Python __iand__() Magic Method

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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:

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