Python __lshift__() Magic Method

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Syntax

object.__lshift__(self, other)

The Python __lshift__() method implements the built-in << operation. So, when you cal x << y, Python attempts to call x.__lshift__(y). If the method is not implemented, Python first attempts to call __rlshift__ 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 lshift()

The Python bitwise left-shift operator x << n shifts the binary representation of integer x by n positions to the left.

For a positive integer, it inserts a 0 bit on the right and shifts all remaining bits by one position to the left. For example, if you left-shift the binary representation 0101 by one position, you’d obtain 01010. Semantically, the bitwise left-shift operator x << n is the same as multiplying the integer x with 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:

Example Custom __lshift__()

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

class Data:
        
    def __lshift__(self, other):
        return '... my result of lshift...'


a = Data()
b = Data()

print(a << b)
# ... my result of lshift...

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

TypeError: unsupported operand type(s) for <<

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

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:\Users\xcent\Desktop\code.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 __lshift__() 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 __lshift__(self, other) method in your class definition as shown previously:

class Data:
        
    def __lshift__(self, other):
        return '... my result of lshift...'

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

Python __lshift__ vs __rlshift__

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

print(x << y)

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

  1. The method x.__lshift__() is not implemented in the first place, or
  2. The method x.__lshift__() 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.__rlshift__() for reverse left-shift on the right operand y. Not that this is not the same as right-shift, it just means that the left-shift operation is called on the second operand y.

If the reverse left-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.__lshift__(x) instead of x.__lshift__(y), the result would be wrong because the operation is non-commutative. That’s why y.__rlshift__(x) is needed.

So, the difference between x.__lshift__(y) and x.__rlshift__(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 __rlshift__(self, other):
        return 'called reverse lshift'


x = Data_1()
y = Data_2()

print(x << y)
# called reverse lshift

References:

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