On Wed, 27 Feb 2008, Keith Suda-Cederquist wrote: > Hi, > > I'm using iPython and I've run into an occasional problem that I don't > understand. Here is what I'm seeing: > > >>aa=range(0,10) > >>bb=aa > >>print aa > [0,1,2,3,4,5,6,7,8,9] > >>print bb > [0,1,2,3,4,5,6,7,8,9] > >> # okay, everything allright at this point > >>bb[5]=0 #change bb > >>print aa > [0,1,2,3,4,0,6,7,8,9] #aa has changed!!! > >>print bb > [0,1,2,3,4,0,6,7,8,9] > > So the problem is that when I change bb, aa also changes even though I > don't want it to. Is this supposed to happen? If it is can someone > explain to me why this is a good thing? and finally, can someone give > me some advice on how to avoid or work-around this problem.
In addition to Luke's comments, let me try it from a slightly different angle: >>> aa=range(0,10) >>> bb=aa >>> id(aa) 12169048 >>> id(bb) 12169048 The id() function provides the unique ID number of an object. Note that aa and bb have the same ID; they are just two names for the same object. Once you realize that, it will be more clear that a change to aa is also a change to bb. In fact, here's something that really drives home they're the same object: >>> aa is bb True Instead of using the assignment statement, which just puts a new label on an object, you can subject your list to an operation that produces another list that is identical to the list it operates on. One example: >>> cc=list(aa) list() takes something as input and produces list from it. if you hand a list to list() you get a new list (i.e., a new object) that is identical to the list that was input. >>> id(cc) # note: new object id 12185688 >>> aa==cc # note: aa is equal to cc True >>> aa is cc # but they're not the same object False Another approach is to take a list "slice" that just happens to slice out the entire list: >>> dd=aa[:] >>> id(dd) # note: new object id 12168408 >>> aa==dd # note: aa is equal to dd True >>> aa is dd # but they're not the same object False _______________________________________________ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
