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pythonclassoperator-overloadingoverridingoperator-keyword
Zoé Martin
Zoé Martin

Reputation: 1907

Define method aliases in Python?

I have a vector class and I defined the __mul__ method to multiply a vector by a number.

Here is the __mul__ method :

def __mul__(self, other):
    x = self.x * other
    y = self.y * other
    new = Vector()
    new.set_pos((x, y))
    return new

My problem is that I don't know which is which between the number and the vector. If self is the number, self.x raises an error. (I'm maybe mistaking on this point : Is "other" always a number ?)

So I found here : Python: multiplication override that I could do :

__rmul__ = __mul__

but how can I do that in a class definition ?

Something like :

def __rmul__ = __mul__

Upvotes: 27

Views: 18923

Answers (3)

kindall
kindall

Reputation: 184280

self will never be the number in __mul__() because the object the method is attached to is not the number, it's the vector, and by definition it's the multiplicand.

other will be a number if your object is being multiplied by a number. Or it could be something else, such as another vector, which you could test for and handle.

When your object is the multiplier, __rmul__() is called if the multiplicand doesn't know how to handle the operation.

To handle the case in which __mul__ and __rmul__ should be the same method, because the operation is commutative, you can just do the assignment in your class definition.

class Vector(object):
    def __mul__(self, other):
        pass

    __rmul__ = __mul__

Upvotes: 33

Katriel
Katriel

Reputation: 123732

The point is that in Python, you can tell objects how to multiply themselves by things. That means that 

a * b

could either mean "tell a to multiply itself by b" or "tell b to multiply itself by a". In code, that translates to

a.__mul__(b)

or

b.__rmul__(a)

Upvotes: 4

Martijn Pieters
Martijn Pieters

Reputation: 1123490

Simply list it as an attribute:

__rmul__ = __mul__

This is the same way you'd create an alias of a function in a module; creating an alias of a method within a class body works the same.

Upvotes: 16

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