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copied!<p>I find it easier to remember how it's works, then I can figure out any specific start/stop/step combination.</p> <p>It's instructive to understand <code>range()</code> first:</p> <pre><code>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 </code></pre> <p>Begin from <code>start</code>, increment by <code>step</code>, do not reach <code>stop</code>. Very simple.</p> <p>The thing to remember about negative step is that <code>stop</code> 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. <code>'abcde'[1:-2][::-1]</code> slices off one char from left, two from right, then reverses. (See also <a href="http://www.python.org/dev/peps/pep-0322/" rel="noreferrer"><code>reversed()</code></a>.)</p> <p>Sequence slicing is same, except it first normalizes negative indexes, and can never go outside the sequence:</p> <p><strong>TODO</strong>: The code below had a bug with "never go outside the sequence" when abs(step)>1; I <em>think</em> I patched it to be correct, but it's hard to understand.</p> <pre><code>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] </code></pre> <p>Don't worry about the <code>is None</code> details - just remember that omitting <code>start</code> and/or <code>stop</code> always does the right thing to give you the whole sequence.</p> <p>Normalizing negative indexes first allows start and/or stop to be counted from the end independently: <code>'abcde'[1:-2] == 'abcde'[1:3] == 'bc'</code> despite <code>range(1,-2) == []</code>. The normalization is sometimes thought of as "modulo the length" but note it adds the length just once: e.g. <code>'abcde'[-53:42]</code> is just the whole string.</p>

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