Python __rrshift__() Magic Method

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Syntax

object.__rrshift__(self, other)

The Python __rrshift__() method implements the reverse bitwise right-shift operation with reflected, swapped operands. So, when you call x >> y, Python attempts to call x.__rshift__(y). If the method is not implemented, Python attempts to call y.__rrshift__(x) on the right operand and if this isn’t implemented either, it raises a TypeError.

The __rrshift__() method is often used in practice to add the right-shift functionality to a class that doesn’t yet provide it and that cannot be modified such as a list or a built-in data type. By implementing the method for the other operand, you can still use the right-shift functionality for the class that doesn’t implement __rshift__().

We call this a “Dunder Method” for “Double Underscore Method” (also called “magic method”). To get a list of all dunder methods with explanation, check out our dunder cheat sheet article on this blog.

Background Bitwise Right-Shift

The Python bitwise right-shift operator x >> n shifts the binary representation of integer x by n positions to the right. It inserts a 0 bit on the left and removes the right-most bit.

For example, if you right-shift the binary representation 0101 by one position, you’d obtain 0010. Semantically, the bitwise right-shift operator is the same as performing integer division by 2**n.

print(8 << 1)
# 16

print(8 << 2)
# 32

print(-3 << 1)
# -6

To understand this operation in detail, feel free to read over our tutorial or watch the following video:

Python __rshift__ vs __rrshift__

Say, you want to calculate the right-shift operation on two custom objects x and y:

print(x >> y)

Python first tries to call the left object’s __rshift__() method x.__rshift__(y). But this may fail for two reasons:

  1. The method x.__rshift__() is not implemented in the first place, or
  2. The method x.__rshift__() is implemented but returns a NotImplemented value indicating that the data types are incompatible.

If this fails, Python tries to fix it by calling the y.__rrshift__() for reverse right-shift on the right operand y. Not that this is not the same as left-shift, it just means that the right-shift operation is called on the second operand y.

If the reverse right-shift method is implemented, Python knows that it doesn’t run into a potential problem of a non-commutative operation. If it would just execute y.__rshift__(x) instead of x.__rshift__(y), the result would be wrong because the operation is non-commutative. That’s why y.__rrshift__(x) is needed.

So, the difference between x.__rshift__(y) and x.__rrshift__(y) is that the former calculates x >> y whereas the latter calculates y >> x — both calling the respective method defined on the object x.

You can see this in effect here where we attempt to call the operation on the left operand x—but as it’s not implemented, Python simply calls the reverse operation on the right operand y.

class Data_1:
    pass

class Data_2:
    def __rrshift__(self, other):
        return 'called reverse rshift'


x = Data_1()
y = Data_2()

print(x >> y)
# called reverse rshift

References:

Where to Go From Here?

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