[issue17854] symmetric difference operation applicable to more than two sets

[Raymond Hettinger]

Change by Raymond Hettinger :


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resolution:  -> rejected
stage: patch review -> resolved
status: pending -> closed

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[issue17854] symmetric difference operation applicable to more than two sets

[Irit Katriel]

Irit Katriel  added the comment:

I agree that the doc is fine as it is.  If there will be no 
objections/suggestions in the next couple of weeks I will close this issue.

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nosy: +iritkatriel
status: open -> pending

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[issue17854] symmetric difference operation applicable to more than two sets

[Terry J. Reedy]

Terry J. Reedy added the comment:

If you take the union/intersection/symmetric difference of n sets, the result 
is a set with all items that appears in one/all/an odd number of the n sets. 
The union and intersection methods actually accept n inputs, because the result 
is obvious, useful, and can be obtained faster that with n-1 binary operations. 
The symmetric_difference method does not, I presume because the result in not 
obvious (but that cuts both ways), not known to be useful, and perhaps would 
not be much faster than than n-1 binary operations.

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[issue17854] symmetric difference operation applicable to more than two sets

[Ezio Melotti]

Ezio Melotti added the comment:

> Return a new set with elements in an odd number of the sets.

This wording is not really clear to me.

IMHO the documentation is fine as is.  The evaluation order works as usual, 
and, since the symmetric difference is an associative (and commutative) 
operation, the order doesn't even matter.

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nosy: +ezio.melotti
type: behavior -> enhancement

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[issue17854] symmetric difference operation applicable to more than two sets

[Terry J. Reedy]

Terry J. Reedy added the comment:

Sets have methods that do not have operators (such as len, isdisjoint),
operators that do not have non-special methods (such as in, <), and 
method-operator pairs that do the same thing (such as (union, |), 
(symmetric_difference, ^)). For the pairs, it gives the method signature and 
the *equivalent* operator expression. Since .union takes multiple 'other' args, 
the equivalent operator expression does too. Since .symmetric_difference only 
takes one 'other' arg, so does the expression.

A coherent proposal would change the method code and doc to the following:

symmetric_difference(other, ...)
set ^ other ^ ...
Return a new set with elements in an odd number of the sets.

s={1,2, 5}
t={2,3, 5}
u={3,4, 5}
print(s^t^u)
>>> 
{1, 4, 5}

I believe the proposal was once considered, and rejected. An argument for is 
that the effect of chained symmetric differences is not obvious, as evidenced 
by Amit's mistaken characterization. I had to think a bit before I was sure of 
the answer. An argument against is that what one actually gets is seldom 
wanted, so that allowing more than two inputs to the method would have little 
benefit. 

What might be done is to document the symmetric different of multiple sets with 
a parenthetical comment such as

"(The symmetric difference of multiple sets, a ^ b ^ c ^ ..., is a new set with 
elements appearing in an odd number of input sets.)"

This would let people know what to expect from such expressions, in a situation 
where the effect is less obvious than usual.

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nosy: +rhettinger, terry.reedy
stage:  -> patch review
versions: +Python 2.7, Python 3.4

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[issue17854] symmetric difference operation applicable to more than two sets

[Amit Saha]

Amit Saha added the comment:

I think the only change I am suggesting is the description of the ^ operator to 
be something like this:

set ^ other ^ ..
Return a new set with elements from the sets which are not present in more than 
one set

I do understand that this is not really what the operator and the corresponding 
operation symmetric_difference() allows semantically. But it does make it 
consistent with the description of operators such as the | or &, but then their 
operation allows multiple sets semantically.

Hmm may be it is fine as it is..

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[issue17854] symmetric difference operation applicable to more than two sets

[Georg Brandl]

Georg Brandl added the comment:

Can you suggest a change?  I don't see a problem here; giving multiple 
operators for the other operations does not imply that they are not treated as 
left-associative.

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nosy: +georg.brandl

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[issue17854] symmetric difference operation applicable to more than two sets

[Amit Saha]

Amit Saha added the comment:

On some more thought, perhaps the description should be updated. Since s^t^u is 
effectively (s^t)^u and hence the implementation does not violate the 
definition.

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[issue17854] symmetric difference operation applicable to more than two sets

[Amit Saha]

New submission from Amit Saha:

The description of the symmetric difference operation implies that it cannot be 
applied to more than two sets 
(http://docs.python.org/3/library/stdtypes.html#set.symmetric_difference).

However, this is certainly possible:

>>> s={1,2}
>>> t={2,3}
>>> u={3,4}
>>> s^t^u
{1, 4}
>>> s.symmetric_difference(t).symmetric_difference(u)
{1, 4}

I am not sure how much of a "semantic" sense that makes, given that symmetric 
difference is by definition for two sets. 
(http://en.wikipedia.org/wiki/Symmetric_difference).

So, either the operator should be fixed to allow only two sets or the 
description be updated.

--
assignee: docs@python
components: Documentation
messages: 187899
nosy: Amit.Saha, docs@python
priority: normal
severity: normal
status: open
title: symmetric difference operation applicable to more than two sets
type: behavior
versions: Python 3.3

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