Can someone please explain why the output of this code is what it is?!

Question

nums = [1, 2]
nums.append(nums)

print(nums)
print(nums[2])
print(nums[2][2])
print(nums[2][2][2])

print(nums==nums[2])

Answer 1

After the call "nums.append(nums)", nums will become a list with the third item being a reference to a list (in this case a reference to itself).
Therefore, nums will look like [1,2,<ref_to_nums>].

When you go to nums[2], you will just get a reference to the nums list.

And, every print you listed (except the last comparison), will just print the nums list itsef, because index 2 of the nums list is the nums list.

The last print will be True, because, again, nums[2] is just a reference to nums.

...

Just remember the third item is just a reference to the list.
Hope that's somewhat clear.

Answer 2

Complementary:

To avoid copying an array by reference and perform "deep" copy, use the slice notation of Python...
In the case of the example, the behavior change if you write:

nums.append( nums[ : ] )

... as now, new element inserted in nums is a real copy by value, not just by reference ;)