On Thu, Oct 18, 2007 at 11:57:10AM -0000, Paul Hankin wrote regarding Re: Appending a list's elements to another list using a list comprehension: > > Not to me: I can never remember which of a.append and a.extend is > which. Falling back to a = a + b is exactly what you want. For > instance: > > a = (1, 2, 3) > a += (4, 5, 6) > > works, whereas: > > a = (1, 2, 3) > a.extend((4, 5, 6)) > > doesn't. So using += makes your code more general. There's no standard > sequence type that has extend and not +=, so worrying that += is > slower isn't a concern unless you're using a user-defined class. Even > then, it's probably a mistake that should be fixed in the class rather > than requiring .extend() to be used instead of +=. >
I was going to argue that in fact += is not more general, it just covers a different set of use cases, but then I tested my hypothesis... >>> a = [1,2,3] >>> b = a >>> c = [4,5,6] >>> d = c >>> e = [7,8,9] >>> a.extend(e) >>> b [1, 2, 3, 7, 8, 9] >>> c += e >>> d # I expected [4, 5, 6] [4, 5, 6, 7, 8, 9] >>> c = c + e # But += doesn't do the same as this >>> c [4, 5, 6, 7, 8, 9, 7, 8, 9] >>> d [4, 5, 6, 7, 8, 9] So I learned something new. Cheers, Cliff -- http://mail.python.org/mailman/listinfo/python-list
