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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 url(self):
 """The url for this collection."""

        if self.parent is None:
            # TODO: differing API Versions?
            pieces = [self.client.base_url, 'api', 'atlas', 'v2']
        else:
            pieces = [self.parent.url]

        pieces.append(self.model_class.path)
        return '/'.join(pieces)