Python-style format strings for Julia
Author bicycle1885
32 Stars
Updated Last
1 Year Ago
Started In
April 2021

This package is still under active development. The API may change anytime. Almost no error checks. Only handful basic types are supported.

Fmt.jl ― Python-style format strings for Julia

Fmt.jl provides a Python-style format language. It is an alternative of Printf.jl and string utility functions in Base. Formats are constructed by a non-standard string literal prefixed by f, called f-strings. In the following example, a part of an f-string surrounded by curly braces { } is replaced with a formatted floating-point number:

julia> using Fmt

julia> pi = float(π)

julia> f"π ≈ {$pi:.4f}"
"π ≈ 3.1416"

The goals of Fmt.jl are:

  • Full-fledged: It supports almost complete features of Python's format strings.
  • Performant: The formatter is much faster than string and other functions.
  • Lightweight: It has no dependencies except the Base library.


The @f_str macro (or f-string) is the only exported binding from the Fmt module. This macro can interpolate variables into a string with format specification. Interpolation happens inside replacement fields surrounded by a pair of curly braces {}; other parts of an f-string are treated as ordinal strings. A replacement field usually has an argument ARG and a specification SPEC separated by a colon: {ARG:SPEC}, although both of them can be omitted.

Let's see some examples.

# load @f_str
using Fmt

# default format
x = 42
f"x is {$x}." == "x is 42."

# binary, octal, decimal, and hexadecimal format
f"{$x:b}" == "101010"
f"{$x:o}" == "52"
f"{$x:d}" == "42"
f"{$x:x}" == "2a"
f"{$x:X}" == "2A"

# format with a minimum width
f"{$x:4}" == "  42"
f"{$x:6}" == "    42"

# left, center, and right alignment
f"{$x:<6}"  == "42    "
f"{$x:^6}"  == "  42  "
f"{$x:>6}"  == "    42"
f"{$x:*<6}" == "42****"
f"{$x:*^6}" == "**42**"
f"{$x:*>6}" == "****42"

# dynamic width
n = 6
f"{$x:<{$n}}" == "42    "
f"{$x:^{$n}}" == "  42  "
f"{$x:>{$n}}" == "    42"

# grouping digits with thousand separator
x = 1234567
f"{$x:,}" == "1,234,567"

In addition to f-strings, Fmt.jl provides two formatting functions:

  • Fmt.format(fstr, args...; kwargs...) creates a formatted string by applying args and kwargs to fstr.
  • Fmt.printf([out,] fstr, args...; kwargs...) prints a formatted string to out (default: stdout) by applying args and kwargs to fstr.

When using these functions, you cannot interpolate replacement fields with $. All replacement values are given as function arguments:

using Fmt

# positional arguments with implicit numbering
Fmt.format(f"{} and {}", "Alice", "Bob") == "Alice and Bob"

# positional arguments with explicit numbering
Fmt.format(f"{1} and {2}", "Alice", "Bob") == "Alice and Bob"
Fmt.format(f"{2} and {1}", "Alice", "Bob") == "Bob and Alice"

# keyword arguments
Fmt.format(f"{A} and {B}", A = "Alice", B = "Bob") == "Alice and Bob"
Fmt.format(f"{B} and {A}", A = "Alice", B = "Bob") == "Bob and Alice"

# box drawing example
┌{1:─^{2}}┐             ┌{1:─^{2}}┐
│{A: ^{2}}│ ──────────> │{B: ^{2}}│
└{1:─^{2}}┘             └{1:─^{2}}┘
""", "", 15, A = "Alice", B = "Bob")
# ┌───────────────┐             ┌───────────────┐
# │     Alice     │ ──────────> │      Bob      │
# └───────────────┘             └───────────────┘

The syntax of f-strings is borrowed from Python's Format String Syntax, which is ported to C++ as C++20 std::format and Rust as std::fmt. See the next sections for details of the syntax and semantic supported by Fmt.jl.


Each replacement field is surrounded by a pair of curly braces. To escape curly braces, double curly braces ({{ and }}) are interpreted as single curly braces ({ and }). Backslash-escaped characters are treated in the same way as in usual strings. However, dollar signs $ are no longer a special character for interpolation; that is, no interpolation happens outside replacement fields.

The syntax of a replacement field is formally defined as follows:

# replacement field
field      = '{'[argument]['/'conv][':'spec]'}'
argument   = number | ['$']identifier | '$('expression')'
number     = digit+
identifier = any valid identifier
expression = any valid expression
digit      = '0' | '1' | '2' | … | '9'
conv       = 's' | 'r'

# format specification
spec       = [[fill]align][sign][altform][zero][width][grouping]['.'precision][type]
fill       = any valid character (except '{' and '}') | '{'[argument]'}'
align      = '<' | '^' | '>'
sign       = '+' | '-' | ' '
altform    = '#'
zero       = '0'
width      = digit+ | '{'[argument]'}'
grouping   = ',' | '_'
precision  = digit+ | '{'[argument]'}'
type       = 'd' | 'X' | 'x' | 'o' | 'B' | 'b' | 'c' | 'p' | 's'
             'F' | 'f' | 'E' | 'e' | 'G' | 'g' | 'A' | 'a' | '%'

Note that syntactic validity does not imply semantic validity. For example, {:,s} is syntactically valid but semantically invalid, because the string type s does not support the thousands separator ,.

A sequence of zero and width may be ambiguous because width may start with 0. To resolve the ambiguity, if 0 is followed by a digit, the leading zero is interpreted as zero and the following digits are interpreted as width. Otherwise, the zero is interpreted as width.


The semantic of the format specification is basically the same as that of Python.

Fields that have an argument prefixed by $ are interpolated like ordinal strings. Currently, mixing interpolated and non-interpolated replacement fields in an f-string is not allowed. The f-string returns a string if there is a field with interpolation. Otherwise, it returns an Fmt.Format object, which can be passed to Fmt.format and Fmt.printf as the formatting template.

f"x is {$x}." isa String      #> true
f"x is {x}."  isa Fmt.Format  #> true
f"x is {}."   isa Fmt.Format  #> true
f"x is x."    isa Fmt.Format  #> true


The argument is either positional or keyword. Positional arguments are numbered from one, and their values are supplied from arguments passed to the Fmt.format function. If numbers are omitted, they are automatically numbered incrementally from left to right, which is independent from other kinds of arguments. Keyword arguments are named by a variable and may be interpolated. If a keyword argument is interpolated (indicated by $), its value is supplied from the context where the replacement field is placed; otherwise, its value is supplied from a keyword argument with the same name passed to the Fmt.format function. Currently, you cannot mix interpolated keyword arguments with other kinds of arguments in a single format.

Interpolated formats immediately return a string of the String type, while other formats are evaluated to an Fmt.Format object. The Fmt.format object can be passed to the Fmt.format function as its first argument to create a formatted string.

# Positional arguments
Fmt.format(f"{1} {2}", "foo", "bar") == "foo bar"

# Positional arguments (implicit numbers)
Fmt.format(f"{} {}", "foo", "bar") == "foo bar"

# Keyword arguments
Fmt.format(f"{x} {y}", x = "foo", y = "bar") == "foo bar"

# Positional and keyword arguments
Fmt.format(f"{1} {x} {2}", "foo", "bar", x = "and") == "foo and bar"

# Keyword arguments with interpolation
x, y = "foo", "bar"
f"{$x} {$y}" == "foo bar"


Conversion is indicated by / followed by s or r. If conversion is specified, the argument is first converted to a string representation using the string or repr function. As the conversion characters suggest, /s converts the argument using the string function and /r with the repr function.

# Conversion
Fmt.format(f"{/s}", 'a') == "a"
Fmt.format(f"{/r}", 'a') == "'a'"

Python uses ! to mark the conversion syntax. Fmt.jl uses / instead to avoid syntactic ambiguity, because Julia allows ! as a valid character for identifiers.

Fill and alignment

The content of a formatted value can be aligned within the specified width. Note that text alignment does not make sense unless width is specified.

The align character indicates an alignment type as follows:

  • < : left alignment
  • ^ : center alignment
  • > : right alignment

The left and right margins are filled with fill. It can be any character except { and }. If omitted, a space character (i.e., U+0020) is used.

# Alignment with the default fill
Fmt.format(f"{:<7}", "foo") == "foo    "
Fmt.format(f"{:^7}", "foo") == "  foo  "
Fmt.format(f"{:>7}", "foo") == "    foo"

# Alignment with a specified fill
Fmt.format(f"{:*<7}", "foo") == "foo****"
Fmt.format(f"{:*^7}", "foo") == "**foo**"
Fmt.format(f"{:*>7}", "foo") == "****foo"


sign controls the character indicating the sign of a number:

  • - : a sign should be used only for negative values (default)
  • + : a sign should be used for both non-negative and negative values
  • space : a sign should be used only for negative values and a space should be used for non-negative values

Note that sign is only meaningful for numbers.

Fmt.format(f"{:-}",  3) == "3"
Fmt.format(f"{:-}", -3) == "-3"
Fmt.format(f"{:+}",  3) == "+3"
Fmt.format(f"{:+}", -3) == "-3"
Fmt.format(f"{: }",  3) == " 3"
Fmt.format(f"{: }", -3) == "-3"

Alternate form (altform)

altform (#) indicates that the value should be formatted in a different way, depending on the type of the value and the type character. For integers, it indicates that the prefix (0b, 0o, 0x, or 0X) should be added before digits:

# Standard form of integers
Fmt.format("{:o}", 42) == "52"
Fmt.format("{:x}", 42) == "2a"

# Alternate form of integers
Fmt.format("{:#o}", 42) == "0o52"
Fmt.format("{:#x}", 42) == "0x2a"

For floating-point numbers, it indicates ... (TBD).


zero (0) indicates that sign-aware zero padding should be added to fill the width specified by width. That is, zeros for padding are added after the sign, not before the sign like fill. The following example illustrates the difference between sign-aware padding and sign-ignorant padding:

# Sign-aware zero padding
Fmt.format(f"{:+08}",  42) == "+0000042"

# Sign-ignorant zero padding
Fmt.format(f"{:0>+8}", 42) == "00000+42"


width indicates the minimum width of a formatted string.

# Format an integer with minimum width 4
Fmt.format(f"{:4}", 123)   == " 123"
Fmt.format(f"{:4}", 1234)  == "1234"
Fmt.format(f"{:4}", 12345) == "12345"

The default alignment depends on the type of a value. For example, numbers are left-aligned while strings are right-aligned unless align is specified.

Fmt.format(f"{:4}", 1)   == "   1"
Fmt.format(f"{:4}", "a") == "a   "


grouping spcifies the way of grouping digits. For integers with the decimal format, , and _ indicates thousand separator (e.g., 1,234,567). For integers with the binary, octal or hexadecimal format, _ indicates four-digit separator (e.g., 0x1234_5678). For floating-point numbers, integral parts are grouped.

# integers
Fmt.format(f"{:,}",   123456789)  == "123,456,789"
Fmt.format(f"{:_}",   123456789)  == "123_456_789"
Fmt.format(f"{:#_x}", 0xdeadbeef) == "0xdead_beef"

# floats
Fmt.format(f"{:,f}", 2.99792458e8) == "299,792,458.000000"
Fmt.format(f"{:_f}", 2.99792458e8) == "299_792_458.000000"


For floating-point numbers, precision specifies the precision of a formatted representation string of a number.


Fmt.format(f"{:.2f}", Float64(pi)) == "3.14"
Fmt.format(f"{:.3f}", Float64(pi)) == "3.142"
Fmt.format(f"{:.4f}", Float64(pi)) == "3.1416"



Type Description
d decimal
X hexadecimal (uppercase)
x hexadecimal (lowercase)
o octal
B binary (uppercase)
b binary (lowecase)
c character
none decimal

Floating-point numbers

Type Description
F fixed-point notation (uppercase)
f fixed-point notation (lowercase)
E scientific notation (uppercase)
e scientific notation (lowercase)
A hexadecimal notation (uppercase)
a hexadecimal notation (lowercase)
G general notation (uppercase)
g general notation (lowercase)
% percentage (multiplied by 100)
none general notation

There are three kinds of notations for floating-point numbers. Fixed-point notation refers to a notation without exponent part, such as 3.14 and 0.001. Scientific notation refers to a notation with exponent part, such as 6.02e+23 and 1e-8. Hexadecimal notation is similar to scientific notation, but it is prefixed by 0x and its fractional part is denoted in hexadecimal digits.

General notation may be in fixed-point notation or scientific notation, depending on the exponent part of a number. It chooses fixed-point notation if the exponent part of the value is within a "reasonable" range. Otherwise, it chooses scientific notation because denoting the value in fixed-point notation will be too long.

F and f force fixed-point notation. The only difference between F and f is that F uses uppercase letters for (positive and negative) infinities and NaNs (i.e., INF and NAN, respectively) whiel f uses lowercase letters (i.e., inf and nan, respectively).

E and e force scientific notation. The difference between E and e is analogous to that of F and f, but the prefix of exponent part is denoted in an uppercase letter (i.e., E) for E and in an lowercase letter (i.e., e) for e.

A and a force hexadecimal notation. A uses uppercase letters and a uses lowercase letters.

G uses F or E, and g uses f or e, depending on the value as already mentioned above.

% multiplies a value by 100, denotes the value in fixed-point notation, and appends the % mark.

If no type specifier is given, the notation is the same as that of g but at least one digit is shown past the decimal point.


If type is F or f, it formats the number in fixed-point notation. If type is %, it formats the number in the same way as f but the number is multiplied by 100, followed by %. If no type is specified, it formats the number with its (reduced) numerator and denominator separated by a slash (e.g., '3/10').

Other values

p is for pointers and s for strings. These are the default for each type and do not specify any special format.


Fmt.jl is carefully optimized and will be faster than naive printing. Let's see the next benchmarking script, which prints a pair of integers to devnull.

using Fmt
using Printf
using Formatting

fmt_print(out, x, y)        = print(out, f"({$x}, {$y})\n")
sprintf_print(out, x, y)    = print(out, @sprintf("(%d, %d)\n", x, y))
naive_print(out, x, y)      = print(out, '(', x, ", ", y, ")\n")
string_print(out, x, y)     = print(out, "($x, $y)\n")
const expr = FormatExpr("({1}, {2})\n")
formatting_print(out, x, y) = print(out, format(expr, x, y))

function benchmark(printer, out, x, y)
    @assert length(x) == length(y)
    for i in 1:length(x)
        printer(out, x[i], y[i])

using Random
x = rand(-999:999, 1_000_000)
y = rand(-999:999, 1_000_000)

using BenchmarkTools
for printer in [fmt_print, sprintf_print, naive_print,
                string_print, formatting_print]
    @btime benchmark($printer, $devnull, $x, $y)

The result on my machine is:

$ julia benchmark/compare.jl
           fmt_print:  37.928 ms (2000000 allocations: 91.55 MiB)
       sprintf_print:  77.613 ms (2000000 allocations: 106.81 MiB)
         naive_print:  202.531 ms (4975844 allocations: 198.00 MiB)
        string_print:  316.838 ms (7975844 allocations: 365.84 MiB)
    formatting_print:  716.088 ms (23878703 allocations: 959.44 MiB)

Benchmark environment:

  • CPU: AMD Ryzen 9 3950X
  • OS: GNU/Linux 5.9.12
  • Julia: v1.6.0
  • Formatting.jl: v0.4.2

Related projects

  • Printf.jl provides C-style formatting macros. In my opinion, it doesn't match dynamic nature of Julia because it needs type specifier.
  • Formatting.jl provides similar functionality with different APIs. Fmt.jl is much simpler and more performant.