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Documentation for issue 5237, auto-numbered format fields. Contributed by Terry J. Reedy.

This commit is contained in:
Eric Smith 2009-04-22 00:47:00 +00:00
parent e56101136c
commit 4c074382bd
2 changed files with 15 additions and 8 deletions
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1
Doc/ACKS.txt
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@ -156,6 +156,7 @@ docs@python.org), and we'll be glad to correct the problem.
* Paul Prescod * Paul Prescod
* Eric S. Raymond * Eric S. Raymond
* Edward K. Ream * Edward K. Ream
* Terry J. Reedy
* Sean Reifschneider * Sean Reifschneider
* Bernhard Reiter * Bernhard Reiter
* Armin Rigo * Armin Rigo

22
Doc/library/string.rst
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@ -221,21 +221,26 @@ The grammar for a replacement field is as follows:
.. productionlist:: sf .. productionlist:: sf
replacement_field: "{" `field_name` ["!" `conversion`] [":" `format_spec`] "}" replacement_field: "{" `field_name` ["!" `conversion`] [":" `format_spec`] "}"
field_name: (`identifier` | `integer`) ("." `attribute_name` | "[" `element_index` "]")* field_name: arg_name ("." `attribute_name` | "[" `element_index` "]")*
arg_name: (`identifier` | `integer`)?
attribute_name: `identifier` attribute_name: `identifier`
element_index: `integer` element_index: `integer`
conversion: "r" | "s" conversion: "r" | "s"
format_spec: <described in the next section> format_spec: <described in the next section>
In less formal terms, the replacement field starts with a *field_name*, which In less formal terms, the replacement field starts with a *field_name* that specifies
can either be a number (for a positional argument), or an identifier (for the object whose value is to be formatted and inserted
keyword arguments). Following this is an optional *conversion* field, which is into the output instead of the replacement field.
The *field_name* is optionally followed by a *conversion* field, which is
preceded by an exclamation point ``'!'``, and a *format_spec*, which is preceded preceded by an exclamation point ``'!'``, and a *format_spec*, which is preceded
by a colon ``':'``. by a colon ``':'``. These specify a non-default format for the replacement value.
The *field_name* itself begins with either a number or a keyword. If it's a The *field_name* itself begins with an *arg_name* that is either either a number or a
number, it refers to a positional argument, and if it's a keyword it refers to a keyword. If it's a number, it refers to a positional argument, and if it's a keyword,
named keyword argument. This can be followed by any number of index or it refers to a named keyword argument. If the numerical arg_names in a format string
are 0, 1, 2, ... in sequence, they can all be omitted (not just some)
and the numbers 0, 1, 2, ... will be automatically inserted in that order.
The *arg_name* can be followed by any number of index or
attribute expressions. An expression of the form ``'.name'`` selects the named attribute expressions. An expression of the form ``'.name'`` selects the named
attribute using :func:`getattr`, while an expression of the form ``'[index]'`` attribute using :func:`getattr`, while an expression of the form ``'[index]'``
does an index lookup using :func:`__getitem__`. does an index lookup using :func:`__getitem__`.
@ -244,6 +249,7 @@ Some simple format string examples::
"First, thou shalt count to {0}" # References first positional argument "First, thou shalt count to {0}" # References first positional argument
"Bring me a {}" # Implicitly references the first positional argument "Bring me a {}" # Implicitly references the first positional argument
"From {} to {}" # Same as "From {0] to {1}"
"My quest is {name}" # References keyword argument 'name' "My quest is {name}" # References keyword argument 'name'
"Weight in tons {0.weight}" # 'weight' attribute of first positional arg "Weight in tons {0.weight}" # 'weight' attribute of first positional arg
"Units destroyed: {players[0]}" # First element of keyword argument 'players'. "Units destroyed: {players[0]}" # First element of keyword argument 'players'.