[Python-Dev] syntactic support for sets (original) (raw)

Josiah Carlson jcarlson at uci.edu
Mon Feb 6 19:39:38 CET 2006

Donovan Baarda <abo at minkirri.apana.org.au> wrote:

On Fri, 2006-02-03 at 11:56 -0800, Josiah Carlson wrote: > Along the lines of "not every x line function should be a builtin", "not > every builtin should have syntax". I think that sets have particular > uses, but I don't believe those uses are sufficiently varied enough to > warrant the creation of a syntax. I suggest that people take a walk > through their code. How often do you use other sequence and/or mapping > types? How many lists, tuples and dicts are there? How many sets? Ok, > now how many set literals? The absence of sets in early Python, the requirement to "import sets" when they first appeared, and the lack of a set syntax now all mean that people tend to avoid using sets and resort to lists, tuples, and "dicts of None" instead, even though they really want a set. Anywhere you see "if value in sequence:", they probably mean sequence is a set, and this code would run much faster if it really was, and might even avoid potential bugs because it would prevent duplicates...

Maybe they mean set, maybe they don't. 'if obj in seq' is used for various reasons.

A quick check of the Python standard library shows that some of the uses of 'if obj in tuple_literal' could certainly be converted into sets, but that ignores the performance impact of using sets instead of short tuples (where short, if I remember correctly, is a length of 3, check the python-dev archives), as well as the module-level contant creation that occurs with tuples. There was probably a good reason why such a thing hasn't happened with lists and dicts (according to my Python 2.4 installation), and why it may not happen with sets.

A nontrivial number of other 'if obj in seq' instances actually need dictionaries, the test is for some sort of data handler or headers with a particular name.

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