Mojo literals reference

A literal is a value written directly in source code: 42, "hello", True. Literals produce values without reading variables or calling functions. Each section below covers one literal type, its syntax, and any rules the lexer enforces.

Integer literals

Integer literals represent whole numbers in four bases:

42        # Decimal
0xFF      # Hexadecimal (0x or 0X prefix)
0o52      # Octal (0o or 0O prefix)
0b101010  # Binary (0b or 0B prefix)

Integer literals follow these lexical rules:

integer     → decinteger | bininteger | octinteger | hexinteger
decinteger  → nonzerodigit ("_" | digit)* | "0"+ ("_" | "0")*
bininteger  → "0" ("b" | "B") ("_" | bindigit)+
octinteger  → "0" ("o" | "O") ("_" | octdigit)+
hexinteger  → "0" ("x" | "X") ("_" | hexdigit)+

Integer literals are always non-negative. -1024 is the unary negation operator - applied to the literal 1024.

Underscores can appear between digits for readability. Mojo is more permissive than Python here. Consecutive and trailing underscores are allowed:

1_000_000  # Readable grouping
1__000_    # Also valid (consecutive and trailing underscores OK)

Leading zeros in decimal literals are not allowed. Use the 0o prefix for octal:

0123   # Error: leading zeros in decimal integer literals are not permitted
0o123  # OK: octal

A base prefix must be followed by at least one digit:

0x  # Error: no digits specified for hex literal
0b  # Error: no digits specified for binary literal
0o  # Error: no digits specified for octal literal

Floating-point literals

Floating-point literals represent numbers with a fractional or exponent part:

1.0
3.14159
.5       # Fraction only (no integer part)
2.       # Integer part with decimal point
2.5e-3   # With exponent
1E10     # Capital E works too

Floating-point literals follow these lexical rules:

floatnumber   → pointfloat | exponentfloat
pointfloat    → digitpart? fraction | digitpart "."
exponentfloat → (digitpart | pointfloat) exponent
fraction      → "." digitpart
exponent      → ("e" | "E") ("+" | "-")? digitpart
digitpart     → digit ("_" | digit)*

Floating-point literals are always non-negative. -3.14 is the unary negation operator - applied to the literal 3.14.

When included, an exponent marker (e or E) must be followed by at least one digit:

2.5e    # Error: expecting a digit after the exponent
2.5e-   # Error: expecting a digit after the exponent
2.5e-3  # OK

Underscores in floating-point literals work as they do in integers. Place them anywhere that enhances readability:

1_000.000_5

String literals

String literals represent text values. Mojo supports single and double quotes, and a triple-quote form for multi-line strings:

"Hello"
'world'
"""Multi-line
string
"""                # Includes final newline
'''Also
    multi-line'''  # Includes 4 spaces at the start of the second line

Triple-quoted strings include any newlines literally. A backslash at the end of a line suppresses the newline, joining the next line directly:

"""\
This string has no leading newline."""

String literals on adjacent lines are joined into a single string. This works on one line or across lines when the continuation is indented:

var x = "Hello, " "World"  # "Hello, World"

var y = "line one "
    "line two"             # "line one line two" (indented continuation)

Prefix with r or R to create a raw string that disables escape processing:

r"C:\path\to\file"  # Backslashes treated literally

Escape sequences

Mojo recognizes these escape sequences in non-raw string literals:

Sequence	Meaning	Sequence	Meaning
\\	Backslash	\a	Bell
\"	Double quote	\b	Backspace
\'	Single quote	\f	Form feed
\n	Newline	\v	Vertical tab
\r	Carriage return	\xHH	Hex value (exactly 2 hex digits)
\t	Tab	\0–\377	Octal value (1–3 octal digits)
\uHHHH	Unicode code point (4 hex digits)	\UHHHHHHHH	Unicode code point (8 hex digits)

Mojo source files are UTF-8. String literals may contain non-ASCII characters directly.

var wave = "👋"

Non-ASCII characters can also be written as Unicode hex escapes:

var wave = "\U0001F44B"   # 8-digit hex escape, 👋
var euro = "\u20AC"       # 4-digit hex escape, €

• \uHHHH accepts code points from U+0000 to U+FFFF
• \UHHHHHHHH accepts the full Unicode range, U+0000 to U+10FFFF

Both forms reject surrogate code points (U+D800 to U+DFFF), which are reserved for UTF-16 encoding. Code points above U+FFFF require \U, not a UTF-16 surrogate pair.

T-string literals

T-string literals support expression interpolation using {}:

var name = "World"
var greeting_template = t"Hello, {name}!"   # "Hello, World!"
var result_template = t"1 + 1 = {1 + 1}"    # "1 + 1 = 2"

Expressions inside {} are evaluated at runtime. Adjacent t-string literals are joined, just like regular string literals. To use them as strings except in print statements, cast them to String:

var name = "Alice"
var greeting = t"Hello, {name}!"    # Type is T-string
print(greeting)                     # Prints "Hello, Alice!"
var greeting_str = String(greeting) # Convert to regular String

T-strings can be triple-quoted and combined with the raw prefix (any case combination of r/R and t/T, in either order):

t"""
Hello, {name}!
"""

rt"Path: {base}\subdir"  # Raw t-string: backslashes are literal

Use {{ and }} to include literal braces in a t-string:

t"Use {{braces}} in t-strings"  # "Use {braces} in t-strings"

T-strings can be nested. An interpolation expression can itself contain t-strings, up to 20 levels deep.

var name = "world"
var greeting = t"Hello, {t"dear {name}"}!"
print(greeting) # "Hello, dear world!"

Boolean literals

True and False represent boolean truth values.

var x = True
var y = False

None literal

None represents the absence of a value. It's the only value of type NoneType.

var x: NoneType = None

A function without an explicit return type returns None. These two declarations are equivalent:

def greet():
    print("hello")

def greet() -> None:
    print("hello")

Self literal

Self refers to the enclosing type inside a struct or trait definition:

from std.math import sqrt

@fieldwise_init
struct Point:
    var x: Float64
    var y: Float64

    @staticmethod
    def create() -> Self:          # Self refers to Point
        return Self(0.0, 0.0)

    def distance(self) -> Float64: # self is an argument name, not Self
        return sqrt(self.x ** 2 + self.y ** 2)

Self (capital S) is a keyword that refers to the type. self (lowercase) is a conventional argument name for the instance.

Discard pattern

The underscore _ discards a value in an assignment:

_, y = get_pair()  # Ignore the first element

Ellipsis literal

... marks a trait method as required. Conforming types must provide their own implementation. It's only valid inside trait definitions:

trait Drawable:
    def draw(self) -> None: ...   # Required: conforming types must implement

... and pass aren't interchangeable. pass is a no-op statement that provides an empty body. ... is a requirement marker that means "you must implement this."