Select an item from a set in Python

Question

I am looking to select one item from a set. It does not matter which item it is, but I want it to be the same item every time the function is called.

For example, if I had the set:

my_set = set(["ABC","ABC inc", "ABC ltd"])

Then I want to return any one of those, but the same one whenever the function is run. [i.e. so I would always get "ABC", or always get "ABC inc" whenever I ran with the above set.] If I was using lists, I would just take the first item, but sets don't have a first per se.

This is in contrast to selecting randomly [e.g. How do I pick 2 random items from a Python set? ] where it will change every time it is run.

Solution 1

What about converting to list and sorting?

my_list = list(my_set)
my_list.sort()
chosen_element = my_list[0]

Solution 2

you could use a function with memoization

def get_random(my_set,memo={}):
    if id(my_set) not in memo:
       memo[id(my_set)] = random.choice(list(my_set))
    return memo[id(my_set)]

a_set = set([1,2,3,4,5])
print get_random(a_set)
print get_random(a_set)

this would always give you the same value as long as you passed in a_set ... (a different set would give a different answer)

if you wanted to make sure the item was still in the set you could change the memo if check

def get_random(my_set,memo={}):
    if id(my_set) not in memo or memo[id(my_set)] not in my_set:
       memo[id(my_set)] = random.choice(list(my_set))
    return memo[id(my_set)]

Comments

[Alan]

Dangerous in so many ways. E.g., remove the memoized item from the set, and it will still show up as the answer. (You'll also need to make a list of my_set before applying random.choice.)

[Joran Beasley]

meh its what OP asked for ... you could always do something like or memo[id(my_set)] not in my_set

[Alan]

Never count on a set iterating through its elements in a certain order.

[hackbot89]

I think you should be good as long as you are using pop to pull the elements out, because after a pop the order still remains the same. Do you have an example that explains why I should "Never count ... " ? : )

[Alex Huszagh]

You can combine this into a single step: sorted(my_set) returns a list in both Python2 and Python3.

[kyrenia]

... I very quickly ran into issues using this [e.g. when re-initialized the set with new value, the prior result was returned]

[Joran Beasley]

if the memory location is the same then its the same set and it returns to you the first value it returned ... ... which is what was requested ... anyway I altered it to check for the items existance still in the set ...

[Alex Huszagh]

If memory is also an issue and you don't want to duplicate the set in memory, you could grab the first value from an iterator rather than random.choice(list(my_set)), which is an advantage of this approach rather than generating a list.

[Alan]

Sort of ... stackoverflow.com/questions/3848091/…