__mod__() method implements the modulo operation
% that per default returns the remainder of dividing the left by the right operand. Internally, Python attempts to call
x.__mod__(y) to implement the modulo operation
x%y. If the method is not implemented, Python first attempts to call
__rmod__ 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.
In the following example, you create a custom class
Data and overwrite the
__mod__() method so that it returns a dummy string when trying to calculate the modulo of two numbers.
class Data: def __mod__(self, other): return '... my result of mod...' a = Data() b = Data() c = a % b print(c) # ... my result of mod...
If you hadn’t defined the
__() method, Python would’ve raised a
How to Resolve TypeError: unsupported operand type(s) for %
Consider the following code snippet where you try to calculate the modulo of two custom objects without defining the dunder method
class Data: pass a = Data() b = Data() c = a % b print(c)
Running this leads to the following error message on my computer:
Traceback (most recent call last): File "C:\Users\xcent\Desktop\code.py", line 7, in <module> c = a % b TypeError: unsupported operand type(s) for %: 'Data' and 'Data'
The reason for this error is that the
__mod__() dunder method has never been defined—and it is not defined for a custom object by default. So, to resolve the
TypeError: unsupported operand type(s) for %, you need to provide the
__mod__(self, other) method in your class definition as shown previously:
class Data: def __mod__(self, other): return '... my result of mod...'
Python __mod__ vs __rmod__
Say, you want to calculate the modulo of two custom objects
print(x % y)
Python first tries to call the left object’s
x.__mod__(y). But this may fail for two reasons:
- The method
x.__mod__()is not implemented in the first place, or
- The method
x.__mod__()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.__rmod__() for reverse modulo on the right operand
If this 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.__mod__(x) instead of
x.__mod__(y), the result would be wrong because the modulo operation is non-commutative. That’s why
y.__rmod__(x) is needed.
So, the difference between
x.__rmod__(y) is that the former calculates
x % y whereas the latter calculates
y % x — both calling the respective modulo method defined on object
You can see this in effect here where we attempt to call the modulo operation on the left operand
x—but as it’s not implemented, Python simply calls the reverse modulo operation on the right operand
class Data_1: pass class Data_2: def __rmod__(self, other): return 'called rmod' x = Data_1() y = Data_2() print(x % y) # called rmod
Explainer Video Modulo
You can also check out my explainer video where I’ll give you a deep dive on the built-in modulo operation and how to use them for various data types. Click to watch:
Where to Go From Here?
Enough theory, let’s get some practice!
To become successful in coding, you need to get out there and solve real problems for real people. That’s how you can become a six-figure earner easily. And that’s how you polish the skills you really need in practice. After all, what’s the use of learning theory that nobody ever needs?
Practice projects is how you sharpen your saw in coding!
Do you want to become a code master by focusing on practical code projects that actually earn you money and solve problems for people?
Then become a Python freelance developer! It’s the best way of approaching the task of improving your Python skills—even if you are a complete beginner.
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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. He’s author of the popular programming book Python One-Liners (NoStarch 2020), coauthor of the Coffee Break Python series of self-published books, 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.