-1 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@ccpgames.com>:
-----Original Message----- From: python-ideas-bounces+kristjan=ccpgames.com@python.org [mailto:python-ideas-bounces+kristjan=ccpgames.com@python.org] On Behalf Of Terry Reedy Sent: 25. júní 2009 17:58 To: python-ideas@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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