On 30.06.20 05:08, Steven D'Aprano wrote:

On Tue, Jun 30, 2020 at 11:10:20AM +0900, Inada Naoki wrote:

On Mon, Jun 29, 2020 at 9:12 PM Hans Ginzel <hans@matfyz.cz> wrote:


      

        

          
What are the reasons, why object dict.items() is not subscriptable – dict.items()[0]?


        

        
Because dict is optimized for random access by key and iteration, but not for
random access by index.


      

      
But we're not talking about *dict*, we're talking about dict.items which
returns a set-like object:

    py> from collections.abc import Set
    py> isinstance({}.items(), Set)
    True

So dict.items isn't subscriptable because it's an unordered set, not a
sequence.


    

    
What is the reason for `dict.items` to return a set-like object?
      The values can be non-hashable an in this case the behavior can be
      surprising:

    
    >>> {'a': []}.items() & {'b'}

          TypeError: unhashable type: 'list'

    
Furthermore [the
documentation](https://docs.python.org/3/library/stdtypes.html#dict-views)
      states the following:

    
> If all
      values are hashable, so that (key, value)
      pairs are unique and hashable,
      then the items view is also set-like.

    
This sounds like the return type of `dict.items` depended on the
      actual values contained (which again would be surprising) but
      actually it doesn't seem to be the case:

    
    >>> from collections.abc import Set

          >>> isinstance({'a': []}.items(), Set)

          True

    
`dict.items` could provide all of its "standard" behavior (like
      membership testing, reversing, etc) without being set-like.

    
The fact that `==` with `dict.items` raises TypeError for
      non-hashable values is also a little surprising since it supports
      membership testing and hence could check `len(self) == len(other)
      and all(x in self for x in other)` (though that drops the type
      comparison, but if you cannot have a set, why would you compare
      them anyway):

    
    >>> self = {'a': []}.items()

          >>> other = {('a', 1)}

          >>> self == other

          TypeError: unhashable type: 'list'

          >>> len(self) == len(other) and all(x in self for x
      in other)

          False

          >>> other = [('a', [])]

          >>> len(self) == len(other) and all(x in self for x
      in other)

          True