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diff --git a/src/seastar/fmt/doc/syntax.rst b/src/seastar/fmt/doc/syntax.rst new file mode 100644 index 00000000..fc27c5e2 --- /dev/null +++ b/src/seastar/fmt/doc/syntax.rst @@ -0,0 +1,415 @@ +.. _syntax: + +******************** +Format String Syntax +******************** + +Formatting functions such as :ref:`fmt::format() <format>` and +:ref:`fmt::print() <print>` use the same format string syntax described in this +section. + +Format strings contain "replacement fields" surrounded by curly braces ``{}``. +Anything that is not contained in braces is considered literal text, which is +copied unchanged to the output. If you need to include a brace character in the +literal text, it can be escaped by doubling: ``{{`` and ``}}``. + +The grammar for a replacement field is as follows: + +.. productionlist:: sf + replacement_field: "{" [`arg_id`] [":" `format_spec`] "}" + arg_id: `integer` | `identifier` + integer: `digit`+ + digit: "0"..."9" + identifier: `id_start` `id_continue`* + id_start: "a"..."z" | "A"..."Z" | "_" + id_continue: `id_start` | `digit` + +In less formal terms, the replacement field can start with an *arg_id* +that specifies the argument whose value is to be formatted and inserted into +the output instead of the replacement field. +The *arg_id* is optionally followed by a *format_spec*, which is preceded +by a colon ``':'``. These specify a non-default format for the replacement value. + +See also the :ref:`formatspec` section. + +If the numerical arg_ids 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. + +Named arguments can be referred to by their names or indices. + +Some simple format string examples:: + + "First, thou shalt count to {0}" // References the first argument + "Bring me a {}" // Implicitly references the first argument + "From {} to {}" // Same as "From {0} to {1}" + +The *format_spec* field contains a specification of how the value should be +presented, including such details as field width, alignment, padding, decimal +precision and so on. Each value type can define its own "formatting +mini-language" or interpretation of the *format_spec*. + +Most built-in types support a common formatting mini-language, which is +described in the next section. + +A *format_spec* field can also include nested replacement fields in certain +positions within it. These nested replacement fields can contain only an +argument id; format specifications are not allowed. This allows the formatting +of a value to be dynamically specified. + +See the :ref:`formatexamples` section for some examples. + +.. _formatspec: + +Format Specification Mini-Language +================================== + +"Format specifications" are used within replacement fields contained within a +format string to define how individual values are presented (see +:ref:`syntax`). Each formattable type may define how the format +specification is to be interpreted. + +Most built-in types implement the following options for format specifications, +although some of the formatting options are only supported by the numeric types. + +The general form of a *standard format specifier* is: + +.. productionlist:: sf + format_spec: [[`fill`]`align`][`sign`]["#"]["0"][`width`]["." `precision`][`type`] + fill: <a character other than '{', '}' or '\0'> + align: "<" | ">" | "=" | "^" + sign: "+" | "-" | " " + width: `integer` | "{" `arg_id` "}" + precision: `integer` | "{" `arg_id` "}" + type: `int_type` | "a" | "A" | "c" | "e" | "E" | "f" | "F" | "g" | "G" | "p" | "s" + int_type: "b" | "B" | "d" | "n" | "o" | "x" | "X" + +The *fill* character can be any character other than '{', '}' or '\\0'. The +presence of a fill character is signaled by the character following it, which +must be one of the alignment options. If the second character of *format_spec* +is not a valid alignment option, then it is assumed that both the fill character +and the alignment option are absent. + +The meaning of the various alignment options is as follows: + ++---------+----------------------------------------------------------+ +| Option | Meaning | ++=========+==========================================================+ +| ``'<'`` | Forces the field to be left-aligned within the available | +| | space (this is the default for most objects). | ++---------+----------------------------------------------------------+ +| ``'>'`` | Forces the field to be right-aligned within the | +| | available space (this is the default for numbers). | ++---------+----------------------------------------------------------+ +| ``'='`` | Forces the padding to be placed after the sign (if any) | +| | but before the digits. This is used for printing fields | +| | in the form '+000000120'. This alignment option is only | +| | valid for numeric types. | ++---------+----------------------------------------------------------+ +| ``'^'`` | Forces the field to be centered within the available | +| | space. | ++---------+----------------------------------------------------------+ + +Note that unless a minimum field width is defined, the field width will always +be the same size as the data to fill it, so that the alignment option has no +meaning in this case. + +The *sign* option is only valid for number types, and can be one of the +following: + ++---------+----------------------------------------------------------+ +| Option | Meaning | ++=========+==========================================================+ +| ``'+'`` | indicates that a sign should be used for both | +| | positive as well as negative numbers. | ++---------+----------------------------------------------------------+ +| ``'-'`` | indicates that a sign should be used only for negative | +| | numbers (this is the default behavior). | ++---------+----------------------------------------------------------+ +| space | indicates that a leading space should be used on | +| | positive numbers, and a minus sign on negative numbers. | ++---------+----------------------------------------------------------+ + +The ``'#'`` option causes the "alternate form" to be used for the +conversion. The alternate form is defined differently for different +types. This option is only valid for integer and floating-point types. +For integers, when binary, octal, or hexadecimal output is used, this +option adds the prefix respective ``"0b"`` (``"0B"``), ``"0"``, or +``"0x"`` (``"0X"``) to the output value. Whether the prefix is +lower-case or upper-case is determined by the case of the type +specifier, for example, the prefix ``"0x"`` is used for the type ``'x'`` +and ``"0X"`` is used for ``'X'``. For floating-point numbers the +alternate form causes the result of the conversion to always contain a +decimal-point character, even if no digits follow it. Normally, a +decimal-point character appears in the result of these conversions +only if a digit follows it. In addition, for ``'g'`` and ``'G'`` +conversions, trailing zeros are not removed from the result. + +.. ifconfig:: False + + The ``','`` option signals the use of a comma for a thousands separator. + For a locale aware separator, use the ``'n'`` integer presentation type + instead. + +*width* is a decimal integer defining the minimum field width. If not +specified, then the field width will be determined by the content. + +Preceding the *width* field by a zero (``'0'``) character enables +sign-aware zero-padding for numeric types. This is equivalent to a *fill* +character of ``'0'`` with an *alignment* type of ``'='``. + +The *precision* is a decimal number indicating how many digits should be +displayed after the decimal point for a floating-point value formatted with +``'f'`` and ``'F'``, or before and after the decimal point for a floating-point +value formatted with ``'g'`` or ``'G'``. For non-number types the field +indicates the maximum field size - in other words, how many characters will be +used from the field content. The *precision* is not allowed for integer, +character, Boolean, and pointer values. + +Finally, the *type* determines how the data should be presented. + +The available string presentation types are: + ++---------+----------------------------------------------------------+ +| Type | Meaning | ++=========+==========================================================+ +| ``'s'`` | String format. This is the default type for strings and | +| | may be omitted. | ++---------+----------------------------------------------------------+ +| none | The same as ``'s'``. | ++---------+----------------------------------------------------------+ + +The available character presentation types are: + ++---------+----------------------------------------------------------+ +| Type | Meaning | ++=========+==========================================================+ +| ``'c'`` | Character format. This is the default type for | +| | characters and may be omitted. | ++---------+----------------------------------------------------------+ +| none | The same as ``'c'``. | ++---------+----------------------------------------------------------+ + +The available integer presentation types are: + ++---------+----------------------------------------------------------+ +| Type | Meaning | ++=========+==========================================================+ +| ``'b'`` | Binary format. Outputs the number in base 2. Using the | +| | ``'#'`` option with this type adds the prefix ``"0b"`` | +| | to the output value. | ++---------+----------------------------------------------------------+ +| ``'B'`` | Binary format. Outputs the number in base 2. Using the | +| | ``'#'`` option with this type adds the prefix ``"0B"`` | +| | to the output value. | ++---------+----------------------------------------------------------+ +| ``'d'`` | Decimal integer. Outputs the number in base 10. | ++---------+----------------------------------------------------------+ +| ``'o'`` | Octal format. Outputs the number in base 8. | ++---------+----------------------------------------------------------+ +| ``'x'`` | Hex format. Outputs the number in base 16, using | +| | lower-case letters for the digits above 9. Using the | +| | ``'#'`` option with this type adds the prefix ``"0x"`` | +| | to the output value. | ++---------+----------------------------------------------------------+ +| ``'X'`` | Hex format. Outputs the number in base 16, using | +| | upper-case letters for the digits above 9. Using the | +| | ``'#'`` option with this type adds the prefix ``"0X"`` | +| | to the output value. | ++---------+----------------------------------------------------------+ +| ``'n'`` | Number. This is the same as ``'d'``, except that it uses | +| | the current locale setting to insert the appropriate | +| | number separator characters. | ++---------+----------------------------------------------------------+ +| none | The same as ``'d'``. | ++---------+----------------------------------------------------------+ + +Integer presentation types can also be used with character and Boolean values. +Boolean values are formatted using textual representation, either ``true`` or +``false``, if the presentation type is not specified. + +The available presentation types for floating-point values are: + ++---------+----------------------------------------------------------+ +| Type | Meaning | ++=========+==========================================================+ +| ``'a'`` | Hexadecimal floating point format. Prints the number in | +| | base 16 with prefix ``"0x"`` and lower-case letters for | +| | digits above 9. Uses ``'p'`` to indicate the exponent. | ++---------+----------------------------------------------------------+ +| ``'A'`` | Same as ``'a'`` except it uses upper-case letters for | +| | the prefix, digits above 9 and to indicate the exponent. | ++---------+----------------------------------------------------------+ +| ``'e'`` | Exponent notation. Prints the number in scientific | +| | notation using the letter 'e' to indicate the exponent. | ++---------+----------------------------------------------------------+ +| ``'E'`` | Exponent notation. Same as ``'e'`` except it uses an | +| | upper-case 'E' as the separator character. | ++---------+----------------------------------------------------------+ +| ``'f'`` | Fixed point. Displays the number as a fixed-point | +| | number. | ++---------+----------------------------------------------------------+ +| ``'F'`` | Fixed point. Same as ``'f'``, but converts ``nan`` to | +| | ``NAN`` and ``inf`` to ``INF``. | ++---------+----------------------------------------------------------+ +| ``'g'`` | General format. For a given precision ``p >= 1``, | +| | this rounds the number to ``p`` significant digits and | +| | then formats the result in either fixed-point format | +| | or in scientific notation, depending on its magnitude. | +| | | +| | A precision of ``0`` is treated as equivalent to a | +| | precision of ``1``. | ++---------+----------------------------------------------------------+ +| ``'G'`` | General format. Same as ``'g'`` except switches to | +| | ``'E'`` if the number gets too large. The | +| | representations of infinity and NaN are uppercased, too. | ++---------+----------------------------------------------------------+ +| none | The same as ``'g'``. | ++---------+----------------------------------------------------------+ + +Floating-point formatting is locale-dependent. + +.. ifconfig:: False + + +---------+----------------------------------------------------------+ + | | The precise rules are as follows: suppose that the | + | | result formatted with presentation type ``'e'`` and | + | | precision ``p-1`` would have exponent ``exp``. Then | + | | if ``-4 <= exp < p``, the number is formatted | + | | with presentation type ``'f'`` and precision | + | | ``p-1-exp``. Otherwise, the number is formatted | + | | with presentation type ``'e'`` and precision ``p-1``. | + | | In both cases insignificant trailing zeros are removed | + | | from the significand, and the decimal point is also | + | | removed if there are no remaining digits following it. | + | | | + | | Positive and negative infinity, positive and negative | + | | zero, and nans, are formatted as ``inf``, ``-inf``, | + | | ``0``, ``-0`` and ``nan`` respectively, regardless of | + | | the precision. | + | | | + +---------+----------------------------------------------------------+ + +The available presentation types for pointers are: + ++---------+----------------------------------------------------------+ +| Type | Meaning | ++=========+==========================================================+ +| ``'p'`` | Pointer format. This is the default type for | +| | pointers and may be omitted. | ++---------+----------------------------------------------------------+ +| none | The same as ``'p'``. | ++---------+----------------------------------------------------------+ + +.. _formatexamples: + +Format examples +=============== + +This section contains examples of the format syntax and comparison with +the printf formatting. + +In most of the cases the syntax is similar to the printf formatting, with the +addition of the ``{}`` and with ``:`` used instead of ``%``. +For example, ``"%03.2f"`` can be translated to ``"{:03.2f}"``. + +The new format syntax also supports new and different options, shown in the +following examples. + +Accessing arguments by position:: + + format("{0}, {1}, {2}", 'a', 'b', 'c'); + // Result: "a, b, c" + format("{}, {}, {}", 'a', 'b', 'c'); + // Result: "a, b, c" + format("{2}, {1}, {0}", 'a', 'b', 'c'); + // Result: "c, b, a" + format("{0}{1}{0}", "abra", "cad"); // arguments' indices can be repeated + // Result: "abracadabra" + +Aligning the text and specifying a width:: + + format("{:<30}", "left aligned"); + // Result: "left aligned " + format("{:>30}", "right aligned"); + // Result: " right aligned" + format("{:^30}", "centered"); + // Result: " centered " + format("{:*^30}", "centered"); // use '*' as a fill char + // Result: "***********centered***********" + +Dynamic width:: + + format("{:<{}}", "left aligned", 30); + // Result: "left aligned " + +Dynamic precision:: + + format("{:.{}f}", 3.14, 1); + // Result: "3.1" + +Replacing ``%+f``, ``%-f``, and ``% f`` and specifying a sign:: + + format("{:+f}; {:+f}", 3.14, -3.14); // show it always + // Result: "+3.140000; -3.140000" + format("{: f}; {: f}", 3.14, -3.14); // show a space for positive numbers + // Result: " 3.140000; -3.140000" + format("{:-f}; {:-f}", 3.14, -3.14); // show only the minus -- same as '{:f}; {:f}' + // Result: "3.140000; -3.140000" + +Replacing ``%x`` and ``%o`` and converting the value to different bases:: + + format("int: {0:d}; hex: {0:x}; oct: {0:o}; bin: {0:b}", 42); + // Result: "int: 42; hex: 2a; oct: 52; bin: 101010" + // with 0x or 0 or 0b as prefix: + format("int: {0:d}; hex: {0:#x}; oct: {0:#o}; bin: {0:#b}", 42); + // Result: "int: 42; hex: 0x2a; oct: 052; bin: 0b101010" + +Padded hex byte with prefix and always prints both hex characters:: + + format("{:#04x}", 0); + // Result: "0x00" + +.. ifconfig:: False + + Using the comma as a thousands separator:: + + format("{:,}", 1234567890); + '1,234,567,890' + + Using type-specific formatting:: + + >>> import datetime + >>> d = datetime.datetime(2010, 7, 4, 12, 15, 58) + Format("{:%Y-%m-%d %H:%M:%S}") << d) + '2010-07-04 12:15:58' + + Nesting arguments and more complex examples:: + + >>> for align, text in zip('<^>', ['left', 'center', 'right']): + ... '{0:{fill}{align}16}") << text, fill=align, align=align) + ... + 'left<<<<<<<<<<<<' + '^^^^^center^^^^^' + '>>>>>>>>>>>right' + >>> + >>> octets = [192, 168, 0, 1] + Format("{:02X}{:02X}{:02X}{:02X}") << *octets) + 'C0A80001' + >>> int(_, 16) + 3232235521 + >>> + >>> width = 5 + >>> for num in range(5,12): + ... for base in 'dXob': + ... print('{0:{width}{base}}") << num, base=base, width=width), end=' ') + ... print() + ... + 5 5 5 101 + 6 6 6 110 + 7 7 7 111 + 8 8 10 1000 + 9 9 11 1001 + 10 A 12 1010 + 11 B 13 1011 + |