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class CallBackDescriptor:
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def __init__(self, name):
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self.callback_name = name
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def __set__(self, instance, value):
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assert getattr(instance, self.callback_name) is None, f"Attribute '{self.callback_name}' has been set."
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instance.__dict__[self.callback_name] = value
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def __get__(self, instance, owner):
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if instance is None:
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return self
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else:
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return instance.__dict__[self.callback_name]
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