CODEWITHSUNDEEP
pythonlistappend
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Reputation: 73

The append() method issue of list in Python3

for example,i got a list:

mylist = [1,2,3]

we all know append() can add a new item at the end of the list like that:

mylist.append(4)

now the mylist is [1,2,3,4]

my issue is what happened when mylist append itself????

mylist.append(mylist)

at first i think it will look like this:

[1,2,3,4,[1,2,3,4]]

but when i print it, output is [1,2,3,4,[...]],so i print the mylist[5] and it's same to the mylist:[1,2,3,4,[...]]

so you can loop the last item of list endless,and the last item always be the same to original mylist!!!!!

anyone can tell me why is that????????

Upvotes: 0

Views: 699

Answers (2)

Cory Madden
Cory Madden

Reputation: 5193

It's doing that because you're telling Python to append a list(my_list) to a list. If you want to extend the list with the contents of the list, you can do this:

new_list = old_list + new_list
output: [1, 2, 3, 4, 1, 2, 3, 4]

or you can use the extend method.

old_list.extend(my_list)

Which will modify the old_list in place.

Upvotes: 0

Robᵩ
Robᵩ

Reputation: 168616

Because you aren't appending a copy of the list, but the actual list object itself. Consider this program:

mylist = [1,2,3,4]
mylist.append(mylist)
assert id(mylist) == id(mylist[4])

The final item of the list is a reference to the list itself, so you have a fully recursive, self-referential data structure.

If you want the result [1,2,3,4,[1,2,3,4]], then you need to append a copy of the original list, like so:

mylist = [1,2,3,4]
mylist.append(list(mylist))
assert mylist == [1,2,3,4,[1,2,3,4]]

Upvotes: 1

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