[Python-Dev] Re: syntactic shortcut - unpack to variably sizedlist (original) (raw)

Nick Coghlan ncoghlan at iinet.net.au
Mon Nov 22 11:36:11 CET 2004

Raymond Hettinger wrote:

Also, if the goal is just to get iunpack(), the good news is that PEP is not required. Only the a,b,*c syntax would need a PEP. The bad news is that I have the final say over what goes into itertools (I've rejected eight proposals so far, including one from Guido).

Actually, the goal (or my goal, anyway) was to avoid having the "a, b, *c = seq" idea proposed again in a few months time. Google searches of the py-dev archive can work sometimes, but a PEP is generally more visible.

However, if you follow through the rambling below, you'll find it doesn't leave much for a PEP to say. . . so, unless Carlos or anyone else wants to run with it, consider my PEPP from this thread to be dead :)

Cheers, Nick.

The basic idea of the "a, b, *c = seq" syntax is to unpack the first few items of a sequence into specific variables, and then drop the rest of the sequence into another variable (sometimes the original variable).

To me, the presented use cases are nowhere near significant enough to justify special syntax. The other argument in favour (symmetry with extended function call syntax) isn't compelling either, since function calls and assignment statements are, well, different.

However, while I didn't find the use cases presented enough to justify a syntax change, I wondered if they pointed towards some potentially missing features. The current PEP draft grew out of the question "Is there a way to provide the functionality without altering Python's syntax?".

For objects that support slicing, the following works now:

a, b, the_rest = seq[:2] + [seq[2:]]

For iterators, roughly equivalent spelling is:

a, b, the_rest = [islice(itr, 2)] + [itr]

Carlos's suggestion gives a common way to spell it for any iterable:

a, b, the_rest = iunpack(iterable, 2)

Alternatively, if we stick with different spellings for mutable sequences and iterators, we can get to:

a, b = seq.pop(slice(2)) a, b = islice(itr, 2)

In both of these cases, the original object (seq, itr) ends up containing "the rest". Neither approach works for an immutable sequence, though - for those, it is still necessary to use the explicit slicing spelling above (or create an iterator and use the islice approach).

Anyway, if you're firmly opposed to itertools.iunpack, there isn't much point in pursuing it (since, in the end, your opinion is the one that really counts). Carlos may choose to post it as a recipe over at ASPN.

That still leaves adding slice handling to list.pop and array.pop. If nobody else steps forward to do it, I expect I'll eventually get around to coming up with a patch for that.

-- Nick Coghlan | Brisbane, Australia Email: ncoghlan at email.com | Mobile: +61 409 573 268

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