Clarify that format strings accept arbitrary tokens as field names

[lgalfaso]

Bug report

Bug description:

Should the code below produce an error?

'{(}'.format(**{'(': 2})

This produces '2' in Python 3.14.5 on MacOS.

Reading
https://docs.python.org/3/library/string.html#format-string-syntax

replacement_field: "{" [field_name] ["!" conversion] [":" format_spec] "}"
field_name:        arg_name ("." attribute_name | "[" element_index "]")*
arg_name:          [identifier | digit+]

this ends up pointing to identifier at https://docs.python.org/3/reference/lexical_analysis.html#grammar-token-python-grammar-identifier

NAME:          name_start name_continue*
name_start:    "a"..."z" | "A"..."Z" | "_" | <non-ASCII character>
name_continue: name_start | "0"..."9"
identifier:    <NAME, except keywords>

Given that '(' is an Ascii character, then ( is not a valid identifier and it should not be allowed.

CPython versions tested on:

3.14

Operating systems tested on:

macOS

[picnixz]

I think it would be too annoying to actually change the parser and it doesn't benefit us to restrict this either. Instead, we can update the docs to make argname allow any token. Note that:

>>> '{(.__class__}'.format(**{'(': int})
"<class 'type'>"

So even the ("." attribute_name | "[" element_index "]")* part is correct. I would still suggest doing a bit more tests to see if the arg_name can be replaced by any token.

[lgalfaso]

A quick check shows that the parser allows for any character other than '!', '.', ':', '[', '{', '}'.

for i in range(0x110000):
    try:
        c = chr(i)
        assert(('{' + c + '}').format(**{c: 2}) == '2')
    except Exception:
        if not c.isdigit(): print("%x -> %c" % (i, c))

21 -> !
2e -> .
3a -> :
5b -> [
7b -> {
7d -> }

Agree that the best way forward would be to update the documentation.

[picnixz]

Thanks for checking! I will update the docs tomorrow.