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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_asset(self, project, atype=None, shot=None, asset=None):
 """Create and return a new asset :param project: the project for the asset :type project: :class:`jukeboxcore.djadapter.models.Project` :param atype: the assettype of the asset :type atype: :class:`jukeboxcore.djadapter.models.Atype` :param shot: the shot to add the asset to :type shot: :class:`jukeboxcore.djadapter.models.Shot` :param asset: the asset to add the new asset to :type asset: :class:`jukeboxcore.djadapter.models.Asset` :returns: The created asset or None :rtype: None | :class:`jukeboxcore.djadapter.models.Asset` :raises: None """

        element = shot or asset
        dialog = AssetCreatorDialog(project=project, atype=atype, parent=self)
        dialog.exec_()
        asset = dialog.asset
        if not atype:
            element.assets.add(asset)
        return asset