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Documentation for issue 5237, auto-numbered format fields. Contributed by Terry J. Reedy.
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@ -156,6 +156,7 @@ docs@python.org), and we'll be glad to correct the problem.
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* Paul Prescod
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* Eric S. Raymond
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* Edward K. Ream
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* Terry J. Reedy
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* Sean Reifschneider
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* Bernhard Reiter
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* Armin Rigo
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@ -221,21 +221,26 @@ The grammar for a replacement field is as follows:
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.. productionlist:: sf
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replacement_field: "{" `field_name` ["!" `conversion`] [":" `format_spec`] "}"
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field_name: (`identifier` | `integer`) ("." `attribute_name` | "[" `element_index` "]")*
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field_name: arg_name ("." `attribute_name` | "[" `element_index` "]")*
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arg_name: (`identifier` | `integer`)?
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attribute_name: `identifier`
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element_index: `integer`
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conversion: "r" | "s"
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format_spec: <described in the next section>
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In less formal terms, the replacement field starts with a *field_name*, which
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can either be a number (for a positional argument), or an identifier (for
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keyword arguments). Following this is an optional *conversion* field, which is
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In less formal terms, the replacement field starts with a *field_name* that specifies
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the object whose value is to be formatted and inserted
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into the output instead of the replacement field.
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The *field_name* is optionally followed by a *conversion* field, which is
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preceded by an exclamation point ``'!'``, and a *format_spec*, which is preceded
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by a colon ``':'``.
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by a colon ``':'``. These specify a non-default format for the replacement value.
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The *field_name* itself begins with either a number or a keyword. If it's a
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number, it refers to a positional argument, and if it's a keyword it refers to a
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named keyword argument. This can be followed by any number of index or
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The *field_name* itself begins with an *arg_name* that is either either a number or a
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keyword. If it's a number, it refers to a positional argument, and if it's a keyword,
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it refers to a named keyword argument. If the numerical arg_names in a format string
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are 0, 1, 2, ... in sequence, they can all be omitted (not just some)
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and the numbers 0, 1, 2, ... will be automatically inserted in that order.
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The *arg_name* can be followed by any number of index or
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attribute expressions. An expression of the form ``'.name'`` selects the named
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attribute using :func:`getattr`, while an expression of the form ``'[index]'``
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does an index lookup using :func:`__getitem__`.
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@ -244,6 +249,7 @@ Some simple format string examples::
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"First, thou shalt count to {0}" # References first positional argument
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"Bring me a {}" # Implicitly references the first positional argument
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"From {} to {}" # Same as "From {0] to {1}"
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"My quest is {name}" # References keyword argument 'name'
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"Weight in tons {0.weight}" # 'weight' attribute of first positional arg
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"Units destroyed: {players[0]}" # First element of keyword argument 'players'.
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