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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.
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 
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The __init__ method for initialization is invoked without any call, when an instance of a class is created, like constructors in certain other programming languages such as C++, Java, C#, PHP etc. These methods are the reason we can add two strings with '+' operator without any explicit typecasting.
__call__ method is used to use the object as a method. __iter__ method is used to generate generator objects using the object.
The __int__ method is called to implement the built-in int function. The __index__ method implements type conversion to an int when the object is used in a slice expression and the built-in hex , oct , and bin functions.
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 You ran a.__rsub__(2.), but that's the wrong __rsub__. You need the right-side operand's __rsub__, not the left-side operand.
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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