format

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

Usage example

import 'package:format/format.dart';
import 'package:intl/intl.dart';

void main() {
  print(format('{}', 'hello world')); // "hello world"
  print('{}'.format('hello world')); // "hello world"
  '{}'.print('hello world'); // "hello world"

  '{} {}'.print('hello', 'world'); // "hello world" (max 10 values)
  '{} {}'
      .print(['hello', 'world']); // "hello world" (unlimited number of values)
  '{0} {1}'.print('hello', 'world'); // "hello world"
  '{1} {0}'.print('hello', 'world'); // "world hello"
  '{h} {w}'.print({'h': 'hello', 'w': 'world'}); // "hello world"
  '{"it\'s hello"} {"it\'s world"}'
      .print({"it's hello": 'hello', "it's world": 'world'}); // "hello world"

  '{:d}'.print(123); // "123"
  '{:7d}'.print(123); // "    123"
  '{:<7d}'.print(123); // "123    "
  '{:^7d}'.print(123); // "  123  "
  '{:*^7d}'.print(123); // "**123**"

  '{:07d}'.print(123); // "0000123"
  '{:09,d}'.print(123); // "0,000,123"
  '{:09_d}'.print(123); // "0_000_123"

  '{:0{},d}'.print(123, 9); // "0,000,123"
  '{:0{},d}'.print(123, 11); // "000,000,123"
  '{value:0{width},d}'.print({'value': 123, 'width': 13}); // "0,000,000,123"

  '{:+d}'.print(123); // "+123"
  '{: d}'.print(123); // " 123"

  // Automatic type inference.
  '{}'.print(123); // "123"
  '{}'.print('aaa'); // "aaa"
  '{:7}'.print(123); // "    123"
  '{:7}'.print('aaa'); // "aaa    "

  const n = 123.4567;
  '{:.2f}'.print(n); // 123.46
  '{:10.2f}'.print(n); // '    123.46'
  '{:010.2f}'.print(n); // 0000123.46
  '{:012,.2f}'.print(n); // 0,000,123.46
  '{:012_.2f}'.print(n); // 0_000_123.46

  '{:0{},.{}f}'.print(n, 12, 2); // 0,000,123.46
  '{value:0{width},.{precision}f}'.print({
    'value': n,
    'width': 12,
    'precision': 2,
  }); // 0,000,123.46

  const n1 = 123456.789;
  const n2 = 1234567.89;
  '{:g}'.print(n1); // 123457
  '{:g}'.print(n2); // 1.23457e+6
  '{:.9g}'.print(n1); // 123456.789
  '{:.9g}'.print(n2); // 1234567.89
  '{:.5g}'.print(n1); // 1.2346e+5
  '{:.5g}'.print(n2); // 1.2346e+6

  '{:g}'.print(double.nan); // nan
  '{:g}'.print(double.infinity); // inf
  '{:g}'.print(double.negativeInfinity); // -inf

  const i = 12345678;
  '{:b}'.print(i); // 101111000110000101001110
  '{:d}'.print(i); // 12345678
  '{:x}'.print(i); // bc614e
  '{:X}'.print(i); // BC614E
  '{:#x}'.print(i); // 0xbc614e
  '{:#X}'.print(i); // 0xBC614E

  '{:_b}'.print(i); // 1011_1100_0110_0001_0100_1110
  '{:,d}'.print(i); // 12,345,678
  '{:_d}'.print(i); // 12_345_678
  '{:_x}'.print(i); // bc_614e
  '{:_X}'.print(i); // BC_614E
  '{:#_x}'.print(i); // 0xbc_614e
  '{:#_X}'.print(i); // 0xBC_614E

  '{:c}+{:c}+{:c}+{:c}={:c}'.print(
    0x1F468, // 👨
    0x1F469, // 👩
    0x1F466, // 👦
    0x1F467, // 👧
    [0x1F468, 0x200D, 0x1F469, 0x200D, 0x1F466, 0x200D, 0x1F467], // 👨‍👩‍👦‍👧
  ); // 👨+👩+👦+👧=👨‍👩‍👦‍👧

  '{:👨>10}'.print('!'); // 👨👨👨👨👨👨👨👨👨!
  '{:👨‍👩‍👦‍👧>10}'.print(
      '!'); // 👨‍👩‍👦‍👧👨‍👩‍👦‍👧👨‍👩‍👦‍👧👨‍👩‍👦‍👧👨‍👩‍👦‍👧👨‍👩‍👦‍👧👨‍👩‍👦‍👧👨‍👩‍👦‍👧👨‍👩‍👦‍👧!
  '{:ä>10}'.print('!'); // äääääääää!

  const m = 12345678.9;
  Intl.defaultLocale = 'ru_RU';
  '{:n}'.print(m); // 1,23457E7
  '{:.9n}'.print(m); // 12345678,9
  '{:012,.9n}'.print(m); // 12 345 678,9
  '{:n}'.print(double.nan); // не число
  '{:n}'.print(double.infinity); // ∞
  '{:n}'.print(double.negativeInfinity); // -∞

  Intl.defaultLocale = 'de_DE';
  '{:n}'.print(m); // 1,23457E7
  '{:.9n}'.print(m); // 12345678,9
  '{:012,.9n}'.print(m); // 12.345.678,9

  Intl.defaultLocale = 'en_IN';
  '{:n}'.print(m); // 1.23457E7
  '{:.9n}'.print(m); // 12345678.9
  '{:012,.9n}'.print(m); // 1,23,45,678.9

  Intl.defaultLocale = 'bn';
  '{:n}'.print(m); // ১.২৩৪৫৭E৭
  '{:.9n}'.print(m); // ১২৩৪৫৬৭৮.৯
  '{:012,.9n}'.print(m); // ১,২৩,৪৫,৬৭৮.৯

  Intl.defaultLocale = 'ar_EG';
  '{:n}'.print(m); // ١٫٢٣٤٥٧اس٧
  '{:.9n}'.print(m); // ١٢٣٤٥٦٧٨٫٩
  '{:012,.9n}'.print(m); // ١٢٬٣٤٥٬٦٧٨٫٩
  '{:n}'.print(double.nan); // ليس رقم

  '{:👨^5}'.print(':'); // 👨👨:👨👨
  '{:👨‍👩‍👦‍👧^5}'
      .print(':'); // 👨‍👩‍👦‍👧👨‍👩‍👦‍👧:👨‍👩‍👦‍👧👨‍👩‍👦‍👧
}

Description

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(...);

Template description

template            ::=  '{' [argId] [':' formatSpec] '}'
argId               ::=  index | identifier | doubleQuotedString | singleQuotedString
index               ::=  digit+
identifier          ::=  idStart idContinue*
idStart             ::=  '_' | letter
idContinue          ::=  '_' | letter | digit
letter              ::=  <any letter of any language> (\ p {Letter})
doubleQuotedString  ::=  '"' <any characters, with replacement" for ""> "'"
singleQuotedString  ::=  "'" <any characters, replacing' with '>' "'
arg_name            ::=  [identifier | digit+]
attribute_name      ::=  identifier
formatSpec          ::=  <in the next section>

Format string syntax

formatSpec      ::=  [[fill]align][sign][#][0][width][grouping_option][.precision][type]
fill            ::=  <any characters>
align           ::=  '<' | '>' | '^'
sign            ::=  '+' | '-' | ' '
width           ::=  digit+ | '{' argId '}'
groupingOption  ::=  '_' | ','
precision       ::=  digit+ | '{' argId '}'
type            ::=  'b' | 'c' | 'd' | 'e' | 'E' | 'f' | 'F' | 'g' | 'G' | 'n' | 'o' | 's' | 'x' | 'X'

fill и align

If align is specified, then the fill line can be anything a combination of characters, while only one character is allowed in Python. This is made in order to be able to use symbols consisting of several Unicode characters (accented characters, surrogate pairs, etc.), and at the same time do not engage in unnecessary analysis.

Python imposes only one constraint on the fill character - not allowed use curly braces {} because they are used to insert values inside the template. In Dart, there is no need for such a function, since he has built into the language is excellent string interpolation.

Possible align values:

Value	Description
'<'	Left justification (this is the default for strings and characters).
'>'	Right justification (this is the default for numbers).
'^'	Center alignment.

Of course, align only matters when a minimum width is given the field is width, and it is larger than the actual width of the field. In this case, the result is is complemented to the specified one from the corresponding side.

Python also includes the '=' value, through which you can insert any character instead of leading zeros between the number sign ('+' or '-') and the first significant digits ('+ 123', '- 42'). '0' before width acts the same principle. I didn’t take this opportunity. First, because I am not I can understand the reasons why this is necessary. If the task was to replace leading zeros any character, then this is not entirely true, because the option of arranging separators thousand (',') is not included if a character other than '0' is specified. Those. Python distinguishes zeros from other characters in this function. Secondly, because of this features all these strange '00000nan', '-0000inf' appear. Although not it is clear why exactly in these cases no exception was made for zeros.

sign

With this function, you can specify how to handle the sign of a number.

Value	Description
'-'	The sign is placed only for negative numbers (this is the default value).
'+'	Positive numbers also have a sign (zero also comes with a plus).
''	Positive numbers have a space character instead of '+'. This can be useful for aligning positive and negative numbers with each other.

...to be continued