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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 shelf(self, shelf=None):
 """ Defines a shelf to use for this recipe """

        if shelf is None:
            self._shelf = Shelf({})
        elif isinstance(shelf, Shelf):
            self._shelf = shelf
        elif isinstance(shelf, dict):
            self._shelf = Shelf(shelf)
        else:
            raise BadRecipe('shelf must be a dict or recipe.shelf.Shelf')

        if self._select_from is None and \
            self._shelf.Meta.select_from is not None:
            self._select_from = self._shelf.Meta.select_from
        return self