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Pesky reverse()


I need to use the built-in method 'reverse', but notice strange behavior.
Foe example:

print my_list.reverse() doesn't work.

new_list = my_list.reverse() doesn't work.

my_list.reverse()
print my_list does work. (displays reversed list)

But I want both a 'forward' and 'reverse' list:

new_list = my+list # should save a 'forward' copy, right? Nope
my_list.reverse() actually reverses both copies, since Python is a bit too
helpful sometimes, and I understand why.

So the question is, how do I get a forward and reverse list?

Thanks......Norm.
Jul 18 '05 #1
3 2043
en*******@ipns.com writes:
print my_list.reverse() doesn't work.
The reverse() method reverses the list in place and returns None.
But I want both a 'forward' and 'reverse' list:

new_list = my_list # should save a 'forward' copy, right? Nope
No, both new_list and my_list are bound to the same list, like in C
you might have two pointers to the same structure. To make a copy, use

new_list = my_list[:]
my_list.reverse() actually reverses both copies, since Python is a bit too
helpful sometimes, and I understand why.

So the question is, how do I get a forward and reverse list?


new_list = my_list[:]
new_list.reverse()
Jul 18 '05 #2
<en*******@ipns.com> wrote in message
news:3t********************************@4ax.com...
I need to use the built-in method 'reverse', but notice strange behavior.
[snip]
new_list = my+list # should save a 'forward' copy, right? Nope
my_list.reverse() actually reverses both copies, since Python is a bit too
helpful sometimes, and I understand why.

So the question is, how do I get a forward and reverse list?

forward = range(10)
reverse = forward[:] # copy
reverse.reverse()
print forward [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] print reverse [9, 8, 7, 6, 5, 4, 3, 2, 1, 0]


HTH
Sean
Jul 18 '05 #3
>>> list1=[1,2,3]
list2=list1[:] ## "cloning"
list1.reverse()==None 1 list1 [3, 2, 1] list2

[1, 2, 3]
<en*******@ipns.com> wrote in message
news:3t********************************@4ax.com...
|
| I need to use the built-in method 'reverse', but notice strange behavior.
| Foe example:
|
| print my_list.reverse() doesn't work.
|
| new_list = my_list.reverse() doesn't work.
|
| my_list.reverse()
| print my_list does work. (displays reversed list)
|
| But I want both a 'forward' and 'reverse' list:
|
| new_list = my+list # should save a 'forward' copy, right? Nope
| my_list.reverse() actually reverses both copies, since Python is a bit too
| helpful sometimes, and I understand why.
|
| So the question is, how do I get a forward and reverse list?
|
| Thanks......Norm.
|
|
Jul 18 '05 #4

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