Pesky reverse()

[engsolnom]

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.

 

[Paul Rubin]

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()

 

[Sean Ross]

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

 

[Elaine Jackson]

list1=[1,2,3]

list2=list1[:] ## "cloning"
list1.reverse()==None 1
list1 [3, 2, 1]
list2

[1, 2, 3]


|
| 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.
|
|