Python __ior__() Magic Method

Syntax

object.__ior__(self, other)

The Python __ior__() magic method implements the in-place bitwise OR x |= y that calculates the result of the bitwise OR 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.__ior__(y).
  • If this is not implemented, it tries the normal bitwise AND operation x.__or__(y).
  • If this is not implemented either, it tries reverse exponentiation operation y.__ror__(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 __ior__

In the following code example, you create a class Data and define the magic method __ior__(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 OR operation.

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


x = Data()
y = Data()

x |= y

print(x)
# finxter 42

In-Place OR |= without __ior__()

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

Here’s an example:

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


x = Data()
y = Data()

x |= y

print(x)
# finxter 42

Even though the __ior__() method is not defined, the in-place bitwise OR operation x |= y still works due to the __or__() “fallback” magic method!

In-Place OR |= without __ior__() and __or__()

To support in-place bitwise OR x |= y on a custom class, you don’t even have to overwrite any of the x.__ior__(y) or x.__or__(y) methods. If both are not defined, Python falls back to the reverse y.__ror__(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 OR operation. Then you define a custom class for the second operand that defines the __ror__() method. For the in-place operation, Python falls back to the __ror__() method defined on the second operand and assigns it to the first operand x:

class Data_1:
    pass

class Data_2:
    def __ror__(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 OR x |= y but neither x.__ior__(y), nor x.__or__(y), nor y.__ror(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 __ior__ 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 OR

Python’s bitwise OR operator x | y performs logical OR on each bit position on the binary representations of integers x and y. Each output bit evaluates to 1 if and only if at least one of the two input bits at the same position are 1.

For example, the integer expression 4 | 3 is translated to the binary operation 0100 | 0011 which results in 0111 because for the last three positions at least one bit is 1.

Related Video Compound Operators

References:

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