Python __rshift__() Magic Method

Syntax

object.__rshift__(self, other)

The Python __rshift__() method implements the built-in >> operation. So, when you cal x >> y, Python attempts to call x.__rshift__(y). If the method is not implemented, Python first attempts to call __rrshift__ on the right operand and if this isn’t implemented either, it raises a TypeError.

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 rshift()

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)
# 4

print(8 >> 2)
# 2

print(-3 >> 1)
# -2

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

Example Custom __rshift__()

In the following example, you create a custom class Data and overwrite the __rshift__() method so that it returns a dummy string when trying to calculate the bitwise right-shift operation.

class Data:
        
    def __rshift__(self, other):
        return '... my result of rshift...'


a = Data()
b = Data()

print(a >> b)
# ... my result of rshift...

If you hadn’t defined the __rshift__() method, Python would’ve raised a TypeError.

TypeError: unsupported operand type(s) for >>

Consider the following code snippet where you try to calculate the right-shift operation on custom objects without defining the dunder method __rshift__():

class Data:
    pass


a = Data()
b = Data()

print(a >> b)

Running this leads to the following error message on my computer:

Traceback (most recent call last):
  File "C:UsersxcentDesktopcode.py", line 8, in <module>
    print(a >> b)
TypeError: unsupported operand type(s) for >>: 'Data' and 'Data'

The reason for this error is that the __rshift__() method has never been defined—and it is not defined for a custom object by default. So, to resolve the TypeError: unsupported operand type(s) for >>, you need to provide the __rshift__(self, other) method in your class definition as shown previously:

class Data:
        
    def __rshift__(self, other):
        return '... my result of rshift...'

Of course, you’d use another return value in practice as explained in the “Background rshift()” section.

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:

• https://docs.python.org/3/reference/datamodel.html

Where to Go From Here?

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