Python In-Place Bitwise AND Operator

Python’s in-place bitwise AND operator x &= y calcualtes bitwise-and x & y and assigns the result to the first operand x. To set it up for your own class, override the magic “dunder” method __iand__(self, other) in your class definition.

>>> x = 1
>>> x &= 2
>>> x
0

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

>>> x = 1
>>> x = x & 2
>>> x
3

Let’s dive into the bitwise AND operator first—because if you’ve mastered bitwise AND, understanding the in-place bitwise AND operator is a breeze in the summer.

Bitwise AND Refresher

Python’s bitwise AND operator x & y performs logical AND on each bit position on the binary representations of integers x and y. Thus, each output bit is 1 if both input bits at the same position are 1, otherwise, it’s 0. For example, the integer expression 4 & 3 is translated to binaries 0100 & 0011 which results in 0000 because all four input bit positions are different.

In this example, you apply the bitwise AND operator to two integers 32 and 16:

>>> 32 & 16
0

The expression 32 & 16 operates on the bit representations "010000" (decimal 32) and "001000" (decimal 16) and performs bitwise AND. As all i-th bit positions are different, the result is 0:

Similarly, let’s have a look at a slightly modified example to showcase the bitwise AND operator:

>>> 31 & 15
15

You can see this in the following visualization:

The decimal representation of the bit sequence "01111" is 15.

Related Article: Python Bitwise AND Operator Deep Dive

Python In-Place Bitwise AND Magic Method

To use the in-place bitwise AND operator &= on custom objects, define the __iand__() 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 calculate the result on two Data objects:

class Data:

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

    def __iand__(self, other):
        self.data &= other.data
        return self

x = Data(1) # b001
y = Data(3) # b011

x &= y # b001
print(x.data)
# 1

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