Define method aliases in Python?

Question

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__

kindall · Accepted Answer · 2017-10-18 15:59:46Z

33

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__

edited Oct 18, 2017 at 15:59

answered Jun 29, 2012 at 15:54

kindall

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4 Comments

gansub Over a year ago

Am I right in concluding that you are doing the assignment in the init method ?

kindall Over a year ago

no, it's in the class definition.

gansub Over a year ago

Ok thanks I googled for an example of this but could not find one. Final question. By putting it in the class definition the method alias assignment has global scope ?

kindall Over a year ago

global as in all instances of that class will have that behavior, yes.

Martijn Pieters · Accepted Answer · 2012-06-29 15:49:00Z

16

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.

answered Jun 29, 2012 at 15:49

Martijn Pieters

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Comments

Katriel · Accepted Answer · 2012-06-29 15:50:30Z

4

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)

answered Jun 29, 2012 at 15:50

Katriel

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3 Comments

Jeremy Pridemore Over a year ago

Is there a way to know which one will be called?

Katriel Over a year ago

__mul__ first, then __rmul__ if that fails.

glglgl Over a year ago

better: or "tell b to multiply a by itself (b)". The multiplication operator doesn't have to be commutative.

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