懒惰地设置词典

问题描述

假设我有一个在模块级别(mysettings.py)定义的python词典：

settings = {
    'expensive1' : expensive_to_compute(1),
    'expensive2' : expensive_to_compute(2),
    ...
}

我希望在访问密钥时计算这些值：

from mysettings import settings # settings is only "prepared"

print settings['expensive1'] # Now the value is really computed.

这可能吗？如何？

---

解决方案

如果不将参数与Callable分开，我认为这是不可能的。但是，这应该是可行的：

class MySettingsDict(dict):

    def __getitem__(self, item):
        function, arg = dict.__getitem__(self, item)
        return function(arg)


def expensive_to_compute(arg):
    return arg * 3

现在：

>>> settings = MySettingsDict({
'expensive1': (expensive_to_compute, 1),
'expensive2': (expensive_to_compute, 2),
})
>>> settings['expensive1']
3
>>> settings['expensive2']
6

编辑：

如果要多次访问expensive_to_compute的结果，您可能还希望缓存这些结果。大概是这样的

class MySettingsDict(dict):

    def __getitem__(self, item):
        value = dict.__getitem__(self, item)
        if not isinstance(value, int):
            function, arg = value
            value = function(arg)
            dict.__setitem__(self, item, value)
        return value

现在：

>>> settings.values()
dict_values([(<function expensive_to_compute at 0x9b0a62c>, 2),
(<function expensive_to_compute at 0x9b0a62c>, 1)])
>>> settings['expensive1']
3
>>> settings.values()
dict_values([(<function expensive_to_compute at 0x9b0a62c>, 2), 3])

您可能还希望重写其他dict方法，具体取决于您希望如何使用词典。