Python Lists Cyclic References by YOKESH

Some days back, I was just digging deep into python lists and i found some thing really interesting and confusing at the same time. ”I will make you confusing too” . Chalooo, Let’s start !….

Before going into weird lists, let’s have some basics on ‘=’ operator that works as a alias operator on lists and on more data types. If i say list1 = list2, both lists are pointing to same memory address of the mutable list object. GOT IT ? …. Nooo ? Look Below !

Why this happens ? Because my_list2 is just an aliased name to my_list1.Changes made to my_list2 will be reflected in my_list1 . So, Be careful while playing with mutable data types in python .

Let’s dig into weird lists behavior,

Guess the output of the above code !

The outputs are not [1,2,3,4,5,[1,2,3,4,5]] and

[1,2,3,4,5,[1,2,3,4,5]] …..feeling weird right ? The correct outputs are

[1, 2, 3, 4, 5, […]] and [1, 2, 3, 4, 5, […]]. Let’s see how this output came..

Initially, both lists are pointing to same object list with items 1,2,3,4,5

NOTE : In the above image , I wrote wierd instead of weird (typo mistake)

After Appending , weird_list2 looks like this,

NOTE : In the above image , I wrote wierd instead of weird (typo mistake)

That means, Weird_list1 is storing in the weird_list2 as a 6th element in the index 5. That means, element at index 5 of weird_list2 is storing whole weird_list1 as it’s 6th element. But, both lists weird_list1 and wierd_list2 are aliases to each other. So, element 6 of weird_list2 is literally equal to the weird_list2 due to aliasing .Changes made to element 6 of the weird_list2 will reflect in the whole weird_list2 .Here, Cyclic Reference happening between the element of a list and the list itself. python handles these kind of cyclic references creating elements using Ellipsis represented by …

python replaces the element that is creating cyclic reference with the list itself with …

So, the output while printing weird_list1 or 2gives [1,2,3,4,5,[…]],meaning that there is a element / item in the list that is creating cyclic reference with the list itself.

Guess what this prints ,

As we know that weird_list1 and weird_list2 has a 5th element that has a cyclic reference, so 1st ,2nd and 3rd print statements prints [1,2,3,4,5,[…]]. the 4th and 5th print statements also prints the same [1,2,3,4,5,[…]] .

How weird_list1[5][5] gives [1,2,3,4,5,[…]]

weird_list2[5] has cyclic reference to wierd_list2 , and weird_list2[5][5] has cyclic reference to weird_list2[5] ……and so on ..for weird_list2[5][5][5]…..

Even If you print weird_list2[5][5][5] , it gives you [1,2,3,4,5,[…]]

This is the process python handles cyclic references.

Please hit the like and follow button if you think I gave some knowledge on python.

Thank YOU