Simple if-statement in Python, testing for mutual exclusiveness

Question

I am doing some stuff in python, but I am not that used to it yet.

This is what I want the code to do (pseudocode):

if x in list or y in list but not x and y:
    #do-stuff

the first part is correct (if x in list or y in list) but I don't know what to write after that. The but not x and ypart.

I am iterating through a set of lists. I want it to only do calculations if the list contains X or Y but not both. — user3685412, Jul 05 '14 at 12:03
If the primary operation you want to perform on `list` is membership tests, you should use a set instead. — Sven Marnach, Jul 05 '14 at 12:11

Jon Clements · Answer 1 · 2014-07-05T12:35:27.870

4

I'd probably go for using a set here instead...

if len({x, y}.intersection(your_list)) == 1:
    # do something

This has the advantage that it only needs to iterate once over your_list.

Example:

examples = [
    [1, 2, 3, 4, 5],
    [2, 3, 4, 5, 6],
    [100, 101, 102, 103, 104]
]

for example in examples:
    overlap = {1, 5}.intersection(example)
    print 'overlap is', overlap, 'with a length of', len(overlap)

# overlap is set([1, 5]) with a length of 2
# overlap is set([5]) with a length of 1
# overlap is set([]) with a length of 0

edited Jul 05 '14 at 12:35

answered Jul 05 '14 at 12:04

Jon Clements

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Thanks. But behzad.nouri's answer solved it for me. I used xor: "`(x in list) ^ (y in list)`" – user3685412 Jul 05 '14 at 12:07
2

@user3685412 indeed... they both work... but `(x in list) ^ (y in list)` has to scan `list` twice to compute the either/or – Jon Clements Jul 05 '14 at 12:08
Oh okay. That's nice. I'll definitely use yours then. The list is pretty big :) – user3685412 Jul 05 '14 at 12:10
1

@user3685412 note: you can't use this approach if `x` and `y` happen to be unhashable... – Jon Clements Jul 05 '14 at 12:13
On the other hand, your solution needs to create a temporary set. It's a classic time vs. space tradeoff, but with no change in complexity. – user4815162342 Jul 05 '14 at 12:34
@user4815162342 indeed, but a two element set is hardly a monstrous overhead and the intersection operation is efficient. – Jon Clements Jul 05 '14 at 12:37
@JonClements Good point, I missed the fact that you're only creating a short constant-length set, so I retract my comment. – user4815162342 Jul 05 '14 at 14:18

score 1 · Answer 2 · edited May 23 '17 at 10:26

1

With "but" spelled and and a pair of parentheses, you can use:

x_in_list = x in list
y_in_list = y in list
if (x_in_list or y_in_list) and not (x_in_list and y_in_list):
    ...

Or, as suggested by behzad.nouri, use xor:

if (x in list) ^ (y in list):
    ...

This is shorter, but possibly less understandable to a non-CS-savvy reader.

edited May 23 '17 at 10:26

Community

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answered Jul 05 '14 at 12:06

user4815162342

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6502 · Answer 3 · 2014-07-05T12:26:10.933

1

Exclusive or means "one or the other, but not both" and maps perfectly in what you want to do:

if (x in list) ^ (y in list):
    ...

looks a bit weird because normally xor is only used for bitwise operations, but here works because Python will implicitly convert True and False to 1 and 0, making the operator work as expected in an if.

Note however that the parenthesis are necessary because the xor operator ^ has an higher precedence than in (it's most often used for bitwise math, so this choice is reasonable).

edited Jul 05 '14 at 12:26

answered Jul 05 '14 at 12:11

6502

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Note that `True` and `False` aren't *converted* to 1 and 0, in a very real sense they [already *are*](http://stackoverflow.com/a/20756275/1600898) 1 and 0. – user4815162342 Jul 05 '14 at 12:33

score 1 · Answer 4 · edited May 23 '17 at 12:29

1

Inspired by Jon Clements's centi-upvoted answer,

items, it = {x, y}, (i in items for i in lst)
print(any(it) and not any(it))

This short-circuit version is better, in the negative cases.

it is an iterator of the original list, and we check if any of it is in items, if yes we check if all the other items are not in items.

Note: This works only if the items are unique in lst

edited May 23 '17 at 12:29

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answered Jul 05 '14 at 12:21

thefourtheye

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Simple if-statement in Python, testing for mutual exclusiveness

4 Answers4