__rsub__() method implements the reverse subtraction operation that is subtraction with reflected, swapped operands. So, when you call
x - y, Python attempts to call
x.__sub__(y). Only if the method is not implemented on the left operand, Python attempts to call
__rsub__ on the right operand and if this isn’t implemented either, it 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.
Python __sub__ vs __rsub__
Say, you want to calculate the
- operation on two custom objects
print(x - y)
Python first tries to call the left object’s
x.__sub__(y). But this may fail for two reasons:
- The method
x.__sub__()is not implemented in the first place, or
- The method
x.__sub__()is implemented but returns a
NotImplementedvalue indicating that the data types are incompatible.
If this fails, Python tries to fix it by calling the
y.__rsub__() for reverse subtraction on the right operand
If the reverse subtraction 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.__sub__(x) instead of
x.__sub__(y), the result would be wrong because the operation may be non-commutative when defined as a custom operation. That’s why
y.__rsub__(x) is needed.
So, the difference between
x.__rsub__(y) is that the former calculates
x - y whereas the latter calculates
y - x — both calling the respective method defined on the object
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
class Data_1: pass class Data_2: def __rsub__(self, other): return 'called reverse -' x = Data_1() y = Data_2() print(x - y) # called reverse -
While working as a researcher in distributed systems, Dr. Christian Mayer found his love for teaching computer science students.
To help students reach higher levels of Python success, he founded the programming education website Finxter.com that has taught exponential skills to millions of coders worldwide. He’s the author of the best-selling programming books Python One-Liners (NoStarch 2020), The Art of Clean Code (NoStarch 2022), and The Book of Dash (NoStarch 2022). Chris also coauthored the Coffee Break Python series of self-published books. He’s a computer science enthusiast, freelancer, and owner of one of the top 10 largest Python blogs worldwide.
His passions are writing, reading, and coding. But his greatest passion is to serve aspiring coders through Finxter and help them to boost their skills. You can join his free email academy here.