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gh-150626: Update "Format String Syntax" documentation block to match actual behaviour #132736

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gh-150626: Update "Format String Syntax" documentation block to match actual behaviour #132736

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4 changes: 2 additions & 2 deletions Doc/library/string.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -216,8 +216,8 @@ The grammar for a replacement field is as follows:
.. productionlist:: format-string
replacement_field: "{" [`field_name`] ["!" `conversion`] [":" `format_spec`] "}"
field_name: `arg_name` ("." `attribute_name` | "[" `element_index` "]")*
arg_name: [`~python-grammar:identifier` | `~python-grammar:digit`+]
attribute_name: `~python-grammar:identifier`
arg_name: [`~python-grammar:digit`+ | `attribute_name`]

@robsdedude robsdedude Jun 8, 2026

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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.

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

@robsdedude robsdedude Jun 8, 2026

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

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

element_index: `~python-grammar:digit`+ | `index_string`
index_string: <any source character except "]"> +
conversion: "r" | "s" | "a"
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