For loop through a nested-list

Question

I noticed something very weird. I tried to look at where it was going wrong and realised that for some reason, the nest[0] list keeps changing midway through the loop.

>>> nest = [['prefix'],['line 1'],['line 2']]
>>> for part in nest[1:]:
...     list = nest[0]
...     list += part
...     print list

The output that I get is:

['prefix', 'line 1']
['prefix', 'line 1', 'line 2']

Whereas, what I need is:

['prefix', 'line 1']
['prefix', 'line 2']

Can somebody explain why this happens? I might be doing something very stupid.

Answer 1

EDIT: with explanation of pointers, as requested

Your problem is that when you assign list to nest[0], you are not creating a new list, your just assigning a pointer. Your pointer is directed to the list containing ['prefix']

On your first iteration, you add something to this list

On your second iteration, you don't make a NEW list, you just repoint to the old one.

Then when you append again, you're appending to the old list!

What you mean is:

nest = [['prefix'],['line 1'],['line 2']]
for part in nest[1:]:
    list = [] + nest[0]
    list += part
    print list

There's several ways to think about this. Here's one. Say you had a deck of cards object:

myobj = Deck(). 

If I then say,

myobj2 = myobj, 

I haven't created a new deck of cards, it would be like someone else looking at the deck I already have. We need to be able to do that to do a lot of programming (it's the fundamentals of object oriented design)! I would need to say

myobj3 = Deck() 

to construct a new deck of cards object.

Consider:

myobj.shuffle #we're shuffling one deck, that two people are looking at

Both myobj and myobj2 will change. Calling myobj3.shuffle leaves the other two untouched. What you've done told someone to re-look at the same deck, where you meant to make a new one!

Answer 2

list = nest[0] mean you assign pointer to nest[0] to variable name list

If you want your expected output, you need to create a new list to make sure it will not effect the original.

nest = [['prefix'],['line 1'],['line 2']]
for part in nest[1:]:
    list = nest[0] + part
    print list

nest[0] + part will create a new value and assign to list

Answer 3

this is just clarification in simple words :

the wrong

'list += part'

in

' for part in nest[1:]:
 list = nest[0]
 list += part
 print list'

the variable list will change its value in each iteration .. I mean the varible list will assigned to the following values :

list = ['prefix'] then after the first iteration it will reassigned to be :

list = ['prefix' , 'line 1'] ## note the loop excute list +=part , if you continue it will get all members of [nest]

so the solution focus on gitting seperate variable which keep its value constant in each iteration

'list = [] + nest[0]'