On Mon, Jun 29, 2020 at 06:42:37PM +0100, Stestagg wrote:
> One simple example of where this is surprising is in the following:
>
> >>> random.choice({'a': 1,}.keys())
> TypeError: 'dict_keys' object is not subscriptable
I don't know why you think that is surprising. It doesn't surprise me in
the least.
> Several times now, I've had the need to 'just get any key/value' from a
> large dictionary. I usually try first to run `var.keys()[0]` only to be
> told that I'm not allowed to do this,
And you needed to be told several times before you learned that dicts,
and dict views, are not sequences?
> and instead have to ask python to
> make a copy of this datastructure with a different type, just so I can
> perform the index operation.
That's a hint that the technique you are using to extract a key may be a
poor choice.
If you need to remove the key from the dict as well, use the popitem
method.
If you don't need to remove it, I think the easiest way would be to use
a helper function:
def get(obj):
return next(iter(obj))
which will work on dicts, dict views, sets, frozensets, strings, bytes,
lists, and any collection that supports iteration.
That's a more powerful technique than requiring subscripting because
it will work on sets as well, and it avoids expensive copying
operations.
The downside of this is that repeated calls on the same object will
return the same element each time, but then your attempt with indexing
will have the same problem.
> Another use-cases is doing variations of reduce() over dictionaries, where
> getting an initial value from the dict, and then performing operations over
> the remaining items is much simpler to do with indexing on the views.
reduce has nothing to do with indexing, only iteration, so we can
already use reduce on dicts.
py> from functools import reduce
py> d = {3: 'a', 5: 'b', 2: 'c'}
py> reduce(lambda a,b: (min(a[0], b[0]), a[1]+b[1]), d.items())
(2, 'abc')
In case that's not clear (reduce often is not :-) that returns the
smallest key with the concatenation of all the values.
--
Steven
_______________________________________________
Python-ideas mailing list -- python-ideas@python.org
To unsubscribe send an email to python-ideas-le...@python.org
https://mail.python.org/mailman3/lists/python-ideas.python.org/
Message archived at
https://mail.python.org/archives/list/python-ideas@python.org/message/MBCV76GQ2IWFERRUUHPMMPEWFC6LNFIQ/
Code of Conduct: http://python.org/psf/codeofconduct/
