[Python-Dev] unsubscriptable vs object does not support indexing

[MRAB]

Ben Finney wrote:
> Steven D'Aprano <steve at pearwood.info> writes:
> 
>> As far as I can see, in practice, people talk about obj[i] as the item
>> at index i, not the item at subscript i -- the term "subscript" in
>> this context seems to be rare to non-existent except for the error
>> message.
> 
> Presumably, the same people would also call ‘obj[i]’ the item at *key*
> ‘i’, if ‘obj’ is a dictionary. For an object that supports neither
> indexes nor keys, though, how is Python to know which the user meant?
> It's a single operation as far as the parser is concerned, so there
> needs to be a single term for it. That term is “subscript”.
> 
> Your point about the awkward word “unsubscriptable” is well-taken,
> though. Perhaps a simple improvement to the message wording:
> 
>     >>> foo = 3
>     >>> foo[6]
>     Traceback (most recent call last):
>       File "<stdin>", line 1, in <module>
>     TypeError: 'int' object does not support subscripts
>     >>> foo['spam']
>     Traceback (most recent call last):
>       File "<stdin>", line 1, in <module>
>     TypeError: 'int' object does not support subscripts
> 
It's called a 'subscript' because conventional mathematical notation
uses subscripting. Newbies might be acquainted with the term 'index'
from books, where the 'value' is non-numeric. It's a bit unfortunate
that dicts have keys+value instead of index+value! :-)