Python __iand__() Magic Method

by

Syntax

object.__iand__(self, other)

The Python __iand__() magic method implements the in-place bitwise AND x &= y that calculates the result of the bitwise AND operation x & y, and assigns it to the first operands’ variable x. This type of in-place operation is also called augmented arithmetic assignment. The method simply returns the new value to be assigned to the first operand.

  • When you call x &= y, Python first attempts to call x.__iand__(y).
  • If this is not implemented, it tries the normal bitwise AND operation x.__and__(y).
  • If this is not implemented either, it tries reverse exponentiation operation y.__rand__(x) with swapped operands.

The result is then assigned to the first operand x. If none of those operations is implemented, Python 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.

Basic Example Overriding __iand__

In the following code example, you create a class Data and define the magic method __iand__(self, other).

  • The “self” argument is the default argument of each method and it refers to the object on which it is called—in our case, the first operand of the in-place operation.
  • The “other” argument of the in-place method refers to the second operand, i.e., y in the in-place operation x &= y.

The return value of the operation returns a dummy string 'finxter 42' to be assigned to the first operand. In practice, this would be the result of the in-place bitwise AND operation.

class Data:
    def __iand__(self, other):
        return 'finxter 42'


x = Data()
y = Data()

x &= y

print(x)
# finxter 42

In-Place AND &= without __iand__()

To support the in-place bitwise AND operation on a custom class, you don’t have to overwrite the __iand__() method. Because if the method is not defined, Python will fall back to the normal __and__() method and assign its result to the first operand.

Here’s an example:

class Data:
    def __and__(self, other):
        return 'finxter 42'


x = Data()
y = Data()

x &= y

print(x)
# finxter 42

Even though the __iand__() method is not defined, the in-place bitwise AND operation x &= y still works due to the __and__() “fallback” magic method!

In-Place AND &= without __iand__() and __and__()

To support in-place bitwise AND x &= y on a custom class, you don’t even have to overwrite any of the x.__iand__(y) or x.__and__(y) methods. If both are not defined, Python falls back to the reverse y.__rand__(x) method and assigns its result to the first operand.

Here’s an example where you create a custom class for the first operand that doesn’t support the bitwise AND operation. Then you define a custom class for the second operand that defines the __rand__() method. For the in-place operation, Python falls back to the __rand__() method defined on the second operand and assigns it to the first operand x:

class Data_1:
    pass

class Data_2:
    def __rand__(self, other):
        return 'finxter 42'

x = Data_1()
y = Data_2()

x &= y

print(x)
# finxter 42

TypeError: unsupported operand type(s) for &=

If you try to perform in-place bitwise AND x &= y but neither x.__iand__(y), nor x.__and__(y), nor y.__rand(x) is defined, Python raises a “TypeError: unsupported operand type(s) for &=". To fix this error, simply define any of those methods before performing the in-place operation.

class Data:
    pass      # ... you should define __iand__ here to prevent error! ... #


x = Data()
y = Data()

x &= y

Output:

Traceback (most recent call last):
  File "C:\Users\xcent\Desktop\code.py", line 8, in <module>
    x &= y
TypeError: unsupported operand type(s) for &=: 'Data' and 'Data'

Background Bitwise AND

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.

Related Video Compound Operators

References:

Where to Go From Here?

Enough theory. Let’s get some practice!

Coders get paid six figures and more because they can solve problems more effectively using machine intelligence and automation.

To become more successful in coding, solve more real problems for real people. That’s how you polish the skills you really need in practice. After all, what’s the use of learning theory that nobody ever needs?

You build high-value coding skills by working on practical coding projects!

Do you want to stop learning with toy projects and focus on practical code projects that earn you money and solve real problems for people?

🚀 If your answer is YES!, consider becoming a Python freelance developer! It’s the best way of approaching the task of improving your Python skills—even if you are a complete beginner.

If you just want to learn about the freelancing opportunity, feel free to watch my free webinar “How to Build Your High-Income Skill Python” and learn how I grew my coding business online and how you can, too—from the comfort of your own home.

Join the free webinar now!