Michael Broe wrote: >I think I understand this sorting-a-list 'in place' stuff, and things >of that kind (reversing for example); but I am finding it very >difficult to get used to, since sorting a list doesn't return the >sorted list as a value, but simply does the work as a side effect. > >The place where it really trips me up is when I want to do something >like pass a sorted list as a value in a program, say. > >Here is a trivial example. Say I want to define a function to always >print lists in sorted form. I can't do it as follows, tho it's what I >want to do intuitively: > >def sort_print(L): > print L.sort() > >This is on a strict analogy to writing a function that prints strings >in upper case only: > >def upper_print(s): > print s.upper() > >(The fact that the second works and the first doesn't really does bug >me as a newbie.) > >Anyway, first question: is the fact that the first doesn't work >purely a function of the fact that lists are mutable? (At least I >could kind of understand that, that methods work differently for >objects of different types). Or is it possible to have methods >like .upper() that return the result of performing the operation even >for lists? > >Second question. Do I really have to write the sort_print() function >like this: > >def sort_print(L): > L.sort() > print L > >i.e. first perform the operation in-place, then pass the variable? Is >this the idiomatic way of doing it? > >Third question: is this in-place behavior of methods in effect >restricted to lists, or are there other places I should be on the >lookout for it? > >Cheers, >Mike > > >_______________________________________________ >Tutor maillist - Tutor@python.org >http://mail.python.org/mailman/listinfo/tutor > > > Well, to answer the specific case of l.sort(), there's a built-in that returns a sorted list - sorted(l). So to do what you want, you'd need:
>>> import random # as you can see, not all operations have >>> examplelist = random.sample(range(10), 10) # non-in-place counterparts, or i would have >>> print sorted(examplelist) # used random.shuffle(range(10)) [1,2,3,4,5,6,7,8,9] Except for a few specific cases, to use a non-in-place version of an in-place operation, you'll probably have to end up writing your own, similar to the way you currently deal with l.sort(). As for why this is, I think you've hit on the answer: the default, if you will, is to perform operations in-place, to avoid the necessity of reassigning a list to it's sorted version, to give one example. However, this only works on mutable objects - you can't modify a string, because, well, strs are immutable, so performing an operation in-place is nonsense. That's why s.upper() returns a value, as opposed to modifying s. To answer your second question, in general, yes. If there isn't already a way to do it in the library, you'll have to write your own function. Btw, another in-place list operation with a non-in-place counterpart is l.reverse(), which has a few: reversed(l) and l[::-1] come to mind. HTH, Orri -- Email: singingxduck AT gmail DOT com AIM: singingxduck Programming Python for the fun of it. _______________________________________________ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
