Using lists and tuples in Python if statements

Question

I'm wondering whether there are any good reasons to prefer a list over a tuple or vice versa in python if statments. So the following are functionally equivalent but is one preferable to the other in terms of performance and coding style or does it not matter?

if x in (1,2,3):
    foo()

if x in [1,2,3]:
    foo()

I seem to have gotten into the habit of using tuples if there are 2 or 3 values and lists for anything longer, I think because in my experience tuples tend to be short and lists long, but this seems a bit arbitrary and probably needlessly inconsistent.

I'd be interested in any examples people can give of where one would be better than the other.

Cheers

Possible dupe of the difference between lists and tuples or python list vs. tuple when to use each. — DSM
– DSM, Commented Sep 13, 2012 at 15:15
sets are more suitable for membership testing, ie if x in {1,2,3}: (I think this syntax only works in py 2.6+, maybe 2.7+) — GP89
– GP89, Commented Sep 13, 2012 at 15:18
@DSM: I don't agree; the context of an if foo in literal_sequence: statement is significant, and the answer is to use a set, not a tuple or a list. Neither linked question has any bearing on that subject. — Martijn Pieters
– Martijn Pieters, Commented Sep 13, 2012 at 15:28
I did some timings and found that testing for membership in a set literal is slower than testing for membership in a tuple literal. This is probably due to the time it takes to create the set. The longer the set literal was, the worse the performance became, even when the items tested were not members of the set. The takeaway is that if you are going to create a set to test for membership only once, you're better off with a tuple. If you need to test more than once, then the set is preferred. — Steven Rumbalski
– Steven Rumbalski, Commented Sep 13, 2012 at 16:06

Jon Clements · Accepted Answer · 2012-09-13 15:22:05Z

5

The initialisation of a tuple (at least in CPython) produces less bytecode than a list - but it's really nothing to worry about. I believe membership testing is pretty much the same (although not tested it).

For purely membership testing the lookup semantics are the same. From Python 2.7 onwards, it's much nicer to write (and adds an implication that it's membership testing only):

if x in {1, 2, 3}:
    pass # do something

While prior to that:

if x in set([1,2,3]):
    pass # do something

just looked a little kludgy...

answered Sep 13, 2012 at 15:22

Jon Clements

143k34 gold badges254 silver badges288 bronze badges

Sign up to request clarification or add additional context in comments.

6 Comments

redrah Over a year ago

Thanks for the explanation. To be honest I hadn't realised you could create a set using {} in 2.7, as you say the implication of membership testing with a set is far more satisfying!

Steven Rumbalski Over a year ago

I did some timings and found that testing for membership in a set literal is slower than testing for membership in a tuple literal. This is probably due to the time it takes to create the set. The longer the set literal was, the worse the performance became, even when the items tested were not members of the set. The takeaway is that if you are going to create a set to test for membership only once, you're better off with a tuple. If you need to test more than once, then the set is preferred.

senderle Over a year ago

@StevenRumbalski, glad you tested that -- I suspected as much. The key is that, as Jon Clements says above, "The initialisation of a tuple (at least in CPython) produces less bytecode than a list." It's not just that it takes longer to produce a tuple than a set; it's that the tuple is produced only once at compile time and then folded into the bytecode as a constant -- which is possible, of course, because tuples are immutable. However, there's yet another complication: if the tuple contains mutable values, constant folding doesn't happen.

redrah Over a year ago

@senderle +1 Very interesting, I'd wondered whether the immutability of tuples might have a bearing on performance. I'm guessing that the performance gains we're looking at here are fairly miniscule in most cases, but I do like to know what's going on behind the scenes!

Steven Rumbalski Over a year ago

@senderle: I just tested creating my set and tuple literals using variables instead of constants. The performance advantage of tuples decreased slightly, but did not go away. I could not test set with mutable values because they are not hashable.

|

ronak · Accepted Answer · 2012-09-13 15:18:27Z

0

The difference between list and tuple is that lists are mutable and tuples are immutable.

People tend to use tuples for heterogenous collections, similar to structs in C, and lists being for homogenous collections, similar to arrays in C.

answered Sep 13, 2012 at 15:18

ronak

1,7983 gold badges20 silver badges35 bronze badges

Collectives™ on Stack Overflow

Using lists and tuples in Python if statements

2 Answers 2

6 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

6 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related