Skipping every other element after the first

Slice notation a[start_index:end_index:step]

return a[::2]

where start_index defaults to 0 and end_index defaults to the len(a).

def altElement(a):
    return a[::2]

Alternatively, you could do:

for i in range(0, len(a), 2):
    #do something

The extended slice notation is much more concise, though.