Python In-Place Bitwise Left-Shift Operator

Python’s in-place bitwise left-shift operator x <<= y calculates the left-shift operation x << y, and assigns the result to the first operands variable name x. You can set up the in-place left-shift behavior in your own class by overriding the magic “dunder” method __ilshift__(self, other) in your class definition.

>>> x = 8
>>> x <<= 2
>>> x
32

The expression x <<= y is syntactical sugar for the longer-form x = x << y:

>>> x = 8
>>> x = x << 2
>>> x
32

Let’s explore some examples on different data types of the operands.

Recap Bitwise Left-Shift

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.

Here’s a minimal example:

print(8 << 1)
# 16

print(8 << 2)
# 32

print(-3 << 1)
# -6

Feel free to watch my explainer video here:

Related. To learn more about the bitwise left-shift operator, and how it works on positive and negative integers, check out our related tutorial: Python Bitwise Left-Shift Operator

Incompatible Data Type

What if two operands have an incompatible data type—unlike floats and integers? For example, if you try to shift a float variable by a list variable (which doesn’t make sense)?

>>> x = 3.0
>>> y = [1, 2]
>>> x << y
Traceback (most recent call last):
  File "<pyshell#11>", line 1, in <module>
    x << y
TypeError: unsupported operand type(s) for <<: 'float' and 'list'

The result of incompatible addition is a TypeError. You can fix it by using only compatible data types for the in-place bitwise left-shift operation.

Can you use the bitwise left-shift operator on custom objects? Yes!

Python In-Place Bitwise Left-Shift Magic Method

To use the in-place bitwise left-shift operator <<= on custom objects, you need to define the __ilshift__() method (“dunder method”, “magic method”) that takes two arguments self and other, updates the first argument self with the result of the operation, and returns the updated object.

In the following code, you combine two Data objects using the in-place left-shift operation:

class Data:

    def __init__(self, data):
        self.data = data

    def __ilshift__(self, other):
        self.data <<= other.data
        return self

x = Data(8)
y = Data(2)

x <<= y
print(x.data)
# 32

You can see that the content of the first operand is updated as a result of the in-place bitwise left-shift operation.

Python In-Place Operators

In-place assignment operators (also called compound assignment operators) perform an operation in-place on a variable provided as first operand. They overwrite the value of the first operand variable with the result of the operation when performing the operator without assignment. For example, x += 3 is the same as x = x + 3 of first calculating the result of x +3 and then assigning it to the variable x.