1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195
|
===============================
cached-property
===============================
.. image:: https://img.shields.io/pypi/v/cached-property.svg
:target: https://pypi.python.org/pypi/cached-property
.. image:: https://img.shields.io/travis/pydanny/cached-property/master.svg
:target: https://travis-ci.org/pydanny/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!
.. code-block:: python
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:
.. code-block:: python
>>> monopoly = Monopoly()
>>> monopoly.boardwalk
550
>>> monopoly.boardwalk
600
Let's convert the boardwalk property into a ``cached_property``.
.. code-block:: python
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.
.. code-block:: python
>>> 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:
.. code-block:: python
>>> 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``:
.. code-block:: python
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:
.. code-block:: python
>>> 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``.
.. code-block:: python
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:
.. code-block:: python
>>> 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 https://github.com/pydanny/cached-property/issues/16.
Credits
--------
* Pip, Django, Werkzueg, Bottle, Pyramid, and Zope for having their own implementations. This package uses an implementation that matches 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
.. _`@audreyr`: https://github.com/audreyr
.. _`cookiecutter`: https://github.com/audreyr/cookiecutter
|