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-   -   Set literals (http://www.velocityreviews.com/forums/t342888-set-literals.html)

 George Sakkis 03-21-2005 10:51 PM

Set literals

>>> aSet = {1,2,3}

- It is the standard mathematic set notation.
- There is no ambiguity or backwards compatibility problem.
- Sets and dicts are in many respects similar data structures, so why not share the same delimiter ?

*ducks*

 George Sakkis 03-21-2005 10:57 PM

Re: Set literals

> - There is no ambiguity or backwards compatibility problem.

....at least if it wasn't for the empty set.. hmm...

 simon@arrowtheory.com 03-22-2005 12:01 AM

Re: Set literals

+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") ?

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.

 George Sakkis 03-22-2005 12:11 AM

Re: Set literals

simon@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.

> 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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