Strange behavior with sort()
Hello box is a list of 3 integer items If I write: box.sort() if box == [1, 2, 3]: the program works as expected. But if I write: if box.sort() == [1, 2, 3]: it doesn't work, the test always fails. Why ? Thx -- https://mail.python.org/mailman/listinfo/python-list
Re: Strange behavior with sort()
ast nom...@invalid.com wrote in message news:530eda1d$0$2061$426a7...@news.free.fr... Hello box is a list of 3 integer items If I write: box.sort() if box == [1, 2, 3]: the program works as expected. But if I write: if box.sort() == [1, 2, 3]: it doesn't work, the test always fails. Why ? Try the following in the interpreter - box = [3, 2, 1] box.sort() box [1, 2, 3] box = [3, 2, 1] print(box.sort()) None box [1, 2, 3] box.sort() sorts box 'in situ', but does not return anything. That is why the second example prints None. In your second example, you are comparing the return value of box.sort() with [1, 2, 3]. As the return value is None, they are unequal. HTH Frank Millman -- https://mail.python.org/mailman/listinfo/python-list
Re: Strange behavior with sort()
On Thu, Feb 27, 2014 at 07:24:24AM +0100, ast wrote: Hello box is a list of 3 integer items If I write: box.sort() if box == [1, 2, 3]: the program works as expected. But if I write: if box.sort() == [1, 2, 3]: it doesn't work, the test always fails. Why ? Thx -- https://mail.python.org/mailman/listinfo/python-list Because when you call the .sort() method on a list, it does the sort in-place, instead of returning a sorted copy of the list. Check this: [2,1,3].sort() The method does not return a value, that's why the direct comparison fails. What you might want is to use the sorted() method on the list, like this: sorted([2,1,3]) [1, 2, 3] sorted([2,1,3]) == [1,2,3] True -- Eduardo Alan Bustamante López -- https://mail.python.org/mailman/listinfo/python-list
Re: Strange behavior with sort()
ast nom...@invalid.com writes: If I write: box.sort() if box == [1, 2, 3]: the program works as expected. But if I write: if box.sort() == [1, 2, 3]: it doesn't work, the test always fails. Why ? Because very often methods **dont't** return the object they are applied (self that is). This works though: box = [1,3,2] sorted(box) == [1,2,3] True hth, ciao, lele. -- nickname: Lele Gaifax | Quando vivrò di quello che ho pensato ieri real: Emanuele Gaifas | comincerò ad aver paura di chi mi copia. l...@metapensiero.it | -- Fortunato Depero, 1929. -- https://mail.python.org/mailman/listinfo/python-list
Re: Strange behavior with sort()
ast nom...@invalid.com: if I write: if box.sort() == [1, 2, 3]: it doesn't work, the test always fails. Why ? The list.sort() method returns None. The builtin sorted() function returns a list: if sorted(box) == [1, 2, 3]: would work. Note that the list.sort() method is often preferred because it sorts the list in place while the sorted() function must generate a fresh, sorted list. Marko -- https://mail.python.org/mailman/listinfo/python-list
Re: Strange behavior with sort()
Thanks for the very clear explanation -- https://mail.python.org/mailman/listinfo/python-list
Re: Strange behavior with sort()
On 02/26/2014 10:24 PM, ast wrote: Hello box is a list of 3 integer items If I write: box.sort() if box == [1, 2, 3]: the program works as expected. But if I write: if box.sort() == [1, 2, 3]: Most such questions can be answered by printing out the values in question and observing first hand what the value is. So, print out box.sort() to see what it is. You might be surprised. Hint: box.sort() does indeed cause box to be sorted, and the sorted list is left in box, but the sorted list is not returned as a function value. it doesn't work, the test always fails. Why ? Thx -- https://mail.python.org/mailman/listinfo/python-list
Re: Strange behavior with sort()
On 02/26/2014 10:24 PM, ast wrote: Hello box is a list of 3 integer items If I write: box.sort() if box == [1, 2, 3]: the program works as expected. But if I write: if box.sort() == [1, 2, 3]: it doesn't work, the test always fails. Why ? Thx sort() sorts the sequence in place, but it _returns_ None. Your second example becomes the equivalent of: box.sort() if None == [1, 2, 3]: So although your box does become sorted, it is NOT what is compared in your if statement. BTW, the sorted() function won't work here either. It will return the sorted sequence, but it leaves the original unchanged. -=- Larry -=- -- https://mail.python.org/mailman/listinfo/python-list
