Python 3.5: subclasses of int don't call the index method?

Question

I am confused as to when the __index__ method of a class is called. I had assumed the method would be called when ever the object was used in an indexing operation. I am not seeing __index__ called when the object is a subclass of int.

In [1]: class foo(int):
...:        def __new__(cls, value):
...:            return int.__new__(cls, value)
...:        def __index__(self):
...:            return int(self)+1

In [2]: i=foo(0)

In [3]: i
Out[3]: 0

In [4]: i.__index__()
Out[4]: 1

In [5]: [0,1,2][:i.__index__()]
Out[5]: [0]

In [6]: [0,1,2][:i]
Out[6]: []

it appears that int(i) is being used as the index not i.__index__(). What have I misunderstood?

Edited to simplify and correct the example.

See the note in the [data model docs](https://docs.python.org/3/reference/datamodel.html#object.__index__) — jonrsharpe, Feb 04 '19 at 21:21
" In order to have a coherent integer type class, when __index__() is defined __int__() should also be defined, and both should return the same value." seems that you have a problem now — Jean-François Fabre, Feb 04 '19 at 21:22
Related: [Make an object that behaves like a slice](https://stackoverflow.com/q/39971030/674039) — wim, Feb 04 '19 at 21:23
@Jean-François Fabre Yes I do have a problem now, thanks all. — Codemonkey, Feb 04 '19 at 21:33
I think that's a XY problem... we don't know what you want to achieve globally — Jean-François Fabre, Feb 04 '19 at 21:33
I'll just use another approach. It was mostly a learning exercise anyway. — Codemonkey, Feb 04 '19 at 21:44

score 0 · Accepted Answer · answered Feb 04 '19 at 21:59

__index__ implements lossless conversion to int. Python only calls __index__ if it actually needs such a conversion. In your case, it doesn't need a conversion, because your i is already an int. Python just uses the int value directly.

If you want to see the code that handles this, it's under PyNumber_Index.

Python 3.5: subclasses of int don't call the __index__ method?

1 Answers1

Python 3.5: subclasses of int don't call the index method?