[EMAIL PROTECTED] wrote: > As a relative new comer to Python, I haven't done a heck of a lot of > hacking around with it. I had my first run in with Python's quirky (to > me at least) tendency to assign by reference rather than by value (I'm > coming from a VBA world so that's the terminology I'm using). I was > surprised that these two cases behave so differently > > test = [[1],[2]] > x = test[0] > x[0] = 5 > test > >>>> [[5],[2]] >>>> > x = 1 > test > >>>> [[5],[2]] >>>> > x > >>>> 1 >>>> > > Now I've done a little reading and I think I understand the problem... > My issue is, "What's the 'best practise' way of assigning just the > value of something to a new name?" > > i.e. > test = [[1,2],[3,4]] > I need to do some data manipulation with the first list in the above > list without changing <test> > obviously x = test[0] will not work as any changes i make will alter > the original... > I found that I could do this: > x = [] + test[0] > > that gets me a "pure" (i.e. unconnected to test[0] ) list but that > concerned me as a bit kludgy > > Thanks for you time and help. >
If you want a *copy* of an object (or portion thereof), you must explicitly *specify* a copy. THere are different ways of doing that depending on the object in question. Lists (and slices thereof) can be copied to a new list withthe slice syntax: L[1:4] will copy a limited portion, and L[:] will copy the whole list. The kind of copy is only one level deep -- the contents of the copy will be references to the contents of L Dictionaries have a copy method that creates a new dictionary. Again this copy is only one level deep. The copy modules provides a more general way to copy objects. It provides the ability to produce both shallow and deep copies. A small note: In a dozen years of programming in Python, I've used these various copy methods perhaps a dozen times or less. If you find you are copying structures often, you are probably not using Python as effectively as you could. Gary Herron -- http://mail.python.org/mailman/listinfo/python-list
