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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 _update(self, layer=None):
 """ Update layers in model. """

        meta = getattr(self, ModelBase._meta_attr)
        if not layer:
            layers = self.layers
        else:
            # convert non-sequence to tuple
            layers = _listify(layer)
        for layer in layers:
            # relative path to layer files from model file
            path = os.path.abspath(os.path.join(meta.modelpath, layer))
            getattr(self, layer).load(path)