Lua-fmt is a fmt like simple C module for Lua to format string.

The format syntax is same with fmtlib, which is same with Python's format routine.

The simplest way to install it is using Luarocks:

luarocks install --server=https://luarocks.org/dev lua-fmt

Or, just compile the single C file:

# Linux gcc -o lfmt.so -O3 -fPIC -shared lfmt.c # macOS gcc -o lfmt.so -O3 -fPIC -shared -undefined dynamic_lookup lfmt.c # Windows cl /Fefmt.dll /LD /MT /O2 /DLUA_BUILD_AS_DLL /Ipath/to/lua/include lfmt.c path/to/lua/lib

local fmt = require "fmt" -- automatic indexed argument print(fmt("{} {} {}", 1,2, 3)) --> "1 2 3" -- manual indexed argument print(fmt("{2}, {1}", "World", "Hello")) --> "Hello, World" -- named indexed argument print(fmt("{name} is {type}", { name = "foo", type = "bar" })) --> "foo is bar" print(fmt("{t.name} is {t.type}", {t = { name = "foo", type = "bar" }})) --> "foo is bar" -- format specifier print(fmt("{:b}", 42)) --> "101010" 

Mostly same as fmtlib.

Format strings contain “replacement fields” surrounded by curly braces {}. Anything that is not contained in braces is considered literal text, which is copied unchanged to the output. If you need to include a brace character in the literal text, it can be escaped by doubling: {{ and }}.

The grammar for a replacement field is as follows:

replacement_field ::= "{" [arg_id] [":" format_spec] "}" arg_id ::= field_name accessor* field_name ::= integer | identifier accessor ::= "." field_name | "[" key_name "]" key_name ::= field_name | <any chars except ']'>* integer ::= digit+ digit ::= "0"..."9" identifier ::= id_start id_continue* id_start ::= "a"..."z" | "A"..."Z" | "_" id_continue ::= id_start | digit 

the mainly difference is the support of accessor, which is supported by Python but not by fmtlib.

“Format specifications” are used within replacement fields contained within a format string to define how individual values are presented. Each formattable type may define how the format specification is to be interpreted.

Most built-in types implement the following options for format specifications, although some of the formatting options are only supported by the numeric types.

The general form of a standard format specifier is:

format_spec ::= [[fill]align][sign]["#"]["0"][width][grouping]["." precision][type] fill ::= <a character other than '{' or '}'> align ::= "<" | ">" | "^" sign ::= "+" | "-" | " " width ::= integer | "{" [arg_id] "}" grouping ::= "_" | "," precision ::= integer | "{" [arg_id] "}" type ::= int_type | flt_type | str_type int_type ::= "b" | "B" | "d" | "o" | "x" | "X" | "c" flt_type ::= "e" | "E" | "f" | "F" | "g" | "G" | "%" str_type ::= "p" | "s" 

Differences (all exists in Python):

• add grouping support for int_type: "{:_}" e.g. "10_000"
• add "%" specifier for float type: "{:%}" e.g. "100.0%"

Lua type vs. type specifiers: