On Monday, August 6, 2012 3:50:13 PM UTC-4, Mok-Kong Shen wrote:
> I ran the following code:
>
>
>
> def xx(nlist):
>
> print("begin: ",nlist)
>
> nlist+=[999]
>
> print("middle:",nlist)
>
> nlist=nlist[:-1]
>
> print("final: ",nlist)
>
>
>
> u=[1,2,3,4]
>
> print(u)
>
> xx(u)
>
> print(u)
>
>
>
> and obtained the following result:
>
>
>
> [1, 2, 3, 4]
>
> begin: [1, 2, 3, 4]
>
> middle: [1, 2, 3, 4, 999]
>
> final: [1, 2, 3, 4]
>
> [1, 2, 3, 4, 999]
>
>
>
> As beginner I couldn't understand why the last line wasn't [1, 2, 3, 4].
>
> Could someone kindly help?
>
>
>
> M. K. Shen
I've modified your code slightly so you can see what's happening with u in the
middle of function xx. Take a look:
u=[1,2,3,4]
def xx(nlist):
print("xx(u)\n")
print("At first, u and nlist refer to the same list")
print("nlist: %s u: %s\n" % (nlist, u))
nlist+=[999]
print("nlist+=[999]\n")
print("The list has been modified in place. u and nlist are still equal")
print("nlist: %s u: %s\n" %(nlist, u))
nlist=nlist[:-1]
print("nlist=nlist[:1]\n")
print("Now nlist refers to a new list object in memory that was created by")
print("taking a slice of u. u and nlist are no longer equal.")
print("nlist: %s u: %s" %(nlist, u))
xx(u)
Here's the output:
xx(u)
At first, u and nlist refer to the same list
nlist: [1, 2, 3, 4] u: [1, 2, 3, 4]
nlist+=[999]
The list has been modified in place. u and nlist are still equal
nlist: [1, 2, 3, 4, 999] u: [1, 2, 3, 4, 999]
nlist=nlist[:1]
Now nlist refers to a new list object in memory that was created by
taking a slice of u. u and nlist are no longer equal.
nlist: [1, 2, 3, 4] u: [1, 2, 3, 4, 999]
Thank you, Rob Day, for explaining a some of what's happening behind the scenes.
--
http://mail.python.org/mailman/listinfo/python-list
