Python __int__() Magic Method

Syntax

object.__int__(x)

The Python __int__() method implements the built-in int() function. So, when you call int(x), Python attempts to call x.__int__(). If the return value is not an integer, Python will raise a TypeError. If it’s not implemented, Python attempts to call x.__index__() instead, and only if this is not implemented either, it raises a TypeError.

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.

Background int()

Python’s built-in int(value) function converts the argument value to an integer number.

For example, int('42') converts the string value '42' into the integer number 42. The int() function on a float argument rounds down to the closest integer.

Example Custom __int__()

In the following example, you create a custom class Data and overwrite the __int__() magic method so that it returns an integer number 42 when you call int(x) on a custom Data object x.

class Data:
    def __int__(self):
        return 42


x = Data()
res = int(x) 

print(res)
# 42

TypeError: int() argument must be a string, a bytes-like object or a number, not ‘…’

If you call the int(x) built-in function without defining the __int__() magic method on a given object x, Python will raise a TypeError:

class Data:
    pass


x = Data()
res = int(x) 

Output:

Traceback (most recent call last):
  File "C:\Users\xcent\Desktop\code.py", line 6, in <module>
    res = int(x)
TypeError: int() argument must be a string, a bytes-like object or a number, not 'Data'

To fix this error, define the x.__int__() method for an object x before calling the built-in int(x) method:

class Data:
    def __int__(self):
        return 42


x = Data()
res = int(x) 

print(res)
# 42

TypeError: __int__ returned non-int (type str)

Consider the following code snippet where you try to return a string, i.e., non-integer value, in the dunder method __int__():

class Data:
    def __int__(self):
        return '42' # not an integer!


x = Data()
res = int(x) 

print(res)

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>
    res = int(x)
TypeError: __int__ returned non-int (type str)

The reason for the TypeError: __int__ returned non-int (type ...) error is that the __int__() method must return an integer value. So, to resolve the error, return an integer value as shown previously:

class Data:
    def __int__(self):
        return 42       # Integer value! :)


x = Data()
res = int(x) 

print(res)
# 42

Fallback Method __index__() for int()

If the __int__() method is not defined on an object x on which you call int(x), Python will first attempt to call x.__index__() method to obtain a numeric value associated with the object.

You can see this in the following example where you override the __index__() method by returning 42 but not the __int__() method. The int(x) method still works and returns the result of the __index__() method, i.e., 42.

class Data:
    def __index__(self):
        return 42


x = Data()
res = int(x) 

print(res)
# 42

References:

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