How can I make an instance of a class act like a dictionary?

Benjamin Kaplan benjamin.kaplan at case.edu
Mon Feb 27 21:37:15 CET 2012


On Mon, Feb 27, 2012 at 3:09 PM, John Salerno <johnjsal at gmail.com> wrote:
> On Feb 27, 1:39 am, Chris Rebert <c... at rebertia.com> wrote:
>> On Sun, Feb 26, 2012 at 11:24 PM, John Salerno <johnj... at gmail.com> wrote:
>> > Hi everyone. I created a custom class and had it inherit from the
>> > "dict" class, and then I have an __init__ method like this:
>>
>> > def __init__(self):
>> >        self = create()
>>
>> > The create function creates and returns a dictionary object. Needless
>> > to say, this is not working. When I create an instance of the above
>> > class, it is simply an empty dictionary rather than the populated
>> > dictionary being created by the create function. Am I doing the
>> > inheritance wrong, or am I getting the above syntax wrong by assigning
>> > the return value to self?
>>
>> Assignment to `self` has no effect outside the method in question;
>> Python uses call-by-object (http://effbot.org/zone/call-by-object.htm
>> ) for argument passing.
>> Even in something like C++, I believe assignment to `this` doesn't work.
>>
>> > I know I could do self.variable = create() and that works fine, but I
>> > thought it would be better (and cleaner) simply to use the instance
>> > itself as the dictionary, rather than have to go through an instance
>> > variable.
>>
>> Call the superclass (i.e. dict's) initializer (which you ought to be
>> doing anyway):
>>     super(YourClass, self).__init__(create())
>>
>> Cheers,
>> Chris
>> --http://rebertia.com
>
> Thanks. This ended up working:
>
> def __init__(self):
>        self = super().__init__(create_board())
>
> Is that what you meant for me to do? Why did assigning to self work in
> this case, but not the original case?


It didn't do anything and still isn't doing anything.

In Python, names are assigned to objects


self -> <Object1>
              ^
foo -------|

Reassigning a name does not change the value, so
self = create()

Just makes an object2

self -> <Object2>

foo ---> <Object1>

The reason it's working here is because the super().__init__() call
modifies the existing object in place. It returns None, so you're
setting self = None but that doesn't matter because of what I
explained before about how assigning to self doesn't actually change
anything.
> --
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