list indices must be integers or slices, not str

[Frank Millman]

On 2022-07-20 12:31 PM, Frank Millman wrote:
> On 2022-07-20 11:37 AM, Chris Angelico wrote:
>> On Wed, 20 Jul 2022 at 18:34, Frank Millman <frank at chagford.com> wrote:
>>>
>>> Hi all
>>>
>>> C:\Users\E7280>python
>>> Python 3.9.7 (tags/v3.9.7:1016ef3, Aug 30 2021, 20:19:38) [MSC v.1929 64
>>> bit (AMD64)] on win32
>>> Type "help", "copyright", "credits" or "license" for more information.
>>>   >>>
>>>   >>> x = list(range(10))
>>>   >>>
>>>   >>> '{x[1]}'.format(**vars())
>>> '1'
>>>   >>>
>>>   >>> '{x[-1]}'.format(**vars())
>>> Traceback (most recent call last):
>>>     File "<stdin>", line 1, in <module>
>>> TypeError: list indices must be integers or slices, not str
>>>   >>>
>>>
>>> Can anyone explain this error? It seems that a negative index is deemed
>>> to be a string in this case.
>>>
>>
>> Yeah, that does seem a little odd. What you're seeing is the same as
>> this phenomenon:
>>
>>>>> "{x[1]} {x[spam]}".format(x={1: 42, "spam": "ham"})
>> '42 ham'
>>>>> "{x[1]} {x[spam]}".format(x={"1": 42, "spam": "ham"})
>> Traceback (most recent call last):
>>    File "<stdin>", line 1, in <module>
>> KeyError: 1
>>
>> But I can't find it documented anywhere that digits-only means
>> numeric. The best I can find is:
>>
>> https://docs.python.org/3/library/string.html#formatstrings
>> """The arg_name can be followed by any number of index or attribute
>> expressions. An expression of the form '.name' selects the named
>> attribute using getattr(), while an expression of the form '[index]'
>> does an index lookup using __getitem__()."""
>>
>> and in the corresponding grammar:
>>
>> field_name        ::=  arg_name ("." attribute_name | "[" 
>> element_index "]")*
>> index_string      ::=  <any source character except "]"> +
>>
>> In other words, any sequence of characters counts as an argument, as
>> long as it's not ambiguous. It doesn't seem to say that "all digits is
>> interpreted as an integer, everything else is interpreted as a
>> string". ISTM that a negative number should be interpreted as an
>> integer too, but that might be a backward compatibility break.
>>
> 
> Thanks for investigating this further. I agree it seems odd.
> 
> As quoted above, an expression of the form '[index]' does an index 
> lookup using __getitem()__.
> 
> The only __getitem__() that I can find is in the operator module, and 
> that handles negative numbers just fine.
> 
> Do you think it is worth me raising an issue, if only to find out the 
> rationale if there is one?
> 
> Frank

I saw this from Paul Rubin - for some reason his posts appear in google 
groups, but not python-list.

"It seems to only want integer constants. x[2+2] and x[k] where k=2
don't work either.

I think the preferred style these days is f'{x[-1]}' which works."

Unfortunately the 'f' option does not work for me in this case, as I am 
using a string object, not a string literal.

Frank