader to implement a game using this method of 'rolling dice'.
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return random.randint(2,12)
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Now use it:
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.. code-block:: python
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>>> monopoly = Monopoly()
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>>> monopoly.dice
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10
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>>> monopoly.dice
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10
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>>> from time import sleep
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>>> sleep(6) # Sleeps long enough to expire the cache
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>>> monopoly.dice
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3
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>>> monopoly.dice
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3
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**Note:** The ``ttl`` tools do not reliably allow the clearing of the cache. This
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is why they are broken out into seperate tools. See https://github.com/pydanny/cached-property/issues/16.
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Credits
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--------
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* Pip, Django, Werkzueg, Bottle, Pyramid, and Zope for having their own implementations. This package originally used an implementation that matched the Bottle version.
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* Reinout Van Rees for pointing out the `cached_property` decorator to me.
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* My awesome wife `@audreyr`_ who created `cookiecutter`_, which meant rolling this out took me just 15 minutes.
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* @tinche for pointing out the threading issue and providing a solution.
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* @bcho for providing the time-to-expire feature
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.. _`@audreyr`: https://github.com/audreyr
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.. _`cookiecutter`: https://github.com/audreyr/cookiecutter
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Support This Project
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---------------------------
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This project is maintained by volunteers. Support their efforts by spreading the word about:
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.. image:: https://cdn.shopify.com/s/files/1/0304/6901/t/2/assets/logo.png?8399580890922549623
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:name: Two Scoops Press
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:align: center
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:alt: Two Scoops Press
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:target: https://www.twoscoopspress.com
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