[Python-ideas] add a list.swap() method

Josiah Carlson josiah.carlson at gmail.com
Thu Jun 25 23:14:34 CEST 2009


I've done some pretty shady things with Python over the years, but
swapping the contents of two lists (or list subclasses) isn't one.
That this can be implemented in a properly formatted 2-line function
(modulo your function naming style):

def SwapLists(a, b):
    a[:], b[:] = b[:], a[:]

... necessarily requires me to invoke "not all x line functions should
be built-in (or methods)".

As for using list.sort(), list.reverse(), or even list.pop() as
examples of why list.swap() should exist, that doesn't fly.  Sorting,
reversing, and popping are *very* common operations in Python code.
Swapping list contents? ...not so common.

If you're really swapping contents that often, you may want to
consider a different design with a better algorithm (swap 2 pointers
rather than n+m), rather than speeding up the slow algorithm.

 - Josiah

2009/6/25 Kristján Valur Jónsson <kristjan at ccpgames.com>:
>> -----Original Message-----
>> From: python-ideas-bounces+kristjan=ccpgames.com at python.org
>> [mailto:python-ideas-bounces+kristjan=ccpgames.com at python.org] On
>> Behalf Of Terry Reedy
>> Sent: 25. júní 2009 17:58
>> To: python-ideas at python.org
>> Subject: Re: [Python-ideas] add a list.swap() method
>> Kristján Valur Jónsson wrote:
>> >
>> >> -----Original Message-----
>> >
>> > The idea is for example to speed up the initialization of lists (and
>> > subclasses of lists) from other lists when the source list's contents
>> > is to be discarded.
>> Which is why 'swap' is confusing.  You want to swap list-body pointers,
>> which is meaningless in Python itself. Nick's 'fromlist' is better. It
>> describes the effect without reference to the backstage magic.
> It is only one application, the other is a fast implementation of
> a[:], b[:] = b[:], a[:]
> There is precedence, I think.
> list.reverse() is semantically identical to
> list[:] = reversed(list)
> but it doesn't invoke any magic methods, and it relies on "backstage magic" to do its work in the most efficient way.
> Same goes for sort(), really.
> The difference with swap(), is that it will also magically affect an operand list.  But I think that both reverse() and sort() can be dangerous to list subclasses with class invariants.
> I think there are other cases.  list.pop() doesn't appear to invoke any magic methods either, and so must be a killer for list invariants.
> K
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