__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
- If this is not implemented, it tries the normal bitwise AND operation
- 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
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
- 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.,
yin 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.__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
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
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
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
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
Where to Go From Here?
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