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 <fr...@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
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