2.1. Types and signatures

2.1.1. Rationale

As an optimizing compiler, Numba needs to decide on the type of each variable to generate efficient machine code. Python’s standard types are not precise enough for that, so we had to develop our own fine-grained type system.

You will encounter Numba types mainly when trying to inspect the results of Numba’s type inference, for debugging or educational purposes. However, you need to use types explicitly if compiling code ahead-of-time.

2.1.2. Signatures

A signature specifies the type of a function. Exactly which kind of signature is allowed depends on the context (AOT or JIT compilation), but signatures always involve some representation of Numba types to specifiy the concrete types for the function’s arguments and, if required, the function’s return type.

An example function signature would be the string "f8(i4, i4)" (or the equivalent "float64(int32, int32)") which specifies a function taking two 32-bit integers and returning a double-precision float.

2.1.3. Basic types

The most basic types can be expressed through simple expressions. The symbols below refer to attributes of the main numba module (so if you read “boolean”, it means that symbol can be accessed as numba.boolean). Many types are available both as a canonical name and a shorthand alias, following Numpy’s conventions.

2.1.3.1. Numbers

The following table contains the elementary numeric types currently defined by Numba and their aliases.

Type name(s)	Shorthand	Comments
boolean	b1	represented as a byte
uint8, byte	u1	8-bit unsigned byte
uint16	u2	16-bit unsigned integer
uint32	u4	32-bit unsigned integer
uint64	u8	64-bit unsigned integer
int8, char	i1	8-bit signed byte
int16	i2	16-bit signed integer
int32	i4	32-bit signed integer
int64	i8	64-bit signed integer
intc	–	C int-sized integer
uintc	–	C int-sized unsigned integer
intp	–	pointer-sized integer
uintp	–	pointer-sized unsigned integer
float32	f4	single-precision floating-point number
float64, double	f8	double-precision floating-point number
complex64	c8	single-precision complex number
complex128	c16	double-precision complex number

2.1.3.2. Arrays

The easy way to declare array types is to subscript an elementary type according to the number of dimensions. For example a 1-dimension single-precision array:

>>> numba.float32[:]
array(float32, 1d, A)

or a 3-dimension array of the same underlying type:

>>> numba.float32[:, :, :]
array(float32, 3d, A)

This syntax defines array types with no particular layout (producing code that accepts both non-contiguous and contiguous arrays), but you can specify a particular contiguity by using the ::1 index either at the beginning or the end of the index specification:

>>> numba.float32[::1]
array(float32, 1d, C)
>>> numba.float32[:, :, ::1]
array(float32, 3d, C)
>>> numba.float32[::1, :, :]
array(float32, 3d, F)

2.1.4. Advanced types

For more advanced declarations, you have to explicitly call helper functions or classes provided by Numba.

Warning

The APIs documented here are not guaranteed to be stable. Unless necessary, it is recommended to let Numba infer argument types by using the signature-less variant of @jit.

2.1.4.1. Inference

numba.typeof(value)

Create a Numba type accurately describing the given Python value. ValueError is raised if the value isn’t supported in nopython mode.

>>> numba.typeof(np.empty(3))
array(float64, 1d, C)
>>> numba.typeof((1, 2.0))
(int64, float64)
>>> numba.typeof([0])
reflected list(int64)

2.1.4.2. Numpy scalars

Instead of using typeof(), non-trivial scalars such as structured types can also be constructed programmatically.

numba.from_dtype(dtype)

Create a Numba type corresponding to the given Numpy dtype:

>>> struct_dtype = np.dtype([('row', np.float64), ('col', np.float64)])
>>> ty = numba.from_dtype(struct_dtype)
>>> ty
Record([('row', '<f8'), ('col', '<f8')])
>>> ty[:, :]
unaligned array(Record([('row', '<f8'), ('col', '<f8')]), 2d, A)

class numba.types.NPDatetime(unit)

Create a Numba type for Numpy datetimes of the given unit. unit should be a string amongst the codes recognized by Numpy (e.g. Y, M, D, etc.).

class numba.types.NPTimedelta(unit)

Create a Numba type for Numpy timedeltas of the given unit. unit should be a string amongst the codes recognized by Numpy (e.g. Y, M, D, etc.).

See also

Numpy datetime units.

2.1.4.3. Arrays

class numba.types.Array(dtype, ndim, layout)

Create an array type. dtype should be a Numba type. ndim is the number of dimensions of the array (a positive integer). layout is a string giving the layout of the array: A means any layout, C means C-contiguous and F means Fortran-contiguous.

2.1.4.4. Optional types

class numba.optional(typ)

Create an optional type based on the underlying Numba type typ. The optional type will allow any value of either typ or None.

>>> @jit((optional(intp),))
... def f(x):
...     return x is not None
...
>>> f(0)
True
>>> f(None)
False