[Python-3000] String formatting: Conversion specifiers

[Talin]

Nick Coghlan wrote:
> Talin wrote:
> 
>> So I decided to sit down and rethink the whole conversion specifier 
>> system. I looked at the docs for the '%' operator, and some other 
>> languages, and here is what I came up with (this is an excerpt from 
>> the revised PEP.)
> 
> 
> Generally nice, but I'd format the writeup a bit differently (see below) 
> and reorder the elements so that an arbitrary character can be supplied 
> as the fill character and the old ' ' sign flag behaviour remains 
> available.

Looks good - thanks for the feedback.

My only comment is that I think that I would still like to have the sign 
field before the width. I'm pretty sure that this can be parsed 
unambiguously.

> I'd also design it so that the standard conversion specifiers are 
> available 'for free' (i.e., they work for any class, unless the class 
> author deliberately replaces them with something else).
> 
> Cheers,
> Nick.
> 
> --------------------------------
> 
> Standard Conversion Specifiers
> 
>      If an object does not define its own conversion specifiers, a standard
>      set of conversion specifiers are used. These are similar in concept to
>      the conversion specifiers used by the existing '%' operator, however
>      there are also a number of significant differences. The standard
>      conversion specifiers fall into three major categories: string
>      conversions, integer conversions and floating point conversions.
> 
>      The general form of a string conversion specifier is:
> 
>          [[fill][align]width][type]
> 
>      The brackets ([]) indicate an optional field.
> 
>      'width' is a decimal integer defining the minimum field width.
>      If not specified, then the field width will be determined by
>      the content.
> 
>      If the minimum field width is defined, then the optional align
>      flag can be one of the following:
> 
>          '<' - Forces the field to be left-aligned within the available
>                space (This is the default.)
>          '>' - Forces the field to be right-aligned within the
>                available space.
> 
>      The optional 'fill' character defines the character to be used to
>      pad the field to the minimum width. The alignment flag must be
>      supplied if the character is a number other than 0 (otherwise the
>      character would be interpreted as part of the field width specifier).
> 
>      Finally, the 'type' determines how the data should be presented.
> 
>      The available string conversion types are:
> 
>          's' - String format. Invokes str() on the object.
>                This is the default conversion specifier type.
>          'r' - Repr format. Invokes repr() on the object.
> 
> 
>      The general form of an integer conversion specifier is:
> 
>          [[fill][align]width][sign]type
> 
>      The 'fill', 'align' and 'width' fields are as for string conversion
>      specifiers.
> 
>      The 'sign' field can be one of the following:
> 
>          '+'  - 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 behaviour)
>          ' '  - indicates that a leading space should be used on
>                 positive numbers
>          '()' - indicates that negative numbers should be surrounded
>                 by parentheses
> 
>      There are several integer conversion types. All invoke int() on the
>      object before attempting to format it.
> 
>      The available integer conversion types are:
> 
>          'b' - Binary. Outputs the number in base 2.
>          'c' - Character. Converts the integer to the corresponding
>                unicode character before printing.
>          '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.
>          'X' - Hex format. Outputs the number in base 16, using upper-
>                case letters for the digits above 9.
> 
>      The general form of a floating point conversion specifier is:
> 
>          [[fill][align]width][.precision][sign]type
> 
>      The 'fill', 'align', 'width' and 'sign' fields are as for
>      integer conversion specifiers.
> 
>      The 'precision' field is a decimal number indicating how many digits
>      should be displayed after the decimal point.
> 
>      There are several floating point conversion types. All invoke 
> float() on
>      the object before attempting to format it.
> 
>      The available floating point conversion types are:
> 
>          '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'.
>          'g' - General format. This prints the number as a fixed-point
>                number, unless the number is too large, in which case
>                it switches to 'e' exponent notation.
>          'G' - General format. Same as 'g' except switches to 'E'
>                if the number gets to large.
>          'n' - Number. This is the same as 'g', except that it uses the
>                current locale setting to insert the appropriate
>                number separator characters.
>          '%' - Percentage. Multiplies the number by 100 and displays
>                in fixed ('f') format, followed by a percent sign.
> 
>      Objects are able to define their own conversion specifiers to replace
>      the standard ones.  An example is the 'datetime' class, whose
>      conversion specifiers might look something like the arguments
>      to the strftime() function:
> 
>          "Today is: {0:a b d H:M:S Y}".format(datetime.now())
> 
> 
>