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copied!<p>I don't know why you're using an xstep variable. You don't really need one with the algorithm you're using.</p> <blockquote> <p>@Gabe: xstep is needed because without it, if x0 > x1, then the for loop will terminate immediately, as the default step for a Python for loop is 1.</p> </blockquote> <p>The reason you don't need an xstep variable is because, if it's going backwards, the coordinates were already switched (in the <code>if backward:</code> conditional at the beginning) so that the end-point is now the start-point and vice-versa, such that we now are <em>still</em> going left-to-right.</p> <p>You just need this:</p> <pre><code>result = [] for x in range(x0, x1): if steep: result.append((y, x)) else: result.append((x, y)) error -= dy if error < 0: y += ystep error += dx return result </code></pre> <p>If you want the list of coordinates in order from start to end-point, then you can do the check at the end:</p> <pre><code>if backward: return result.reverse() else: return result </code></pre> <p><strong>EDIT</strong>: The problem is that the <code>backward</code> boolean is being evaluated <em>before</em> it needs to be. If the <code>steep</code> conditional executes, then the values change, but by then your <code>backward</code> conditional is different. To fix this, instead of using a <code>backward</code> boolean, make it an explicit expression:</p> <pre><code>if x0 > x1: # swapping here </code></pre> <p>Then again, since you end up using the boolean later on, you could just define it before the conditional:</p> <pre><code>backward = x0 > x1 if backward: </code></pre>

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