I find it easier to remember how it works, and then I can figure out any specific start/stop/step combination.
It's instructive to understand range() first:
def range(start=0, stop, step=1): # Illegal syntax, but that's the effect
i = start
while (i < stop if step > 0 else i > stop):
yield i
i += step
Begin from start, increment by step, do not reach stop. Very simple.
The thing to remember about negative step is that stop is always the excluded end, whether it's higher or lower. If you want same slice in opposite order, it's much cleaner to do the reversal separately: e.g. 'abcde'[1:-2][::-1] slices off one char from left, two from right, then reverses. (See also reversed().)
Sequence slicing is same, except it first normalizes negative indexes, and it can never go outside the sequence:
TODO: The code below had a bug with "never go outside the sequence" when abs(step)>1; I think I patched it to be correct, but it's hard to understand.
def this_is_how_slicing_works(seq, start=None, stop=None, step=1):
if start is None:
start = (0 if step > 0 else len(seq)-1)
elif start < 0:
start += len(seq)
if not 0 <= start < len(seq): # clip if still outside bounds
start = (0 if step > 0 else len(seq)-1)
if stop is None:
stop = (len(seq) if step > 0 else -1) # really -1, not last element
elif stop < 0:
stop += len(seq)
for i in range(start, stop, step):
if 0 <= i < len(seq):
yield seq[i]
Don't worry about the is None details - just remember that omitting start and/or stop always does the right thing to give you the whole sequence.
Normalizing negative indexes first allows start and/or stop to be counted from the end independently: 'abcde'[1:-2] == 'abcde'[1:3] == 'bc' despite range(1,-2) being empty.
The normalization is sometimes thought of as "modulo the length", but note it adds the length just once: e.g. 'abcde'[-53:42] is just the whole string.