esoteric question about dict keys

[Reinhold Birkenfeld]

Mel Wilson wrote:
> In article <mailman.2023.1092997148.5135.python-list at python.org>,
> Alexis Roda <alexis.roda at urv.es> wrote:
>>Now the question. In "normal" dicts its not possible to use dictionaries
>>(nor other kinds of mutable objects) as keys, if I undersand correctly
>>this is a technical requirement. If I write my own dictionary-like
>>object, on wich the mutability of the keys is not a technical issue, is
>>considered blasphemous the use of dicts as keys? For example, querying
>>an SQL table can be partially modelled as a dictionary access:
> 
>    As I understand it, you can use any object as a key if
> you give it a __hash__ method and an __eq__ or __cmp__
> method.

You can, after all, use dicts as dict keys if you create your own
subclass of dict:

class hashdict(dict):
   def __hash__(self):
      return id(self)

x = hashdict()
testdic = {x : "test"}

But in order to retrieve the "test" value, you would need to index the
testdic with exactly the same instance of the hashdict. And that's why
dictionaries are unhashable.

Reinhold

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