394 lines
19 KiB
ReStructuredText
394 lines
19 KiB
ReStructuredText
.. highlight:: c++
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.. _syntax:
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********************
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Format String Syntax
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********************
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Formatting functions such as :ref:`fmt::format() <format>` and :ref:`fmt::print() <print>`
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use the same format string syntax described in this section.
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Format strings contain "replacement fields" surrounded by curly braces ``{}``.
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Anything that is not contained in braces is considered literal text, which is
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copied unchanged to the output. If you need to include a brace character in the
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literal text, it can be escaped by doubling: ``{{`` and ``}}``.
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The grammar for a replacement field is as follows:
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.. productionlist:: sf
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replacement_field: "{" [`arg_index`] [":" `format_spec`] "}"
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arg_index: `integer`
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In less formal terms, the replacement field can start with an *arg_index*
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that specifies the argument whose value is to be formatted and inserted into
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the output instead of the replacement field.
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The *arg_index* is optionally followed by a *format_spec*, which is preceded
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by a colon ``':'``. These specify a non-default format for the replacement value.
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See also the :ref:`formatspec` section.
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If the numerical arg_indexes in a format string are 0, 1, 2, ... in sequence,
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they can all be omitted (not just some) and the numbers 0, 1, 2, ... will be
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automatically inserted in that order.
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Some simple format string examples::
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"First, thou shalt count to {0}" // References the first argument
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"Bring me a {}" // Implicitly references the first argument
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"From {} to {}" // Same as "From {0} to {1}"
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The *format_spec* field contains a specification of how the value should be
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presented, including such details as field width, alignment, padding, decimal
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precision and so on. Each value type can define its own "formatting
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mini-language" or interpretation of the *format_spec*.
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Most built-in types support a common formatting mini-language, which is
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described in the next section.
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A *format_spec* field can also include nested replacement fields within it.
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These nested replacement fields can contain only an argument index;
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format specifications are not allowed. Formatting is performed as if the
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replacement fields within the format_spec are substituted before the
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*format_spec* string is interpreted. This allows the formatting of a value
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to be dynamically specified.
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See the :ref:`formatexamples` section for some examples.
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.. _formatspec:
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Format Specification Mini-Language
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==================================
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"Format specifications" are used within replacement fields contained within a
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format string to define how individual values are presented (see
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:ref:`formatstrings`). Each formattable type may define how the format
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specification is to be interpreted.
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Most built-in types implement the following options for format specifications,
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although some of the formatting options are only supported by the numeric types.
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The general form of a *standard format specifier* is:
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.. productionlist:: sf
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format_spec: [[`fill`]`align`][`sign`]["#"]["0"][`width`]["." `precision`][`type`]
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fill: <a character other than '{' or '}'>
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align: "<" | ">" | "=" | "^"
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sign: "+" | "-" | " "
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width: `integer`
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precision: `integer` | "{" `arg_index` "}"
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type: `int_type` | "c" | "e" | "E" | "f" | "F" | "g" | "G" | "p" | "s"
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int_type: "b" | "B" | "d" | "o" | "x" | "X"
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The *fill* character can be any character other than '{' or '}'. The presence
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of a fill character is signaled by the character following it, which must be
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one of the alignment options. If the second character of *format_spec* is not
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a valid alignment option, then it is assumed that both the fill character and
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the alignment option are absent.
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The meaning of the various alignment options is as follows:
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+---------+----------------------------------------------------------+
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| Option | Meaning |
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+=========+==========================================================+
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| ``'<'`` | Forces the field to be left-aligned within the available |
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| | space (this is the default for most objects). |
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+---------+----------------------------------------------------------+
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| ``'>'`` | Forces the field to be right-aligned within the |
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| | available space (this is the default for numbers). |
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+---------+----------------------------------------------------------+
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| ``'='`` | Forces the padding to be placed after the sign (if any) |
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| | but before the digits. This is used for printing fields |
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| | in the form '+000000120'. This alignment option is only |
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| | valid for numeric types. |
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+---------+----------------------------------------------------------+
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| ``'^'`` | Forces the field to be centered within the available |
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| | space. |
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+---------+----------------------------------------------------------+
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Note that unless a minimum field width is defined, the field width will always
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be the same size as the data to fill it, so that the alignment option has no
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meaning in this case.
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The *sign* option is only valid for number types, and can be one of the
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following:
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+---------+----------------------------------------------------------+
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| Option | Meaning |
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+=========+==========================================================+
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| ``'+'`` | indicates that a sign should be used for both |
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| | positive as well as negative numbers. |
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+---------+----------------------------------------------------------+
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| ``'-'`` | indicates that a sign should be used only for negative |
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| | numbers (this is the default behavior). |
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+---------+----------------------------------------------------------+
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| space | indicates that a leading space should be used on |
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| | positive numbers, and a minus sign on negative numbers. |
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+---------+----------------------------------------------------------+
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The ``'#'`` option causes the "alternate form" to be used for the
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conversion. The alternate form is defined differently for different
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types. This option is only valid for integer and floating-point types.
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For integers, when binary, octal, or hexadecimal output is used, this
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option adds the prefix respective ``"0b"`` (``"0B"``), ``"0"``, or
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``"0x"`` (``"0X"``) to the output value. Whether the prefix is
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lower-case or upper-case is determined by the case of the type
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specifier, for example, the prefix ``"0x"`` is used for the type ``'x'``
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and ``"0X"`` is used for ``'X'``. For floating-point numbers the
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alternate form causes the result of the conversion to always contain a
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decimal-point character, even if no digits follow it. Normally, a
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decimal-point character appears in the result of these conversions
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only if a digit follows it. In addition, for ``'g'`` and ``'G'``
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conversions, trailing zeros are not removed from the result.
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.. ifconfig:: False
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The ``','`` option signals the use of a comma for a thousands separator.
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For a locale aware separator, use the ``'n'`` integer presentation type
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instead.
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*width* is a decimal integer defining the minimum field width. If not
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specified, then the field width will be determined by the content.
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Preceding the *width* field by a zero (``'0'``) character enables
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sign-aware zero-padding for numeric types. This is equivalent to a *fill*
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character of ``'0'`` with an *alignment* type of ``'='``.
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The *precision* is a decimal number indicating how many digits should be
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displayed after the decimal point for a floating-point value formatted with
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``'f'`` and ``'F'``, or before and after the decimal point for a floating-point
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value formatted with ``'g'`` or ``'G'``. For non-number types the field
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indicates the maximum field size - in other words, how many characters will be
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used from the field content. The *precision* is not allowed for integer values.
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Finally, the *type* determines how the data should be presented.
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The available string presentation types are:
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+---------+----------------------------------------------------------+
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| Type | Meaning |
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+=========+==========================================================+
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| ``'s'`` | String format. This is the default type for strings and |
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| | may be omitted. |
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+---------+----------------------------------------------------------+
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| none | The same as ``'s'``. |
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+---------+----------------------------------------------------------+
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The available character presentation types are:
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+---------+----------------------------------------------------------+
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| Type | Meaning |
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+=========+==========================================================+
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| ``'c'`` | Character format. This is the default type for |
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| | characters and may be omitted. |
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+---------+----------------------------------------------------------+
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| none | The same as ``'c'``. |
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+---------+----------------------------------------------------------+
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The available integer presentation types are:
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+---------+----------------------------------------------------------+
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| Type | Meaning |
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+=========+==========================================================+
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| ``'b'`` | Binary format. Outputs the number in base 2. Using the |
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| | ``'#'`` option with this type adds the prefix ``"0b"`` |
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| | to the output value. |
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+---------+----------------------------------------------------------+
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| ``'B'`` | Binary format. Outputs the number in base 2. Using the |
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| | ``'#'`` option with this type adds the prefix ``"0B"`` |
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| | to the output value. |
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+---------+----------------------------------------------------------+
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| ``'d'`` | Decimal integer. Outputs the number in base 10. |
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+---------+----------------------------------------------------------+
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| ``'o'`` | Octal format. Outputs the number in base 8. |
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+---------+----------------------------------------------------------+
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| ``'x'`` | Hex format. Outputs the number in base 16, using |
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| | lower-case letters for the digits above 9. Using the |
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| | ``'#'`` option with this type adds the prefix ``"0x"`` |
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| | to the output value. |
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+---------+----------------------------------------------------------+
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| ``'X'`` | Hex format. Outputs the number in base 16, using |
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| | upper-case letters for the digits above 9. Using the |
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| | ``'#'`` option with this type adds the prefix ``"0X"`` |
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| | to the output value. |
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+---------+----------------------------------------------------------+
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| none | The same as ``'d'``. |
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+---------+----------------------------------------------------------+
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The available presentation types for floating-point values are:
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+---------+----------------------------------------------------------+
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| Type | Meaning |
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+=========+==========================================================+
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| ``'a'`` | Hexadecimal floating point format. Prints the number in |
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| | base 16 with prefix ``"0x"`` and lower-case letters for |
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| | digits above 9. Uses 'p' to indicate the exponent. |
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+---------+----------------------------------------------------------+
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| ``'A'`` | Same as ``'a'`` except it uses upper-case letters for |
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| | the prefix, digits above 9 and to indicate the exponent. |
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+---------+----------------------------------------------------------+
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| ``'e'`` | Exponent notation. Prints the number in scientific |
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| | notation using the letter 'e' to indicate the exponent. |
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+---------+----------------------------------------------------------+
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| ``'E'`` | Exponent notation. Same as ``'e'`` except it uses an |
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| | upper-case 'E' as the separator character. |
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+---------+----------------------------------------------------------+
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| ``'f'`` | Fixed point. Displays the number as a fixed-point |
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| | number. |
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+---------+----------------------------------------------------------+
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| ``'F'`` | Fixed point. Same as ``'f'``, but converts ``nan`` to |
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| | ``NAN`` and ``inf`` to ``INF``. |
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+---------+----------------------------------------------------------+
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| ``'g'`` | General format. For a given precision ``p >= 1``, |
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| | this rounds the number to ``p`` significant digits and |
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| | then formats the result in either fixed-point format |
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| | or in scientific notation, depending on its magnitude. |
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| | |
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| | A precision of ``0`` is treated as equivalent to a |
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| | precision of ``1``. |
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+---------+----------------------------------------------------------+
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| ``'G'`` | General format. Same as ``'g'`` except switches to |
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| | ``'E'`` if the number gets too large. The |
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| | representations of infinity and NaN are uppercased, too. |
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+---------+----------------------------------------------------------+
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| none | The same as ``'g'``. |
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+---------+----------------------------------------------------------+
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.. ifconfig:: False
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+---------+----------------------------------------------------------+
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| | The precise rules are as follows: suppose that the |
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| | result formatted with presentation type ``'e'`` and |
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| | precision ``p-1`` would have exponent ``exp``. Then |
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| | if ``-4 <= exp < p``, the number is formatted |
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| | with presentation type ``'f'`` and precision |
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| | ``p-1-exp``. Otherwise, the number is formatted |
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| | with presentation type ``'e'`` and precision ``p-1``. |
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| | In both cases insignificant trailing zeros are removed |
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| | from the significand, and the decimal point is also |
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| | removed if there are no remaining digits following it. |
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| | |
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| | Positive and negative infinity, positive and negative |
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| | zero, and nans, are formatted as ``inf``, ``-inf``, |
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| | ``0``, ``-0`` and ``nan`` respectively, regardless of |
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| | the precision. |
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| | |
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+---------+----------------------------------------------------------+
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The available presentation types for pointers are:
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+---------+----------------------------------------------------------+
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| Type | Meaning |
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+=========+==========================================================+
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| ``'p'`` | Pointer format. This is the default type for |
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| | pointers and may be omitted. |
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+---------+----------------------------------------------------------+
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| none | The same as ``'p'``. |
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+---------+----------------------------------------------------------+
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.. _formatexamples:
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Format examples
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===============
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This section contains examples of the format syntax and comparison with
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the printf formatting.
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In most of the cases the syntax is similar to the printf formatting, with the
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addition of the ``{}`` and with ``:`` used instead of ``%``.
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For example, ``"%03.2f"`` can be translated to ``"{:03.2f}"``.
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The new format syntax also supports new and different options, shown in the
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following examples.
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Accessing arguments by position::
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format("{0}, {1}, {2}", 'a', 'b', 'c');
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// Result: "a, b, c"
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format("{}, {}, {}", 'a', 'b', 'c');
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// Result: "a, b, c"
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format("{2}, {1}, {0}", 'a', 'b', 'c');
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// Result: "c, b, a"
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format("{0}{1}{0}", "abra", "cad"); // arguments' indices can be repeated
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// Result: "abracadabra"
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Aligning the text and specifying a width::
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format("{:<30}", "left aligned");
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// Result: "left aligned "
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format("{:>30}", "right aligned");
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// Result: " right aligned"
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format("{:^30}", "centered");
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// Result: " centered "
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format("{:*^30}", "centered"); // use '*' as a fill char
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// Result: "***********centered***********"
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Replacing ``%+f``, ``%-f``, and ``% f`` and specifying a sign::
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format("{:+f}; {:+f}", 3.14, -3.14); // show it always
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// Result: "+3.140000; -3.140000"
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format("{: f}; {: f}", 3.14, -3.14); // show a space for positive numbers
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// Result: " 3.140000; -3.140000"
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format("{:-f}; {:-f}", 3.14, -3.14); // show only the minus -- same as '{:f}; {:f}'
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// Result: "3.140000; -3.140000"
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Replacing ``%x`` and ``%o`` and converting the value to different bases::
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format("int: {0:d}; hex: {0:x}; oct: {0:o}; bin: {0:b}", 42);
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// Result: "int: 42; hex: 2a; oct: 52; bin: 101010"
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// with 0x or 0 or 0b as prefix:
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format("int: {0:d}; hex: {0:#x}; oct: {0:#o}; bin: {0:#b}", 42);
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// Result: "int: 42; hex: 0x2a; oct: 052; bin: 0b101010"
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.. ifconfig:: False
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Using the comma as a thousands separator::
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format("{:,}", 1234567890);
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'1,234,567,890'
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Expressing a percentage::
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>>> points = 19
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>>> total = 22
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Format("Correct answers: {:.2%}") << points/total)
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'Correct answers: 86.36%'
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Using type-specific formatting::
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>>> import datetime
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>>> d = datetime.datetime(2010, 7, 4, 12, 15, 58)
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Format("{:%Y-%m-%d %H:%M:%S}") << d)
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'2010-07-04 12:15:58'
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Nesting arguments and more complex examples::
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>>> for align, text in zip('<^>', ['left', 'center', 'right']):
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... '{0:{fill}{align}16}") << text, fill=align, align=align)
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...
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'left<<<<<<<<<<<<'
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'^^^^^center^^^^^'
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'>>>>>>>>>>>right'
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>>>
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>>> octets = [192, 168, 0, 1]
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Format("{:02X}{:02X}{:02X}{:02X}") << *octets)
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'C0A80001'
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>>> int(_, 16)
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3232235521
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>>>
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>>> width = 5
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>>> for num in range(5,12):
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... for base in 'dXob':
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... print('{0:{width}{base}}") << num, base=base, width=width), end=' ')
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... print()
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...
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5 5 5 101
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6 6 6 110
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7 7 7 111
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8 8 10 1000
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9 9 11 1001
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10 A 12 1010
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11 B 13 1011
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