Python __truediv__() Magic Method

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Syntax

object.__truediv__(self, other)

The Python __truediv__() method is called to implement the normal division operation / called true division—as opposed to the floor division operation //. For example to evaluate the expression x / y, Python attempts to call x.__truediv__(y).

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.

Example

In the following example, you create a custom class Data and overwrite the __truediv__() method so that it returns a dummy string when trying to divide two Data objects using the true division operation a / b.

class Data:
        
    def __truediv__(self, other):
        return '... my result of truediv...'


a = Data()
b = Data()
c = a / b

print(c)
# ... my result of truediv...

If you hadn’t defined the __truediv__() method, Python would’ve raised a TypeError.

How to Resolve TypeError: unsupported operand type(s) for /

Consider the following code snippet where you try to divide two custom objects without defining the dunder method __truediv__():

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 __truediv__() 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 __truediv__(self, other) method in your class definition as shown previously:

class Data:
        
    def __truediv__(self, other):
        return '... my result of truediv...'

Python __truediv__ vs __div__

  • The Python __truediv__() dunder method is called to implement the normal division operation in Python 3.
  • The Python __div__() dunder method is called to implement the normal division operation in Python 2. It doesn’t work in Python 3 anymore.

So, although both methods __truediv__ and __div__ have the same semantics as operators, they are different in that they are used for Python version 3 and 2, repsectively.

Python __truediv__ vs __rtruediv__

Say, you want to divide two objects x and y using true division:

print(x / y)

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

  1. The method x.__truediv__() is not implemented in the first place, or
  2. The method x.__truediv__() 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.__rtruediv__() for reverse true division on the right operator y. 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.__truediv__(x) instead of x.__truediv__(y), it could cause an error if the operation is non-commutative. That’s why y.__rtruediv__(x) is needed which indicates that true division is possible after all.

So, the difference between x.__truediv__(y) and x.__rtruediv__(y) is that the former calculates x / y whereas the latter calculates y / x — both calling the respective true division method defined on object x.

References:

Explainer Video Division Operators

You can also check out my explainer video where I’ll give you a deep dive on the integer and true division operators and how to use them for various data types. Click to watch:

Where to Go From Here?

Enough theory. Let’s get some practice!

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