for loops and iterating through lists

What's happening here is a list is mutated during looping.

Let's consider following code snippet:

a = [0, 1, 2, 3]
for a[-1] in a:
    print a

Output is:

[0, 1, 2, 0]
[0, 1, 2, 1]
[0, 1, 2, 2]
[0, 1, 2, 2]

Each iteration:

• reads value from position currently pointed by internal pointer
• immediately assigns it to last element in list
• after that last element is printed on standard output

So it goes like:

• internal pointer points to first element, it's 0, and last element is overwritten with that value; list is [0, 1, 2, 0]; printed value is 0
• internal pointer points to second element, it's 1, and last element is overwritten with that value; list is [0, 1, 2, 1]; printed value is 1
• (...)
• at last step, internal pointer points to last element; last element is overwritten by itself - list does not change on last iteration; printed element also does not change.

While doing for a[-1] in a, you actually iterate through the list and temporary store the value of the current element into a[-1].

You can see the loop like these instructions:

a[-1] = a[0] # a = [0, 1, 2, 0]
print(a[-1]) # 0
a[-1] = a[1] # a = [0, 1, 2, 1]
print(a[-1]) # 1
a[-1] = a[2] # a = [0, 1, 2, 2]
print(a[-1]) # 2
a[-1] = a[3] # a = [0, 1, 2, 2]
print(a[-1]) # 2

So, when you are on the third element, then 2 is stored to a[-1] (which value is 1, but was 0 before and 3 on start).

Finally, when it comes to the last element (and the end of the iteration), the last value stored into a[-1] is 2 which explains why it is printed twice.