Setup dictionary lazily

If you don't separe the arguments from the callable, I don't think it's possible. However, this should work:

class MySettingsDict(dict):

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


def expensive_to_compute(arg):
    return arg * 3

And now:

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

Edit:

You may also want to cache the results of expensive_to_compute, if they are to be accessed multiple times. Something like this

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

And now:

>>> 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])

You may also want to override other dict methods depending of how you want to use the dict.

Don't inherit build-in dict. Even if you overwrite dict.__getitem__() method, dict.get() would not work as you expected.

The right way is to inherit abc.Mapping from collections.

from collections.abc import Mapping

class LazyDict(Mapping):
    def __init__(self, *args, **kw):
        self._raw_dict = dict(*args, **kw)

    def __getitem__(self, key):
        func, arg = self._raw_dict.__getitem__(key)
        return func(arg)

    def __iter__(self):
        return iter(self._raw_dict)

    def __len__(self):
        return len(self._raw_dict)

Then you can do:

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

I also list sample code and examples here: https://gist.github.com/gyli/9b50bb8537069b4e154fec41a4b5995a