__itruediv__() magic method implements the in-place division operation
x /= y that calculates the division operation
x / y, and assigns the result to the first operands variable
x. This 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 true division operation
- If this is not implemented either, it tries reverse true division operation
y.__rtruediv__(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.
Note that this is different from the integer division operator
x // y. You can learn about integer division in this Finxter blog tutorial.
Basic Example Overriding __itruediv__
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 true division operation.
class Data: def __itruediv__(self, other): return 'finxter 42' x = Data() y = Data() x /= y print(x) # finxter 42
In-Place True Division /= Without __itruediv__()
To support in-place true division on a custom class, you don’t have to overwrite the in-place
__itruediv__() method. Because if the method is not defined, Python will fall back to the normal
__truediv__() method and assign its result to the first operand.
Here’s an example:
class Data: def __truediv__(self, other): return 'finxter 42' x = Data() y = Data() x /= y print(x) # finxter 42
Even though the
__itruediv__() method is not defined, the in-place true division operation
x /= y still works due to the
__truediv__() “fallback” magic method!
In-Place True Division /= Without __itruediv__() and __truediv__()
To support in-place true division
x /= y on a custom class, you don’t even have to overwrite any of the
x.__truediv__(y) methods. If both are not defined, Python falls back to the reverse
y.__rtruediv__(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 true division operation. Then you define a custom class for the second operand that defines the
__rtruediv__() method. For the in-place operation, Python falls back to the
__rtruediv__() method defined on the second operand and assigns it to the first operand
class Data_1: pass class Data_2: def __rtruediv__(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 division
x /= y but neither
y.__rtruediv(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 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 True Division
In Python 3, the single front-slash “/” is a float division operator that returns a float value as a result. For example, the expression
2.5 instead of
2.0 instead of
>>> # Python 3 >>> 10/4 2.5 >>> 4/2 2.0
Be careful to use the most updated type of Python available. For example, Python 2.0 returns an integer instead of a float value for the
/ operator. Also when we perform division in Python we want to be careful what value we divide by. We find that if we divide by a whole number, it will round to an integer.
>>> 10 / 90 0
You can find full tutorials on related operators (including videos) here:
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. 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.