Converting a one-item list to an integer

Use Slice Notation:

def addFirstAndLast(x):  
    return x[0] + x[-1]

x[0] = will give you 0^th index of the list, first value.

x[-1] = will give you the last element of the list.

It all boils down to this:

def addFirstAndLast(x): 
    return x[0] + x[-1]

In Python, a negative list index means: start indexing from the right of the list in direction to the left, where the first position from right-to-left is -1, the second position is -2 and the last position is -len(lst).

Use indexes

You're slicing the list, which return lists. Here, you should use indexes instead:

firstDigit = x[0]
lastDigit = x[-1]

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Why is slicing wrong for you:

When you do x[0:1], you're taking the list of items from the beginning of the list to the first interval.

 item0, item1, item2, item3
^ interval 0
        ^ interval 1
              ^ interval 2 
                     ^ interval 3    

Doing x[0:2], for example, would return items 0 and 1.