format

It is a package for formatting Dart strings. It contains: the format function and the extension methods of the String class: format and print.

Function format similar to format method in Python, std::format function from C++20, which in turn became the development of the popular function sprintf from C. Its essence is that instead of templates enclosed in curly braces {}, substitute the values of the passed arguments, formatting them as required.

String result = '{...}'.format(...);

Usage example

Hello world:

format('{}', 'hello world'); // hello world

Values as function arguments (max 10 values):

format('{} {}', 'hello', 'world'); // hello world

Values as positional arguments (unlimited number of values):

format('{} {}', ['hello', 'world']);

Manual numbering of positional arguments:

format('{0} {1}', 'hello', 'world'); // hello world
format('{1} {0}', 'hello', 'world'); // world hello

Values as named arguments:

format('{h} {w}', {'h': 'hello', 'w': 'world'}); // hello world
format('{w} {h}', {#h: 'hello', #w: 'world'}); // world hello

Number formatting:

print('| Name     |    Price |    Count |      Sum |');
print(
  '| {name:8s} | {price:8.2f} | {count:8d} | {sum:8.2f} |'.format({
    #name: 'Apple',
    #price: 1.2,
    #count: 9,
    #sum: 10.8,
  }),
);

// | Name     |    Price |    Count |      Sum |
// | Apple    |     1.20 |        9 |    10.80 |

Unicode:

format('{:🇺🇦^12}', ' No war '); // 🇺🇦🇺🇦 No war 🇺🇦🇺🇦

Locale:

Intl.defaultLocale = 'uk_UA';
format('{:,.8n}', 123456.789); // 123 456,79

Intl.defaultLocale = 'bn';
format('{:,.8n}', 123456.789); // ১,২৩,৪৫৬.৭৯

Differences from Python

  • Python supports only one type of argument numbering at a time: either automatic numbering ({} {} {}) or manual numbering ({0} {1} {2}). I've added the ability to use both types of numbering at the same time:

    format('{0} {}->1 {}->2 {5} {}->6'); // 0 1 2 5 6

  • Named parameters can be specified in national languages according to the same principle as Latin characters: the identifier must start with a letter or an underscore and then digits can be added (Unicode is used). Parameter names can be specified in quotes (both single and double). There are no restrictions in this case. Quotation marks inside names must be double-quoted to avoid being perceived as the end of the identifier.

    format('{가격}', {'가격': 123.45});
format("{'It''s a name'}", {"It's a name": 'Name'});

  • The alternative format for X outputs 0x instead of 0X (0xABCDEF instead of 0XABCDEF).

    format('{0:#x} {0:#X}', 0xabcdef); // 0xabcdef 0xABCDEF

  • I did not support an alternative format for b (0b...) and o (0o...), since Dart does not support such literals. Let me know if you need it.

  • nan and inf are not complemented with zeros when the zero flag is set (as Python does, but msvc:spintf does not). sign does not work for nan (nan cannot become +nan) (Python and msvc:sprintf output +nan). In short, neither nan nor inf changes in any way. Only the width alignment works. And this is done deliberately. Nobody, as I think, needs +nan, 000nan and 000inf.

  • In g and n formats, the default precision is 6.

  • To support surrogate pairs and other combinations of Unicode characters, fill can accept any number of characters, as long as it ends with one of the align characters (>, <, ^).

  • Not supported in named arguments .key and index. That's an interesting solution. I like it. But I didn't.

  • In exponential notation (e and g) in Python, the exponent is returned with at least two digits. I have one.

  • In n format, E5 is output instead of e+05. This is how NumberFormat works by default.

  • Does not support = in align.

  • Doesn't support %.

  • {} only supports width and precision, while Python supports {} anywhere to form a pattern. But this can easily be replaced with ${}.

Libraries

format