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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 create_shipment(self, data=None):
 """Create a shipment for an order. When no data arg is given, a shipment for all order lines is assumed."""

        if data is None:
            data = {'lines': []}
        return Shipments(self.client).on(self).create(data)