The 'expand this here' * operator in python

September 29th, 2009
programming, python, tech
The python docs (2.6.2) say this about the use of *:

If the syntax *expression appears in the function call, expression must evaluate to a sequence. Elements from this sequence are treated as if they were additional positional arguments; if there are positional arguments x1,..., xN, and expression evaluates to a sequence y1, ..., yM, this is equivalent to a call with M+N positional arguments x1, ..., xN, y1, ..., yM


If the * is present [in the function definition argument list], it is initialized to a tuple receiving any excess positional parameters, defaulting to the empty tuple
This is very handy, but is also limited. In python 3.0, PEP 3132 added some related syntax:
first, *rest = seq
a, *b, c = range(5)
This is again, handy but limited. This star syntax only works in these two cases: functions (star-args) and assignment targets. There are logical extensions that make a lot of sense to me, though. If one can do:
first, *rest = seq
then I want to be able to do:
seq = first, *rest
While the following is ambiguous and so clearly needs to be illegal:
first, *some, *rest = seq
this is not ambiguous:
seq = first, *some, *rest
While we're at it, lets make the following legal:
foo(first, *some, *rest)
All these changes together would make for a very consistent and powerful interpretation of * as 'expand this sequence here'.

Update 2013-04-05: Danner points out a 2008 patch. This was put on hold because of the moratotium (to "allow non-CPython implementations to 'catch up'") but with 3.3 out I think the moratorium is over. I just poked the bug.

Recent posts on blogs I like:

A Big Problem With The Going To Bed Book

One day my dad was reading this book called the "Going to Bed Book" to my sister Nora. The book is basically about a bunch of animals who are getting ready for bed on a boat. They go down the stairs, take a bath, hang their towels on the wall, find…

via Lily Wise's Blog Posts September 18, 2023

Investing in boundaries with young kids

Putting in some work to get the behavior you want The post Investing in boundaries with young kids appeared first on Otherwise.

via Otherwise August 15, 2023

Self-driving car bets

This month I lost a bunch of bets. Back in early 2016 I bet at even odds that self-driving ride sharing would be available in 10 US cities by July 2023. Then I made similar bets a dozen times because everyone disagreed with me. The first deployment to pot…

via The sideways view July 29, 2023

more     (via openring)