[Python-ideas] Star assignment in iterator way?

Steven D'Aprano steve at pearwood.info
Tue Nov 21 05:53:38 EST 2017

On Tue, Nov 21, 2017 at 12:27:32PM +0300, Kirill Balunov wrote:

> Backward compatibility is an important issue, but at the same time it is
> the main brake on progress.

"Progress just means bad things happen faster."
-- Terry Pratchett, "Witches Abroad"

> And how do you look at something like this (deferred star evaluation)?:
> a, ?*b, c, d = something_iterable

A waste of effort?

How do you defer evaluating the second and subsequent items if you 
evaluate the final two? Given:

def gen():
    yield 999
    for i in range(100):
        yield random.random()
    yield 999
    yield 999


a, ?*b, c, d = gen()

has to evaluate all 100 random numbers in order to assign a, c, d all 
equal to 999. Making b an iterator instead of a list doesn't actually 
avoid evaluating anything, and it will still require as much storage as 
a list. The most likely implementation would:

- store the evaluated items in a list;
- assign iter(the list) as b.

I suppose that there could be some way of delaying the calls to 
random.random() by returning a thunk, but that is likely to be more 
expensive in both memory and time than a simple list of floats.


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