ColorDict

class ColorDict(object):
    """Provides a color for each key, trying to create as distinct a set
    of colors as possible.
    """

    # Minimum difference in hue between unique colors, before we begin
    # to recycle old colors rather than generating new ones.
    THRESHOLD = 0.05

    def __init__(self):
        self._data = {}
        self._index = 0

        # Pregenerate the colors. It would be nicer if this were done
        # one demand, but since there is a rather limited set of color,
        # it probably ain't not worth it.
        self._colors = []
        divcount = 1
        while True:
            step = 1.0 / (divcount*2)
            if step < self.THRESHOLD:
                break
            for i in range(divcount):
                color = (i*2*step+step, 0.5, 0.5)
                self._colors.append(colorsys.hls_to_rgb(*color))
            divcount *= 2

    def _next_color(self):
        if self._index >= len(self._colors):
            self._index = 0
        self._index += 1
        return self._colors[self._index-1]

    def __getitem__(self, key):
        if not key in self._data:
            self._data[key] = self._next_color()
        return self._data[key]