In the 5*[[]] example, the issue is again mutable objects. (list * int) does a shallow copy. This is the only thing that makes sense. There is no universal clone operation that you could use instead of the shallow copy. Issues with mutable objects are not going to go away just because we wish they would. I like to be able to write an initialization with immutable objects, like 5 *[None], and would not want to give that up because of the gotcha's with mutable objects.
What one probably intends by 5 * [ [ ] ] is [ [ ] for i in range(5)] and this only works because [ ] is a literal, with a new version created each time through the list comprehension. Andy On Fri, Apr 23, 2010 at 4:41 PM, David MacQuigg <macqu...@ece.arizona.edu>wrote: > Mark Engelberg wrote: > >> That reminds me of a similar gotcha I've unfortunately run into on >> more than one occasion. >> >> # intialize a to be a list of 5 empty lists >> a = 5*[[]] >> # push a value onto the first list. >> a[0].append(1) >> >> What's a? >> >> > > The result actually makes sense, although I did guess it wrong. :>( > > >>> a = b = [1,2,3] > >>> c = [a,b] # a list with two references to the same object > > >>> id(c[0]) > 3626848 > >>> id(c[1]) > 3626848 > > >>> c[0][0] = 5 > >>> a > [5, 2, 3] > > What is b? > > Would you rather have Python do something different? > > > When taking shortcuts like a = 5*[[]], never trust, always verify. > > -- Dave > > > _______________________________________________ > Edu-sig mailing list > Edu-sig@python.org > http://mail.python.org/mailman/listinfo/edu-sig > -- Andrew N. Harrington Director of Academic Programs Computer Science Department Loyola University Chicago 512B Lewis Towers (office) Snail mail to Lewis Towers 416 820 North Michigan Avenue Chicago, Illinois 60611 http://www.cs.luc.edu/~anh Phone: 312-915-7982 Fax: 312-915-7998 g...@cs.luc.edu for graduate administration u...@cs.luc.edu for undergrad administration ahar...@luc.edu as professor
_______________________________________________ Edu-sig mailing list Edu-sig@python.org http://mail.python.org/mailman/listinfo/edu-sig
