問題描述

當我從整數中減去浮點數時(例如 1-2.0)，Python 會進行隱式類型轉換(我認為).但是當我使用魔術方法 __sub__ 調用我認為是相同的操作時，它突然不再存在了.

When I subtract a float from an integer (e.g. 1-2.0), Python does implicit type conversion (I think). But when I call what I thought was the same operation using the magic method __sub__, it suddenly does not anymore.

我在這里缺少什么?當我為自己的類重載運算符時，除了將輸入顯式轉換為我需要的任何類型之外，還有其他方法嗎?

What am I missing here? When I overload operators for my own classes, is there a way around this other than explicitly casting input to whatever type I need?

a=1
a.__sub__(2.)
# returns NotImplemented
a.__rsub__(2.)
# returns NotImplemented
# yet, of course:
a-2.
# returns -1.0

推薦答案

a - b 不僅僅是 a.__sub__(b).如果 a 無法處理該操作，它也會嘗試 b.__rsub__(a)，在 1 - 2. 的情況下，它是float 的 __rsub__ 處理操作.

a - b isn't just a.__sub__(b). It also tries b.__rsub__(a) if a can't handle the operation, and in the 1 - 2. case, it's the float's __rsub__ that handles the operation.

>>> (2.).__rsub__(1)
-1.0

您運行了 a.__rsub__(2.)，但這是錯誤的 __rsub__.您需要右側操作數的 __rsub__，而不是左側操作數.

You ran a.__rsub__(2.), but that's the wrong __rsub__. You need the right-side operand's __rsub__, not the left-side operand.

減法運算符沒有內置隱式類型轉換.float.__rsub__ 必須手動處理整數.如果您想在自己的運算符實現中進行類型轉換，您也必須手動處理.

There is no implicit type conversion built into the subtraction operator. float.__rsub__ has to handle ints manually. If you want type conversion in your own operator implementations, you'll have to handle that manually too.

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