python list.index() ValueError not IndexError

Question

I suspect this is historical but nothing in the docs explains it.

demo = 'zero', 'one', 'two'
a = demo.index('three')

will raise a ValueError.

dict()['three'] and demo[100]

raises KeyError, IndexError

I find IndexError more logical for list.index failing. You failed to find the index of the requested key.

Am I right? And they can't change it after all this time? Or is ValueError the right one for the failing list.index

I also notice LookupError has parents: KeyError, IndexError

but not AttributeError which is effectively a keyError from dict

demo typed as tuple not dict, please check your example and update the question. — Feras Alfrih
– Feras Alfrih, Commented Jan 2, 2022 at 12:36
demo is a tuple. dict() is a dict. Not sure what you're warning me about — Francis Cagney
– Francis Cagney, Commented Jan 2, 2022 at 12:42

Adid · Accepted Answer · 2022-01-02 12:40:19Z

4

The problem that raises ValueError here is that the value was not found in the list (or tuple, or whatever).
In your second example, the reason it's a KeyError or IndexError is because the key or index was not found in your search (in these cases you're searching for a key or index).
When running list.index(), you're searching for a value.
Thus, when it is not found, a ValueError is raised.

answered Jan 2, 2022 at 12:40

Adid

1,5845 silver badges13 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Francis Cagney Over a year ago

That does indeed justify things; meaning the answer is I'm wrong to find IndexError more logical. Thanks!

Collectives™ on Stack Overflow

python list.index() ValueError not IndexError

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related