ways to declare empty set variable

[Sun]

Maybe this is a very primative question, but I just get a bit confused about
'set' and 'Set' module in python.

I understand 'set' is a build in type in python after 2.4(or 2.3) and Set a
seperate module, anyhow, I gonna use build in 'set'.

then the question is how can I declare a empty set variable as a 'var= []'
do to a list variable?

[Marc 'BlackJack' Rintsch]

Re: ways to declare empty set variable

On Tue, 12 Feb 2008 14:45:43 +0100, Sun wrote:

then the question is how can I declare a empty set variable as a 'var= []'
do to a list variable?

You don't declare variables in Python. Just create an instance of `set`
and bind it to a name:

var = set()

Ciao,
Marc 'BlackJack' Rintsch

[Chris]

Re: ways to declare empty set variable

On Feb 12, 3:45 pm, "Sun" <a...@hut.atwro te:

Maybe this is a very primative question, but I just get a bit confused about
'set' and 'Set' module in python.
>
I understand 'set' is a build in type in python after 2.4(or 2.3) and Set a
seperate module, anyhow, I gonna use build in 'set'.
>
then the question is how can I declare a empty set variable as a 'var= []'
do to a list variable?

>>test = set()
>>test

set([])

>>type(test)

<type 'set'>

You looking for that ?

[Sun]

Re: ways to declare empty set variable


"Chris" <cwitts@gmail.c omwrote in message
news:f30e545f-4485-4c0a-a9b2-476939818bc6@y5 g2000hsf.google groups.com...

On Feb 12, 3:45 pm, "Sun" <a...@hut.atwro te:

>Maybe this is a very primative question, but I just get a bit confused
>about
>'set' and 'Set' module in python.
>>
>I understand 'set' is a build in type in python after 2.4(or 2.3) and Set
>a
>seperate module, anyhow, I gonna use build in 'set'.
>>
>then the question is how can I declare a empty set variable as a 'var=
>[]'
>do to a list variable?

>

>>>test = set()
>>>test

set([])

>>>type(test)

<type 'set'>
>
You looking for that ?

yeah, that 's what I am looking for, thanks all for such prompt answers!

I was wondering why can't I use a format as "var = {} " to "var=list() " in
set variable, and decided not to bother with it.

Thanks.

[Paul Rubin]

Re: ways to declare empty set variable

"Sun" <as@hut.atwrite s:

I was wondering why can't I use a format as "var = {} " to "var=list() " in
set variable, and decided not to bother with it.

In 3.0 you may be able to say {,} but there is a contingent that would
just as soon get rid of all that special syntax, so you'd say list()
instead of [], dict() instead of {}, etc.

[Gabriel Genellina]

Re: ways to declare empty set variable

En Tue, 12 Feb 2008 12:04:43 -0200, Sun <as@hut.atescri bió:

"Chris" <cwitts@gmail.c omwrote

>>>>test = set()
>>>>test

>set([])

>
yeah, that 's what I am looking for, thanks all for such prompt answers!
>
I was wondering why can't I use a format as "var = {} " to "var=list() "
in
set variable, and decided not to bother with it.

Python 3.0 has set literals {1,2,3} (perhaps they become frozensets
instead). But {} still is, and will be, an empty dict.
In reply to the n-th proposal to define a literal for empty sets, Guido
van Rossum said, in python-ideas:

"All possible proposals have already been discussed at length. Really,
writing set() isn't so bad. Get used to it."

http://mail.python.org/pipermail/python-ideas/2008-January/001316.html

--
Gabriel Genellina

[bearophileHUGS@lycos.com]

Re: ways to declare empty set variable

Paul Rubin:

In 3.0 you may be able to say {,} but there is a contingent that would
just as soon get rid of all that special syntax, so you'd say list()
instead of [], dict() instead of {}, etc.

For Python 3.0 I'd like {} for the empty set and {:} for the empty
dict, but that idea was refused time ago, probably for some mental
backward compatibility. Missing that, I think dict() and set() and
tuple() and list() look better than using {} for the empty dict and
{/} for the empty set and () for empty tuple (or {} for the empty dict
and set() for the empty set).
dict() is a bit more verbose than {}, but it doesn't matter much. With
those dict(), set(), tuple(), list() the only little wart left is the
unary tuple literal: x, that I don't like much, maybe I'd like tuple
to be identified by a pair of delimiters, maybe like [|x|] or
something like that as in the Fortress language. I don't know...

Bye,
bearophile

[Ben Finney]

Re: ways to declare empty set variable

bearophileHUGS@ lycos.com writes:

For Python 3.0 I'd like {} for the empty set and {:} for the empty
dict, but that idea was refused time ago, probably for some mental
backward compatibility.

I agree with not breaking that backward compatibility; it seems
wanton.

Missing that, I think dict() and set() and tuple() and list()

I often use these myself. They're slightly more explicit, which can
help when I want the reader not to have to think too much, and they're
not particularly verbose because the names are well-chosen and short.

look better than using {} for the empty dict and {/} for the empty
set and () for empty tuple

Note that '()' is syntactically null. Parentheses don't declare a
tuple literal, commas do. Parentheses are for grouping within
expressions, not specifying type.

(or {} for the empty dict and set() for the empty set).

I thought you said above that you preferred 'set()' for an empty set?
I'm not sure what it is you're saying now.

the only little wart left is the unary tuple literal: x, that I
don't like much

I agree that it's a wart, but I think the harm done by trying to
change it would be more than the harm done by leaving it in.

--
\ “[W]e are still the first generation of users, and for all that |
`\ we may have invented the net, we still don’t really get it.” |
_o__) —Douglas Adams |
Ben Finney

[bearophileHUGS@lycos.com]

Re: ways to declare empty set variable

Ben Finney:

I often use these myself. They're slightly more explicit, which can
help when I want the reader not to have to think too much, and they're
not particularly verbose because the names are well-chosen and short.

I'd like "list" be called "array" ;-)

Note that '()' is syntactically null. Parentheses don't declare a
tuple literal, commas do.

() is the literal for the empty tuple:

>>t = ()
>>type(t)

<type 'tuple'>

>>(1, 2)[0:0]

()

>Parentheses are for grouping within expressions, not specifying type.<

I know, but I prefer Fortress in that regard, where each container has
its specific delimiter(s). In Python ( ) denote:
- expression grouping
- they are very often used to denote tuples (despite being necessary
only for the empty one)
- generators (x for x in ...).
The Boo language shows that () aren't that necessary for the
generators.

I thought you said above that you preferred 'set()' for an empty set?
I'm not sure what it is you're saying now.

Your language isn't my first one, and for me sometimes it's not easy
to express complex things :-) I can try again. Here are syntax pairs
(for empty dict && empty set) sorted from the (IMHO) the best one to
the worst one:
{:} && {}
dict() && set()
{} && set()
{} && {/}

I think the harm done by trying to change it would be more
than the harm done by leaving it in.

I agree.

Bye,
bearophile

[Steve Holden]

Re: ways to declare empty set variable

Ben Finney wrote:
[...]

>
Note that '()' is syntactically null. Parentheses don't declare a
tuple literal, commas do. Parentheses are for grouping within
expressions, not specifying type.
>

Tell that to the interpreter:

>>type(())

<type 'tuple'>

>>tuple() is ()

True

>>>

regards
Steve
--
Steve Holden +1 571 484 6266 +1 800 494 3119
Holden Web LLC http://www.holdenweb.com/

[Ben Finney]

Re: ways to declare empty set variable

Steve Holden <steve@holdenwe b.comwrites:

Ben Finney wrote:
[...]

Note that '()' is syntactically null. Parentheses don't declare a
tuple literal, commas do. Parentheses are for grouping within
expressions, not specifying type.

Tell that to the interpreter:
>

>type(())

<type 'tuple'>

>tuple() is ()

True

>>

Well, knock me down with a kipper.

That makes it even more a violation of principle-of-least-astonishment
that the '(foo)' form doesn't give a one-element tuple literal.

--
\ "If consumers even know there's a DRM, what it is, and how it |
`\ works, we've already failed." —Peter Lee, Disney corporation, |
_o__) 2005 |
Ben Finney

[Ben Finney]

Re: ways to declare empty set variable

bearophileHUGS@ lycos.com writes:

In Python ( ) denote:
- expression grouping
- they are very often used to denote tuples (despite being necessary
only for the empty one)
- generators (x for x in ...).
The Boo language shows that () aren't that necessary for the
generators.

Now, that one I *am* sure of. Generator literals do not require the
parens at all. However, the syntax of where the generator literal
*appears* can make it necessary to explicitly group the expression
using parens.

>>import string
>>list(char for char in string.digits)

['0', '1', '2', '3', '4', '5', '6', '7', '8', '9']

>>char for char in string.digits

File "<stdin>", line 1
char for char in string.digits
^
SyntaxError: invalid syntax

So, it's not that parens are "used for generator literals". It's that
parens can be used to make grouping explicit where the syntax would
otherwise be ambiguous.

--
\ "If you're a horse, and someone gets on you, and falls off, and |
`\ then gets right back on you, I think you should buck him off |
_o__) right away." -- Jack Handey |
Ben Finney

[bearophileHUGS@lycos.com]

Re: ways to declare empty set variable

Ben Finney:

Generator literals do not require the
parens at all. However, the syntax of where the generator literal
*appears* can make it necessary to explicitly group the expression
using parens.

Have you taken a look at Boo?
In Python this isn't possible:
s = char for char in string.digits
You need ( ) there, while in Boo they aren't necessary there.

Bye,
bearophile

[cokofreedom@gmail.com]

Re: ways to declare empty set variable

The irony that, x = (,) produces an error.

Personally I would of thought it would be a better example of an empty
tuple than anything else, but it still isn't that readable.

The use of dict/list/tuple/set seems to stand out a lot better, makes
it readable! Else in a few years you'll have §x§ = !^!()

Or maybe I am going crazy...

[Ben Finney]

Re: ways to declare empty set variable

Nick Craig-Wood <nick@craig-wood.comwrites:

Ben Finney <bignose+hate s-spam@benfinney. id.auwrote:

bearophileHUGS@ lycos.com writes:

Missing that, I think dict() and set() and tuple() and list()

I often use these myself. They're slightly more explicit, which can
help when I want the reader not to have to think too much, and they're
not particularly verbose because the names are well-chosen and
short.

>
You can also do this with the dict() syntax

As was implied by the plural "these" in "I often use these myself".

--
\ "My house is made out of balsa wood, so when I want to scare |
`\ the neighborhood kids I lift it over my head and tell them to |
_o__) get out of my yard or I'll throw it at them." -- Steven Wright |
Ben Finney