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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 list(self, mask=None):
 """List existing placement groups Calls SoftLayer_Account::getPlacementGroups """

        if mask is None:
            mask = "mask[id, name, createDate, rule, guestCount, backendRouter[id, hostname]]"
        groups = self.client.call('Account', 'getPlacementGroups', mask=mask, iter=True)
        return groups