Mojo traits cheat sheet

What each standard library trait does, when you need it, and where it bites.

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v1.1.0dev

Lifecycle

AnyTypethe root; every type conforms automaticallytraits/anytype.mojo

no requirements

Deinitablehas an implicit destructor, called at last usetraits/deinitable.mojo

Signature	Remark
__deinit__(deinit self, /)	provided
comptime __del__is_trivial: Bool	compile-time flag

Automatically added to every eligible type (all fields are also Deinitable). When the type is trivial, Mojo skips the destructor.

Movabletransfer ownership; enables the ^ operatortraits/movable.mojo

Signature	Remark
__init__(out self, *, deinit move: Self)	provided
comptime __move_ctor_is_trivial: Bool	compile-time flag

Required to store a type in List, Optional, or Variant, or to return it by move.

Copyableexplicit copytraits/copyable.mojo

Refines: Movable

Signature	Remark
__init__(out self, *, copy: Self)	provided
copy(self) -> Self	provided
comptime __copy_ctor_is_trivial: Bool	compile-time flag

The copy constructor is synthesized if all fields are Copyable.

ImplicitlyCopyable(MARKER) lets copies be inserted implicitlytraits/copyable.mojo

Refines: Copyable < Movable · no new requirements

Can mask logical errors and hide code reasoning. Prefer Movable or Copyable.

Defaultablecreate with no argumentsbuiltin/value.mojo

Signature	Remark
__init__(out self)

Reach for this when you need parameterized default-construction without arguments.

RegisterPassable(MARKER) stored in registers, not memory; no stable address or identitybuiltin/value.mojo

Refines: Movable · no requirements (marker)

No stable address: you can't take the address of self in imm-convention methods. Identifiable is meaningless for these types.

Moved trivially; all fields must also conform.

TrivialRegisterPassable(MARKER) register-passable, copyable by moving bits, no side effectsbuiltin/value.mojo

Refines: ImplicitlyCopyable < Copyable < Movable, Deinitable, RegisterPassable · no requirements (marker)

A type whose values are treated as basic bit patterns. No constructors or destructors needed. All fields must also conform.

Format

Writableformat itself as text; works with print(), String(), format stringsformat/__init__.mojo

Signature	Remark
write_to(self, mut writer: Some[Writer])	provided
write_repr_to(self, mut writer: Some[Writer])	provided

If all fields conform, you inherit both methods through reflection.

Writera destination for a custom output targetformat/__init__.mojo

String, FileHandle, FileDescriptor conform

Signature	Remark
write_string(mut self, string: StringSpan)
write[*Ts: Writable](mut self, *args: *Ts)	provided

Use for loggers, network streams, string builders, etc.

Testing

Strategyproduces random inputs for property-based teststesting/prop/strategy/__init__.mojo

Refines: Deinitable, Movable

Signature	Remark
Value: Copyable & Deinitable	associated type
value(mut self, mut rng: Rng) raises -> Self.Value

Allows strategies to carry and advance state between draws. value() draws one sample from the random number generator.

Accelerator traits

DevicePassablemarks a type as passable to an acceleratorbuiltin/device_passable.mojo

Signature	Remark
comptime device_type: AnyType	the on-device type
_to_device_type(self, mut enc, ...)	DeviceContext hook

A host type implements this so it can be handed to a GPU or other accelerator. DeviceContext calls the conversion hook to turn host into device at kernel launch.

DeviceTypeEncoderencodes host values into device layoutbuiltin/device_passable.mojo

Signature	Remark
target() -> _TargetType	device target
encode_device_ptr(mut self, ...)	required
encode[T](mut self, value, dst)	provided (+ fields, tuple, array)

Encodes a value's fields into the accelerator's data layout.

Compare & hash

Equatableequality; enables == and !=builtin/comparable.mojo

Signature	Remark
__eq__(self, other: Self) -> Bool	provided
__ne__(self, other: Self) -> Bool	provided

Don't use with floating-point values (use isclose()). NaN != NaN.

Mojo provides a fieldwise default. Override for caches, internal metadata, and custom behavior.

Comparableordered comparison; < > ≤ ≥, and sort()builtin/comparable.mojo

Refines: Equatable

Signature	Remark
__lt__(self, rhs: Self) -> Bool
__gt__(self, rhs: Self) -> Bool	provided
__le__(self, rhs: Self) -> Bool	provided
__ge__(self, rhs: Self) -> Bool	provided

Implement __lt__() unless it's expensive. If so, override all four.

Hashableproduces a hash; needed for Dict keys and Set elementshashlib/hash.mojo

KeyElement = Hashable + Equatable + Movable

Signature	Remark
__hash__(self, mut hasher: Some[Hasher])	provided

Hasherimplements a hash algorithm (the algorithm, not a hashable type)hashlib/hasher.mojo

Signature	Remark
__init__(out self)
_update_with_bytes(mut self, data: Span[Byte, _])
_update_with_simd(mut self, value: SIMD[_,_])
update(mut self, value: Some[Hashable])
finish(var self) -> UInt64

Hashers remain alive after finalization. All three update methods are required.

Identifiableidentity; same-object test, enables is / is notbuiltin/identifiable.mojo

Signature	Remark
__is__(self, rhs: Self) -> Bool
__isnot__(self, rhs: Self) -> Bool	provided

Excludes register-passable types, which don't have stable addresses.

Convert

Boolable, Intable, Floatableconvert with Bool(), Int(), Float64()builtin/bool.mojo · builtin/int.mojo · builtin/floatable.mojo

Signature	Remark
__bool__(self) -> Bool	Boolable
__int__(self) -> Int	Intable
__int__(self) raises -> Int	IntableRaising
__float__(self) -> Float64	Floatable
__float__(self) raises -> Float64	FloatableRaising

If the method raises, use the Raising variant.

Boolable unlocks if / while / and / or usage.

Math

Absable, Powable, Roundableunary math operatorsmath/math.mojo

Signature	Remark
__abs__(self) -> Self	Absable · abs()
__pow__(self, exp: Self) -> Self	Powable · pow(), **
__round__(self) -> Self	Roundable · round()
__round__(self, ndigits: Int) -> Self	Roundable · round(), precision

Ceilable, Floorable, Truncableround toward a boundmath/math.mojo

Signature	Remark
__ceil__(self) -> Self	Ceilable · ceil()
__floor__(self) -> Self	Floorable · floor()
__trunc__(self) -> Self	Truncable · trunc()

CeilDivable, CeilDivableRaisingceiling division (rounds up instead of down)math/math.mojo

Signature	Remark
__ceildiv__(self, denominator: Self) -> Self	CeilDivable
__ceildiv__(self, denominator: Self) raises -> Self	CeilDivableRaising

DivModablecombined division and modulo; enables divmod()math/math.mojo

Refines: ImplicitlyCopyable < Copyable < Movable

Signature	Remark
__divmod__(self, denominator: Self) -> Tuple[Self, Self]

Math outlier. The tuple is (quotient, remainder).

Iterate

Sized, SizedRaisinghas a length; enables len()builtin/len.mojo

Signature	Remark
__len__(self) -> Int	Sized
__len__(self) raises -> Int	SizedRaising

Iterableiterate by borrowingiter/__init__.mojo

Signature	Remark
IteratorType[iterable_mut: Bool, //, iterable_origin: Origin[mut=iterable_mut]]: Iterator	associated type
__iter__(ref self) -> Self.IteratorType[origin_of(self)]

Parameterized on mutability and origin. Yields references tied to the source lifetime.

IterableOwnediterate by consuming and owningiter/__init__.mojo

Signature	Remark
IteratorOwnedType: Iterator	associated type
__iter__(var self) -> Self.IteratorOwnedType

No origin tracking.

Iteratorproduces elements one at a time; the for-loop workhorseiter/__init__.mojo

Refines: Deinitable, Movable

Signature	Remark
Element: Movable	associated type
__next__(mut self) raises StopIteration -> Self.Element
bounds(self) -> Tuple[Int, Optional[Int]]	provided
nth(var self, n: Int) -> Optional[Self.Element]	provided

Requires an Iterable on the collection, and Iterator on the iterator. Don't rely on bounds() for safety checks. It's a hint.

Typed raises (StopIteration).

Interop

PathLikerepresents a file system pathos/pathlike.mojo

Path conforms

Signature	Remark
__fspath__(self) -> String

ConvertibleToPythoncan be sent to Pythonpython/conversions.mojo

Refines: Deinitable

Signature	Remark
to_python_object(var self) raises -> PythonObject

ConvertibleFromPythoncan be created from a Python objectpython/conversions.mojo

Refines: Copyable < Movable, Deinitable

Signature	Remark
__init__(out self, *, py: PythonObject) raises