Python 3.4 set intersection

Question

I am trying to understand why my intersection does not return an empty list when I run this code.

n = ([1,2,3,4,5],[3,4,5,6],[5,6,7],[7,8,9,10,11,12],[10,22,33,44,45])
w = set(n[0]).intersection(*n[:1])
print(w)
#Returns (1,2,3,4,5)

However this returns the correct set

n = ([1,2,3,4,5],[3,4,5,6],[5,6,7],[7,8,9,10,11,12],[10,22,33,44,45])
w = set(n[0]).intersection(*n)
print(w)
#Returns empty set.

This question gave the correct results for both:
Python -Intersection of multiple lists?

Why do I not get the correct result when I compare the first set to the remaining lists?

`set.intersection(*map(set, n)` would be a more memory efficient way to do what you want. — Padraic Cunningham, Feb 01 '16 at 16:35
Padraic. I have not looked at memory but your method is about 30% slower. 1.5 to 1.7 seconds per 100,000 searches vs 0.9 to 1.0 seconds for what I was attempting. — Dennis Simpson, Feb 01 '16 at 16:55
yes that is why I said more memory efficient, it is not always possible to make copies of data so sometimes memory is more important than speed. — Padraic Cunningham, Feb 01 '16 at 17:10

Tim Pietzcker · Accepted Answer · 2016-02-01T16:31:08.330

7

*n[:1] unpacks to (n[0],) - the first element of the set.

So you're intersecting n[0] with itself, and the result is what you see. You probably intended to write

set(n[0]).intersection(*n[1:])

edited Feb 01 '16 at 16:31

answered Feb 01 '16 at 16:29

Tim Pietzcker

That was it. I cannot believe I made that mistake. Thank you. – Dennis Simpson Feb 01 '16 at 16:33

1 Answers1