Python __ror__() Magic Method


object.__ror__(self, other)

The Python __ror__() method implements the reverse Bitwise OR | operation with reflected, swapped operands. So, when you call x | y, Python attempts to call x.__or__(y). If the method is not implemented, Python attempts to call __ror__ on the right operand and if this isn’t implemented either, it 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.

Background Bitwise OR |

Python’s bitwise OR 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.

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

>>> 32 | 16 

The expression 32 | 16 operates on the bit representations "010000" (decimal 32) and "001000" (decimal 16) and performs bitwise OR. Each "1" position propagates remains in the result "110000" (decimal 48):

First Operand x100000
Second Operand y010000
x | y110000

To understand this operation in detail, feel free to read over our tutorial or watch the following video:

Python __or__ vs __ror__

Say, you want to calculate the | operation on two custom objects x and y:

print(x | y)

Python first tries to call the left object’s __or__() method x.__or__(y). But this may fail for two reasons:

  1. The method x.__or__() is not implemented in the first place, or
  2. The method x.__or__() is implemented but returns a NotImplemented value indicating that the data types are incompatible.

If this fails, Python tries to fix it by calling the y.__ror__() for reverse bitwise OR on the right operand y.

If the reverse bitwise OR method is implemented, Python knows that it doesn’t run into a potential problem of a non-commutative operation. If it would just execute y.__or__(x) instead of x.__or__(y), the result would be wrong because the operation may be non-commutative when defined as a custom operation. That’s why y.__ror__(x) is needed.

So, the difference between x.__or__(y) and x.__ror__(y) is that the former calculates x | y whereas the latter calculates y | x — both calling the respective method defined on the object x.

You can see this in effect here where we attempt to call the operation on the left operand x—but as it’s not implemented, Python simply calls the reverse operation on the right operand y.

class Data_1:

class Data_2:
    def __ror__(self, other):
        return 'called reverse bitwise OR'

x = Data_1()
y = Data_2()

print(x | y)
# called reverse bitwise OR


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