[Python-ideas] get method for sets?

[Steven D'Aprano]

On Wed, May 16, 2012 at 02:32:15AM -0400, Mike Meyer wrote:
> Is there some reason that there isn't a straightforward way to get an
> element from a set without removing it? Everything I find either
> requires multiple statements or converting the set to another data
> type.
> 
> It seems that some kind of get method would be useful. The argument
> that "getting an arbitrary element from a set isn't useful" is refuted
> by 1) the existence of the pop method, which does just that,

pop returns an arbitrary element, and removes it. That's a very 
different operation to "get this element from the set".

The problem is, if there was a set.get(x) method, you have to pass x as 
argument, and it returns, what? x. So what's the point? You already have 
the return value before you call the function.

def get(s, x):
    """Return element x from set s."""
    if x in s: return x
    raise KeyError('not found')


As I see it, this is only remotely useful if:

- you care about identity, e.g. caching/interning

- you care about types, e.g. get(s, 42) may return 42.0 as the element 
of the set instead.

In either case, a dict is the more obvious data structure to use.

def intern(d, x):
    """Intern element x in dict d, and return the interned version."""
    return d.setdefault(x, x)
    

But with sets? Seems pretty pointless to me. I can't help but feel that 
set.get() is a poorly thought out operation, much requested but rarely 
useful.


> and 2)
> the fact that I (and a number of other people) have run into such a
> need.
> 
> My search for such a reason kept finding people asking how
> to get an element instead. Of course, my key words (set and get) are
> heavily overloaded.

What's your use-case?



-- 
Steven