[issue35338] set union/intersection/difference could accept zero arguments

[Josh Rosenberg]

Josh Rosenberg  added the comment:

Given the "feature" in question isn't actually an intended feature (just an 
accident of how unbound methods work), I'm closing this. We're not going to try 
to make methods callable without self.

--
resolution:  -> wont fix
stage:  -> resolved
status: open -> closed

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue35338] set union/intersection/difference could accept zero arguments

[Mark Dickinson]

Mark Dickinson  added the comment:

> set.union() without constructing the set you call union on only happens to 
> work for the set.union(a) case because `a` is already a set.

Good point. I wasn't thinking clearly about the unbound-methodness of this.

> I'm -1 on making any changes to set.union to support this misuse case.

Agreed.

--

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue35338] set union/intersection/difference could accept zero arguments

[Josh Rosenberg]

Josh Rosenberg  added the comment:

set.union() without constructing the set you call union on only happens to work 
for the set.union(a) case because `a` is already a set. union takes arbitrary 
iterables, not just sets, and you're just cheating by explicitly passing `a` as 
the expected self argument. If you'd set `a = [1, 2]` (a list, not a set), 
set.union(a) would fail, because set.union(a) was only working by accident of a 
being interpreted as self; any such use is misuse.

Point is, the zero args case isn't a unique corner case;

args = ([1, 2], ANY OTHER ITERABLES HERE)
set.union(*args)

fails too, because the first argument is interpreted as self, and must be a set 
for this to work.

SilentGhost's solution of constructing the set before union-ing via 
set().union(*args) is the correct solution; it's free of corner cases, removing 
the specialness of the first element in args (because self is passed in 
correctly), and not having any troubles with empty args.

intersection is the only interesting case here, where preconstruction of the 
empty set doesn't work, because that would render the result the empty set 
unconditionally. The solution there is set(args[0]).intersection(*args) (or 
*args[1:]), but that's obviously uglier.

I'm -1 on making any changes to set.union to support this misuse case.

--
nosy: +josh.r

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue35338] set union/intersection/difference could accept zero arguments

[Mark Dickinson]

Mark Dickinson  added the comment:

> The intersection of an empty set of sets is either the empty set [...]

Nope, it's never the empty set, unless you're using a *very* unusual definition.

The intersection of a collection X of sets is the set of all x in the universe 
such that x is in S for all S in X. If X is empty, that condition is vacuously 
true, and you simply get the set of all x.

There are universe problems here, but giving the empty set is definitely wrong.

OTOH, the union of an empty collection of sets _is_ unambiguously the empty 
set, and the request for `set.union(*args)` to be valid regardless of the 
length of args seems reasonable to me. Without that validity, there's a 
potential corner-case bug where `set.union(*args)` potentially fails when 
`args` is empty.

--

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue35338] set union/intersection/difference could accept zero arguments

[Mark Dickinson]

Mark Dickinson  added the comment:

Some reading: http://mathforum.org/library/drmath/view/62503.html

--

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue35338] set union/intersection/difference could accept zero arguments

[Rémi Lapeyre]

Rémi Lapeyre  added the comment:

The intersection of an empty set of sets is either the empty set or the 
universe sets so if set.union() is defined to support set.union(*sequence), 
wouldn't it be less surprising for set.intersection() to return the empty set 
too?

--
nosy: +remi.lapeyre

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue35338] set union/intersection/difference could accept zero arguments

[Mark Dickinson]

Mark Dickinson  added the comment:

What would the intersection of zero sets be? The only reasonable mathematical 
answer is either "not defined", or "the universe" - i.e., the set containing 
everything, but what would "everything" be in a Python context?

I also don't see how a difference of zero sets makes any sense; you need a set 
to start subtracting items from.

So I think this feature request only really applies to union.

--
nosy: +mark.dickinson

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue35338] set union/intersection/difference could accept zero arguments

[Raymond Hettinger]

Change by Raymond Hettinger :


--
assignee:  -> rhettinger

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue35338] set union/intersection/difference could accept zero arguments

[Karthikeyan Singaravelan]

Karthikeyan Singaravelan  added the comment:

I agree with @SilentGhost to use set().union(*sequence) which is compatible 
with Python 2 too.

--
nosy: +rhettinger, xtreak

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue35338] set union/intersection/difference could accept zero arguments

[SilentGhost]

SilentGhost  added the comment:

You can write your code like this:

   set().union(*sequence)

as well as: a.union(b, c) if you already have a set object available.

--
nosy: +SilentGhost
versions: +Python 3.8 -Python 3.7

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue35338] set union/intersection/difference could accept zero arguments

[David Miguel Susano Pinto]

New submission from David Miguel Susano Pinto :

set union, intersection, difference methods accept any non-zero number of sets 
and return a new set instance, like so:

>>> a = set([1, 2])
>>> b = set([1, 3])
>>> c = set([3, 5])
>>> set.union(a, b, c)
{1, 2, 3, 5}

even if it's only one argument:

>>> set.union(a)
{1, 2}

I think it would be nice if zero arguments were not an error:

>>> set.union()
Traceback (most recent call last):
  File "", line 1, in 
TypeError: descriptor 'union' of 'set' object needs an argument

This would allow to handle any sequence of sets which otherwise requires this:

if len(sequence):
return set.union(*sequence)
else:
return set()

--
messages: 330601
nosy: carandraug
priority: normal
severity: normal
status: open
title: set union/intersection/difference could accept zero arguments
type: enhancement
versions: Python 3.7

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com