Set operations in python

Question

I am looking for a single set operation to accomplish both the cases below. Is there a way to do this python?

Case 1:
a = set([1,2]) and b = set([1,2,3])
I want a result [1,2], which is a straightforward intersection. Now set(a) could be empty and performing intersection on empty set with any other set would lead to empty set.

Case 2:
a = set([]) and b = set([1,2,3]) => set([1,2,3]) i.e. set b

How can I achieve case 1 and case 2 with one set operation.

Do you want both `union` and `intersection` to happen in one operation ? — Shreevardhan, May 24 '16 at 09:14
It's unclear what you're asking -- it *looks* like you want a single set operation that has contradictory behavior. Can you elaborate? — Frédéric Hamidi, May 24 '16 at 09:14
In one case you want an intersection, in the other you essentially want to do nothing. I don't think there's any way to express that other than as "if `a` is empty, do nothing". — deceze, May 24 '16 at 09:14
My bad, let's simplify can we do "intersection" and "union" according to the set(a) value. I don't want to use "if" conditional to check it's value. — Program Questions, May 24 '16 at 09:16
What do you mean intersection *and* union? And why these restrictions like no if? You think there's a builtin operation for every conceivable weird construction? — Alex Hall, May 24 '16 at 09:17
This seems like an [XY Problem](http://meta.stackexchange.com/questions/66377/what-is-the-xy-problem) to me. Maybe you can tell us **why** you want what you want. — exilit, May 24 '16 at 09:20

score 3 · Answer 1 · answered May 24 '16 at 09:20

If what you want is "if a is non-empty, give me the intersection; otherwise just give me b", then what you want is:

result = (a&b) if a else b

"I don't want to use if conditional to check its value" does not make much sense as a requirement. This is what if is for: varying behaviour based on some condition.

score 1 · Accepted Answer · answered May 24 '16 at 09:50

1

Use and and or, which will short-circuit to produce the desired results:

>>> a = {1,2}
>>> b = {1,2,3}
>>> a and a&b or b
{1, 2}
>>> a = {}
>>> b = {1,2,3}
>>> a and a&b or b
{1, 2, 3}

answered May 24 '16 at 09:50

TigerhawkT3

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I don't think this gives the intended result (the empty intersection) for `a={1,2}`, `b={3,4}` – khelwood May 24 '16 at 10:06
1

You're right. I didn't think to test that case. @ProgramQuestions, can you unaccept this so I can delete it? – TigerhawkT3 May 24 '16 at 10:27

Set operations in python

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