[Python-ideas] random.choice on non-sequence

Guido van Rossum guido at python.org
Tue Apr 12 19:04:11 EDT 2016

This still seems wrong *unless* we add a protocol to select a random
item in O(1) time. There currently isn't one for sets and mappings --
only for sequences. It would be pretty terrible if someone wrote code
to get N items from a set of size N and their code ended up being

On Tue, Apr 12, 2016 at 3:57 PM, Ben Finney <ben+python at benfinney.id.au> wrote:
> Rob Cliffe <rob.cliffe at btinternet.com> writes:
>> It surprised me a bit the first time I realised that random.choice did
>> not work on a set. (One expects "everything" to "just work" in Python!
>> :-) )
> I am +1 on the notion that an instance of ‘tuple’, ‘list’, ‘set’, and
> even ‘dict’, should each be accepted as input for ‘random.choice’.
>> On 2011-06-23, Sven Marnach posted 'A few suggestions for the random
>> module' on Python-Ideas, one of which was to allow random.choice to
>> work on an arbitrary iterable.
> I am −1 on the notion of an arbitrary iterable as ‘random.choice’ input.
> Arbitrary iterables may be consumed merely by being iterated. This will
> force the caller to make a copy, which may as well be a normal sequence
> type, defeating the purpose of accepting that “arbitrary iterable” type.
> Arbitrary iterables may never finish iterating. This means the call
> to ‘random.choice’ would sometimes never return.
> The ‘random.choice’ function should IMO not be responsible for dealing
> with those cases.
> Instead, a more moderate proposal would be to have ‘random.choice’
> accept an arbitrary container.
> If the object implements the container protocol (by which I think I mean
> that it conforms to ‘collections.abc.Container’), it is safe to iterate
> and to treat its items as a collection from which to choose a random
> item.
> Rob, does that proposal satisfy the requirements that motivated this?
> --
>  \        “Some people, when confronted with a problem, think ‘I know, |
>   `\       I'll use regular expressions’. Now they have two problems.” |
> _o__)                           —Jamie Zawinski, in alt.religion.emacs |
> Ben Finney
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--Guido van Rossum (python.org/~guido)

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