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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def image_url(self, pixel_size=None):
 """ Get the URL for the user icon in the desired pixel size, if it exists. If no size is supplied, give the URL for the full-size image. """

        if "profile" not in self._raw:
            return
        profile = self._raw["profile"]
        if (pixel_size):
            img_key = "image_%s" % pixel_size
            if img_key in profile:
                return profile[img_key]
        return profile[self._DEFAULT_IMAGE_KEY]