解读Django框架中的低层次缓存API
有些时候，对整个经解析的页面进行缓存并不会给你带来太多好处，事实上可能会过犹不及。
比如说，也许你的站点所包含的一个视图依赖几个费时的查询，每隔一段时间结果就会发生变化。 在这种情况下，使用站点级缓存或者视图级缓存策略所提供的整页缓存并不是最理想的，因为你可能不会想对整个结果进行缓存（因为一些数据经常变化），但你仍然会想对很少变化的部分进行缓存。
针对这样的情况，Django提供了简单低级的缓存API。 你可以通过这个API，以任何你需要的粒度来缓存对象。 你可以对所有能够安全进行 pickle 处理的 Python 对象进行缓存： 字符串、字典和模型对象列表等等。 （查阅 Python 文档可以了解到更多关于 pickling 的信息。）
缓存模块django.core.cache拥有一个自动依据CACHE_BACKEND设置创建的django.core.cache对象。
>>> from django.core.cache import cache
基本的接口是 set(key, value, timeout_seconds) 和 get(key) :
>>> cache.set('my_key', 'hello, world!', 30)
>>> cache.get('my_key')
'hello, world!'
timeout_seconds 参数是可选的, 并且默认为前面讲过的 CACHE_BACKEND 设置中的 timeout 参数.
如果缓存中不存在该对象，那么cache.get()会返回None。
# Wait 30 seconds for 'my_key' to expire...
>>> cache.get('my_key')
None
我们不建议在缓存中保存 None 常量，因为你将无法区分你保存的 None 变量及由返回值 None 所标识的缓存未命中。
cache.get() 接受一个 缺省 参数。 它指定了当缓存中不存在该对象时所返回的值：
>>> cache.get('my_key', 'has expired')
'has expired'
使用add()方法来新增一个原来没有的键值。 它接受的参数和set()一样，但是并不去尝试更新已经存在的键值。
>>> cache.set('add_key', 'Initial value')
>>> cache.add('add_key', 'New value')
>>> cache.get('add_key')
'Initial value'
如果想确定add()是否成功添加了缓存值，你应该测试返回值。 成功返回True，失败返回False。
还有个get_many()接口。 get_many() 所返回的字典包括了你所请求的存在于缓存中且未超时的所有键值。
>>> cache.set('a', 1)
>>> cache.set('b', 2)
>>> cache.set('c', 3)
>>> cache.get_many(['a', 'b', 'c'])
{'a': 1, 'b': 2, 'c': 3}
最后,你可以用 cache.delete() 显式地删除关键字。
>>> cache.delete('a')
也可以使用incr()或者decr()来增加或者减少已经存在的键值。 默认情况下，增加或减少的值是1。可以用参数来制定其他值。 如果尝试增减不存在的键值会抛出ValueError。
>>> cache.set('num', 1)
>>> cache.incr('num')
2
>>> cache.incr('num', 10)
12
>>> cache.decr('num')
11
>>> cache.decr('num', 5)
6
注意
incr()/decr()方法不是原子操作。 在支持原子增减的缓存后端上（最著名的是memcached），增减操作才是原子的。 然而，如果后端并不原生支持增减操作，也可以通过取值/更新两步操作来实现。
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