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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 _ensure_object_id(cls, id):
 """Checks whether the given id is an ObjectId instance, and if not wraps it."""

        if isinstance(id, ObjectId):
            return id

        if isinstance(id, basestring) and OBJECTIDEXPR.match(id):
            return ObjectId(id)

        return id