Types and Constants

Type wrappers, predefined value constants, and type aliases that appear in public method signatures.

Data type

NcclDataType

class nccl.core.NcclDataType(*values)

Bases: IntEnum

NCCL data type, mirroring ncclDataType_t.

Used as the dtype of buffer specs and as the datatype argument of NCCL collective operations. Supports conversion to/from NumPy dtypes via from_numpy_dtype() and numpy_dtype.

INT8 = 0

CHAR = 0

UINT8 = 1

INT32 = 2

INT = 2

UINT32 = 3

INT64 = 4

UINT64 = 5

FLOAT16 = 6

HALF = 6

FLOAT32 = 7

FLOAT = 7

FLOAT64 = 8

DOUBLE = 8

BFLOAT16 = 9

FLOAT8E4M3 = 10

FLOAT8E5M2 = 11

classmethod from_numpy_dtype(dtype: numpy.dtype) → NcclDataType

Maps a NumPy dtype to its NCCL equivalent.

Parameters:: dtype – A NumPy dtype. Mapped first by name (for ml-dtypes like bfloat16, float8_e4m3fn, float8_e5m2) and then by (kind, itemsize) for standard types.
Returns:: Corresponding NcclDataType member.
Raises:: NcclInvalid – If the dtype has no NCCL equivalent.

property itemsize: int: Size in bytes of a single element of this data type.

property numpy_dtype: numpy.dtype

Equivalent NumPy dtype.

Returns:: NumPy dtype corresponding to this NCCL data type. For BFLOAT16 and the float8 variants, ml-dtypes must be installed.
Raises:: NcclInvalid – If ml-dtypes is required but not installed.

Predefined data type constants

Module-level NcclDataType instances for use as the dtype argument of buffer specs.

Constant	Maps to
`nccl.core.INT8`	`NcclDataType.INT8`
`nccl.core.CHAR`	`NcclDataType.CHAR`
`nccl.core.UINT8`	`NcclDataType.UINT8`
`nccl.core.INT32`	`NcclDataType.INT32`
`nccl.core.INT`	`NcclDataType.INT`
`nccl.core.UINT32`	`NcclDataType.UINT32`
`nccl.core.INT64`	`NcclDataType.INT64`
`nccl.core.UINT64`	`NcclDataType.UINT64`
`nccl.core.FLOAT16`	`NcclDataType.FLOAT16`
`nccl.core.HALF`	`NcclDataType.HALF`
`nccl.core.FLOAT32`	`NcclDataType.FLOAT32`
`nccl.core.FLOAT`	`NcclDataType.FLOAT`
`nccl.core.FLOAT64`	`NcclDataType.FLOAT64`
`nccl.core.DOUBLE`	`NcclDataType.DOUBLE`
`nccl.core.BFLOAT16`	`NcclDataType.BFLOAT16`
`nccl.core.FLOAT8E4M3`	`NcclDataType.FLOAT8E4M3`
`nccl.core.FLOAT8E5M2`	`NcclDataType.FLOAT8E5M2`

Reduction operator

NcclRedOp

class nccl.core.NcclRedOp(*values)

Bases: IntEnum

NCCL reduction operator, mirroring ncclRedOp_t.

Used as the op argument of reduction collectives (Communicator.allreduce(), Communicator.reduce(), Communicator.reduce_scatter()).

SUM = 0

PROD = 1

MAX = 2

MIN = 3

AVG = 4

Predefined reduction operators

Module-level NcclRedOp instances for use as the op argument of reduction collectives. User-defined operators are created via Communicator.create_pre_mul_sum().

Constant	Maps to
`nccl.core.SUM`	`NcclRedOp.SUM`
`nccl.core.PROD`	`NcclRedOp.PROD`
`nccl.core.MAX`	`NcclRedOp.MAX`
`nccl.core.MIN`	`NcclRedOp.MIN`
`nccl.core.AVG`	`NcclRedOp.AVG`

Team

NCCLTeam

class nccl.core.NCCLTeam(n_ranks: int, rank: int, stride: int)

Bases: LowppSpec

A NCCL team: (n_ranks, rank, stride) view over a communicator.

Produced by team_world, team_lsa, and team_rail.

n_ranks: int

rank: int

stride: int

Type aliases

Aliases naming the argument types accepted by public methods. Each expands to a union of concrete types, so a value of any member type is accepted.

nccl.core.NcclBufferSpec

Represent a union type

E.g. for int | str

nccl.core.NcclScalarSpec

Represent a union type

E.g. for int | str

nccl.core.NcclDeviceSpec

Represent a union type

E.g. for int | str

nccl.core.NcclStreamSpec

Represent a union type

E.g. for int | str

Exceptions

NcclInvalid

Python-side validation exception, raised when a public API receives a malformed argument before it reaches NCCL itself.

exception nccl.core.NcclInvalid(msg)

Bases: Exception

Raised when an argument provided to an NCCL4Py API is invalid.

Used for argument validation errors that the Python layer detects before forwarding the call to NCCL (e.g. unsupported dtype, mismatched buffer counts, wrong device). Errors raised by NCCL itself are reported as NCCLError from the bindings layer.