gh-150626: Update "Format String Syntax" documentation block to match actual behaviour

[pyctrl]

Hello, let me try to help a bit with docs.

Here is an string formatting example:

In [29]: "{a-b} {-}".format(**{"-": "_-_", "a-b": "ABC"})
Out[29]: 'ABC _-_'

This behaviour differs from Format String Syntax:

arg_name: [identifier | digit+]
attribute_name: identifier

So I updated arg_name and attribute_name syntax in that section to follow the actual behaviour:

• replaced identifier with attribute_name in arg_name
• set syntax for attribute_name as anything that doesn’t contain "." or "]"

You can find original discussion thread here

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📚 Documentation preview 📚: https://cpython-previews--132736.org.readthedocs.build/en/132736/library/string.html#format-string-syntax

[python-cla-bot]

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[StanFromIreland]

See: #127833

We could update the whole snippet now. I'd ask Petr for what his plan was but he's unavailable at the moment.

[skirpichev]

Any?

Are you sure that can use both N and ℕ as attribute names?

[pyctrl]

Yes, any. 👌

It works the same way as kwargs in functions.

In [1]: def f(**kwargs):
   ...:     print(kwargs)
   ...: 

In [2]: d = {"-": 1, "N": 2, "ℕ": 3, "փ": 4, " ": 5}

In [3]: f(**d)
{'-': 1, 'N': 2, 'ℕ': 3, 'փ': 4, ' ': 5}

In [4]: "{-} {N} {ℕ} {փ} { }".format(**d)
Out[4]: '1 2 3 4 5'

[encukou]

You can even use ]:

>> class C:
...     def __init__(self, path=()): self.path = path
...     def __getitem__(self, key): return C((*self.path, key))
...     def __getattr__(self, name): return C((*self.path, name))
...     def __str__(self): return str(self.path)
...     
>>> '{].][{].\0!s}'.format_map(C())
"(']', ']', '{', '\\x00')"
>>> '{.][{].\0!s}'.format(C())
"(']', '{', '\\x00')"

Suggested change

	attribute_name: <any source character except "]" or "."> +
	attribute_name: <any source character except "{", "}", "[", or "."> +

[pyctrl]

You are right, thank you! 👍

Updated the PR. 👌

[pyctrl]

Can I help here more with something?

[github-actions]

This PR is stale because it has been open for 30 days with no activity.

[robsdedude]

Suggested change

	attribute_name: <any source character except "{", "}", "[", or "."> +
	attribute_name: <any source character except "{", "}", "[", ".", "!", or ":"> +

":" and "!" are also invalid attribute names.

[pyctrl]

fixed 👌

[robsdedude]

cf: #151064 (comment)

TL;DR: any str.isdecimal string works for indexing, not just roman decimal digits ("0"..."9") . "١" for example is also valid.

[pyctrl]

Please check my wording and solution with digital_number.

[read-the-docs-community]

Documentation build overview

📚 cpython-previews | 🛠️ Build #33277785 | 📁 Comparing ac7f8c9 against main (a8d74c0)

  🔍 Preview build  

32 files changed · ± 32 modified

± Modified

• glossary.html
• py-modindex.html
• c-api/bytes.html
• deprecations/index.html
• extending/extending.html
• faq/gui.html
• library/codecs.html
• library/csv.html
• library/curses.ascii.html
• library/curses.html
• and 22 more...

[robsdedude]

As a side-note: I (and a fair few others I've interacted with on GH) don't like force pushes on PRs once a review has started. It makes it harder for reviewers to follow what has changed. For a ±4 lines PR this isn't such a big deal, but still.

Other than that and the nit pick, I'd say this is looking good 👌