Set literals

George Sakkis gsakkis at rutgers.edu
Tue Mar 22 01:11:00 CET 2005


simon at arrowtheory.com> wrote:

> +1 from me.
>
> The other possible meaning for {1,2,3} would be {1:None,2:None,3:None},
> but that is usually meant to be a set anyway (done with a dict).
>
> So what is this: {1:2, 3, 4 } (apart from "nearly useless") ?

Syntax error; you'll have to decide whether you want a set or a dict.

> hmmm, thinking a bit more about this, it seems
> you can build a set from a dict's keys, but not the other
> way around. Is this odd, or what ?
>
> >>> a = set({1:0,2:0,3:0})
> >>> a
> set([1, 2, 3])
> >>>
> >>> dict(a)
> Traceback (most recent call last):
>   File "<stdin>", line 1, in ?
> TypeError: cannot convert dictionary update sequence element #0 to a
> sequence
>
> Simon.

Nothing odd here. The set constructor takes an iterable, and dict.__iter__ iterates through the
dict's keys, so the set a in your example doesn't know anything about the dict's values. You can go
back and forth a set and a dict if you store the dict's items instead:
>>> a = set({1:0,2:0,3:0}.iteritems())
>>> a
set([(1,0), (2,0), (3,0)])
>>> dict(a)
{1:0, 2:0, 3:0}

Regards,
George





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