list indices must be integers or slices, not str

[Chris Angelico]

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.

ChrisA