__round__() method implements the built-in
round() function. For example, if you attempt to call
round(x, ndigits), Python will run the
x.__round__(ndigits) method, respectively.
The following code snippet overrides the
__round__() dunder method to return the rounded age of a
Person when you pass an object of type
Person into the
class Person: def __init__(self, age): self.age = age def __round__(self, ndigits=0): return round(self.age) alice = Person(42.42424242) print(round(alice)) # 42
How to fix TypeError: type XXX doesn’t define __round__ method
Note that without defining the
__round__() method, Python would’ve raised a
class Person: def __init__(self, age): self.age = age alice = Person(42.42424242) print(round(alice))
Traceback (most recent call last): File "C:\Users\...\code.py", line 7, in <module> print(round(alice)) TypeError: type Person doesn't define __round__ method
To fix this
TypeError, simply define the
__round__() method as outlined in the first code snippet in this article.
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.
round() function takes two input arguments: a
number and an optional
precision in decimal digits. It rounds the number to the given precision and returns the result. The return value has the same type as the input number—or integer if the
precision argument is omitted. Per default, the precision is set to 0 digits, so
round(3.14) results in
Here are some examples:
>>> round(3.14) 3 >>> round(3.14, ndigits=1) 3.1 >>> round(3.13, ndigits=-1) 0.0 >>> round(4458.1242, ndigits=-1) 4460.0 >>> round(3.14159, ndigits=3) 3.142
To understand this operation in detail, feel free to read over our tutorial or watch the following video:
Where to Go From Here?
Enough theory. Let’s get some practice!
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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 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.