#format #string #str #dynamic #static


strfmt: rust library for formatting dynamic strings

12 releases

Uses old Rust 2015

0.2.4 Feb 15, 2023
0.2.3 Feb 1, 2023
0.2.2 Aug 10, 2022
0.2.1 Jul 29, 2022
0.1.3 May 27, 2016

#72 in Rust patterns

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Used in 64 crates (46 directly)

MIT license

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strfmt: rust library for formatting dynamic strings

This library is for rust developers who want to bring rust-like formatting to non-static strings.

Basic use of formatting Display types

extern crate strfmt;
use strfmt::strfmt;
use std::collections::HashMap;

fn it_works() {
    let mut vars = HashMap::new();
    vars.insert("name".to_string(), "bob");
    vars.insert("job".to_string(), "python developer");

    let fmt = "hi, my name is {name} and I am a {job}!".to_string();
      strfmt(&fmt, &vars).unwrap(),
      "hi, my name is bob and I am a python developer!")

In addition to the strfmt function, this library has the Format trait which adds the format method to str and String types.

assert_eq!("hi, my name is {name}".format(&vars), "hi, my name is bob")

You can use this library any time you have dynamic strings you want to format, such as if you are providing your users a ui or are reading configuration files.

strfmt does not support empty identifiers (i.e. {} or {:<10}. Integer identifiers will be read as str keys to the hashmap (i.e. {1:<10} will have key == "1")


In the 0.2.0 update, the signature of strfmt and Format::format changed to fix a bug with numeric formatting. For easy migration the strfmt_display and Format::format_display function provide the old behaviour.

BETA: Formatting numeric types

This feature is in Beta and may change. I expect it to be fairly stable at this point but would appreciate feedback on development.

In addition, "signed 0 padding" (i.e. +000042) is not yet supported for numeric types

Using strfmt_map it is also possible to format integers and floats:

let mut vars: HashMap<String, f64> = HashMap::new();
vars.insert("x".to_string(), 42.4242);
vars.insert("y".to_string(), -100.11111);
vars.insert("z".to_string(), 0.);

let f = |mut fmt: Formatter| {
assert_eq!(strfmt_map("{x:<7.2}", f).unwrap(), "42.42  ");
assert_eq!(strfmt_map("{y:+.2E}", f).unwrap(), "-1.00E2");
assert_eq!(strfmt_map("{z:+.2E}", f).unwrap(), "+0.00E0");

Status and Goals

strfmt aims to support all of the formatting options defined in std::fmt. Currently it officially only supports the format options for strings (beta support for i64 and f64)

See the syntax for how to create a formatted string

Current Status (in order of priority)

  • get strfmt_map out of Beta and create Format.format_map method
  • handle sign aware zero padding for numeric types
  • format any Display type
  • stabilize strfmt_map and add format_map to the Format trait.
  • add f64 method to Formatter allowing those using strfmt_map to format f64s according to the spec
  • add i64 method to Formatter allowing those using strfmt_map to format i64s according to the spec
  • use DisplayStr trait for formatting, permitting proper formatting of integer types.
  • look for a rust library has "unbounded float" (like python) and add that to the formatter
  • look for a rust library has "unbounded integer" (like python) and add that to the formatter
  • Implement vec method to Formatter allowing those usin strfmt_map to format types of Vec<Display> in a way that uses precision and width (precision will limit the number of elements displayed, width the width of each element)
  • special suppport to format HashMap<String, String> for improved speed
  • special suppport to format HashMap<String, &str> for improved speed
  • special suppport to format HashMap<&str, &str> for improved speed


I (@vitiral) am no longer an active maintainer of this library or any rust code, but I accept pull requests that fix bugs or implement the above features. All pull requests must be tested appropriately.

Adding functionality should be fairly easy, the main piece of work is checking and handling the flags correctly and creating comprehensive tests. Hopefully I will be creating the f64 method soon to show how it can be done, but I could really use all the help I can get on making this libray complete.

No runtime deps