Pythonic way to convert a list of integers into a string of comma-separated ranges

I have a list of integers which I need to parse into a string of ranges.

For example:

 [0, 1, 2, 3] -> "0-3"
 [0, 1, 2, 4, 8] -> "0-2,4,8"

And so on.

I'm still learning more pythonic ways of handling lists, and this one is a bit difficult for me. My latest thought was to create a list of lists which keeps track of paired numbers:

[ [0, 3], [4, 4], [5, 9], [20, 20] ]

I could then iterate across this structure, printing each sub-list as either a range, or a single value.

I don't like doing this in two iterations, but I can't seem to keep track of each number within each iteration. My thought would be to do something like this:

Here's my most recent attempt. It works, but I'm not fully satisfied; I keep thinking there's a more elegant solution which completely escapes me. The string-handling iteration isn't the nicest, I know -- it's pretty early in the morning for me :)

def createRangeString(zones):
        rangeIdx = 0
        ranges   = [[zones[0], zones[0]]]
        for zone in list(zones):
            if ranges[rangeIdx][1] in (zone, zone-1):
                ranges[rangeIdx][1] = zone
            else:
                ranges.append([zone, zone])
                rangeIdx += 1

        rangeStr = ""
        for range in ranges:
            if range[0] != range[1]:
                rangeStr = "%s,%d-%d" % (rangeStr, range[0], range[1])
            else:
                rangeStr = "%s,%d" % (rangeStr, range[0])

        return rangeStr[1:]

Is there a straightforward way I can merge this into a single iteration? What else could I do to make it more Pythonic?