__index__(self) method is called on an object to get its associated integer value. The returned integer is used in slicing or as the basis for the conversion in the built-in functions
__index__() method is also used as a fallback for
complex() functions when their corresponding magic methods are not defined.
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 Custom __index__()
In the following example, you create a custom class
Data and overwrite the
__index__() magic method so that it returns an integer 2 on a custom
Data object. We then print the result of the function calls of six built-in functions that all rely on
class Data: def __index__(self): return 2 x = Data() # All those functions may use __index__(): print(bin(x)) print(oct(x)) print(hex(x)) print(complex(x)) print(int(x)) print(float(x))
The output of those functions shows that all use the value 2 for their conversions, returned by the
0b10 0o2 0x2 (2+0j) 2 2.0
You can see the same output when passing the integer value 2 directly into those functions:
>>> bin(2) '0b10' >>> oct(2) '0o2' >>> hex(2) '0x2' >>> complex(2) (2+0j) >>> int(2) 2 >>> float(2) 2.0
How to Use __index__() for Slicing and Indexing
You can use the magic method
__index__() on a custom class to make it possible for objects of this class to be used in a slicing or indexing operation on an iterable. Python will internally call the
__index__() method to obtain the integer associated with the custom object. This integer is then used as the basis for the slicing and indexing operation.
See this example where we create a custom class
My_Integer and use objects of this class as arguments for the slicing operation on a specific list
class My_Integer: def __init__(self, i): self.i = i def __index__(self): return self.i x = My_Integer(1) y = My_Integer(8) z = My_Integer(3) my_list = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] print(my_list[x]) # 2 print(my_list[y]) # 9 print(my_list[x:y:z]) # [2, 5, 8]
z are of type
My_Integer but they can still be used for the indexing and slicing operations as shown in the last three print statements.
How to Fix “TypeError: __index__ returned non-int (type …)”
If you override the
__index__() method so that it returns a non-integer type
x, Python will raise a
TypeError: __index__ returned non-int (type ...x).
You can see this in the following example:
class Data: def __index__(self): return 'finxter' x = Data() print(int(x))
Traceback (most recent call last): File "C:\Users\xcent\Desktop\code.py", line 8, in <module> print(int(x)) TypeError: __index__ returned non-int (type str)
To fix this error, simply return an integer value from the
__index__() method like so:
class Data: def __index__(self): return 42 x = Data() print(int(x)) # 42
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.