Supported NumPy features

One objective of Numba is having a seamless integration with NumPy. NumPy arrays provide an efficient storage method for homogeneous sets of data. NumPy dtypes provide type information useful when compiling, and the regular, structured storage of potentially large amounts of data in memory provides an ideal memory layout for code generation. Numba excels at generating code that executes on top of NumPy arrays.

NumPy support in Numba comes in many forms:

  • Numba understands calls to NumPy ufuncs and is able to generate equivalent native code for many of them.

  • NumPy arrays are directly supported in Numba. Access to Numpy arrays is very efficient, as indexing is lowered to direct memory accesses when possible.

  • Numba is able to generate ufuncs and gufuncs. This means that it is possible to implement ufuncs and gufuncs within Python, getting speeds comparable to that of ufuncs/gufuncs implemented in C extension modules using the NumPy C API.

The following sections focus on the Numpy features supported in nopython mode, unless otherwise stated.

Scalar types

Numba supports the following Numpy scalar types:

  • Integers: all integers of either signedness, and any width up to 64 bits

  • Booleans

  • Real numbers: single-precision (32-bit) and double-precision (64-bit) reals

  • Complex numbers: single-precision (2x32-bit) and double-precision (2x64-bit) complex numbers

  • Datetimes and timestamps: of any unit

  • Character sequences (but no operations are available on them)

  • Structured scalars: structured scalars made of any of the types above and arrays of the types above

The following scalar types and features are not supported:

  • Arbitrary Python objects

  • Half-precision and extended-precision real and complex numbers

  • Nested structured scalars the fields of structured scalars may not contain other structured scalars

The operations supported on NumPy scalars are almost the same as on the equivalent built-in types such as int or float. You can use a type’s constructor to convert from a different type or width. In addition you can use the view(np.<dtype>) method to bitcast all int and float types within the same width. However, you must define the scalar using a NumPy constructor within a jitted function. For example, the following will work:

>>> import numpy as np
>>> from numba import njit
>>> @njit
... def bitcast():
...     i = np.int64(-1)
...     print(i.view(np.uint64))
...
>>> bitcast()
18446744073709551615

Whereas the following will not work:

>>> import numpy as np
>>> from numba import njit
>>> @njit
... def bitcast(i):
...     print(i.view(np.uint64))
...
>>> bitcast(np.int64(-1))
---------------------------------------------------------------------------
TypingError                               Traceback (most recent call last)
    ...
TypingError: Failed in nopython mode pipeline (step: ensure IR is legal prior to lowering)
'view' can only be called on NumPy dtypes, try wrapping the variable with 'np.<dtype>()'

File "<ipython-input-3-fc40aaab84c4>", line 3:
def bitcast(i):
    print(i.view(np.uint64))

Structured scalars support attribute getting and setting, as well as member lookup using constant strings. Strings stored in a local or global tuple are considered constant strings and can be used for member lookup.

import numpy as np
from numba import njit

arr = np.array([(1, 2)], dtype=[('a1', 'f8'), ('a2', 'f8')])
fields_gl = ('a1', 'a2')

@njit
def get_field_sum(rec):
    fields_lc = ('a1', 'a2')
    field_name1 = fields_lc[0]
    field_name2 = fields_gl[1]
    return rec[field_name1] + rec[field_name2]

get_field_sum(arr[0])  # returns 3

It is also possible to use local or global tuples together with literal_unroll:

import numpy as np
from numba import njit, literal_unroll

arr = np.array([(1, 2)], dtype=[('a1', 'f8'), ('a2', 'f8')])
fields_gl = ('a1', 'a2')

@njit
def get_field_sum(rec):
    out = 0
    for f in literal_unroll(fields_gl):
        out += rec[f]
    return out

get_field_sum(arr[0])   # returns 3

Record subtyping

Warning

This is an experimental feature.

Numba allows width subtyping of structured scalars. For example, dtype([('a', 'f8'), ('b', 'i8')]) will be considered a subtype of dtype([('a', 'f8')], because the second is a strict subset of the first, i.e. field a is of the same type and is in the same position in both types. The subtyping relationship will matter in cases where compilation for a certain input is not allowed, but the input is a subtype of another, allowed type.

import numpy as np
from numba import njit, typeof
from numba.core import types
record1 = np.array([1], dtype=[('a', 'f8')])[0]
record2 = np.array([(2,3)], dtype=[('a', 'f8'), ('b', 'f8')])[0]

@njit(types.float64(typeof(record1)))
def foo(rec):
    return rec['a']

foo(record1)
foo(record2)

Without subtyping the last line would fail. With subtyping, no new compilation will be triggered, but the compiled function for record1 will be used for record2.

See also

Numpy scalars reference.

Array types

Numpy arrays of any of the scalar types above are supported, regardless of the shape or layout.

Array access

Arrays support normal iteration. Full basic indexing and slicing is supported. A subset of advanced indexing is also supported: only one advanced index is allowed, and it has to be a one-dimensional array (it can be combined with an arbitrary number of basic indices as well).

See also

Numpy indexing reference.

Structured array access

Numba presently supports accessing fields of individual elements in structured arrays by attribute as well as by getting and setting. This goes slightly beyond the NumPy API, which only allows accessing fields by getting and setting. For example:

from numba import njit
import numpy as np

record_type = np.dtype([("ival", np.int32), ("fval", np.float64)], align=True)

def f(rec):
    value = 2.5
    rec[0].ival = int(value)
    rec[0].fval = value
    return rec

arr = np.ones(1, dtype=record_type)

cfunc = njit(f)

# Works
print(cfunc(arr))

# Does not work
print(f(arr))

The above code results in the output:

[(2, 2.5)]
Traceback (most recent call last):
  File "repro.py", line 22, in <module>
    print(f(arr))
  File "repro.py", line 9, in f
    rec[0].ival = int(value)
AttributeError: 'numpy.void' object has no attribute 'ival'

The Numba-compiled version of the function executes, but the pure Python version raises an error because of the unsupported use of attribute access.

Note

This behavior will eventually be deprecated and removed.

Attributes

The following attributes of Numpy arrays are supported:

The flags object

The object returned by the flags attribute supports the contiguous, c_contiguous and f_contiguous attributes.

The flat object

The object returned by the flat attribute supports iteration and indexing, but be careful: indexing is very slow on non-C-contiguous arrays.

The real and imag attributes

Numpy supports these attributes regardless of the dtype but Numba chooses to limit their support to avoid potential user error. For numeric dtypes, Numba follows Numpy’s behavior. The real attribute returns a view of the real part of the complex array and it behaves as an identity function for other numeric dtypes. The imag attribute returns a view of the imaginary part of the complex array and it returns a zero array with the same shape and dtype for other numeric dtypes. For non-numeric dtypes, including all structured/record dtypes, using these attributes will result in a compile-time (TypingError) error. This behavior differs from Numpy’s but it is chosen to avoid the potential confusion with field names that overlap these attributes.

Calculation

The following methods of Numpy arrays are supported in their basic form (without any optional arguments):

The corresponding top-level Numpy functions (such as numpy.prod()) are similarly supported.

Other methods

The following methods of Numpy arrays are supported:

  • argsort() (kind key word argument supported for values 'quicksort' and 'mergesort')

  • astype() (only the 1-argument form)

  • copy() (without arguments)

  • dot() (only the 1-argument form)

  • flatten() (no order argument; ‘C’ order only)

  • item() (without arguments)

  • itemset() (only the 1-argument form)

  • ptp() (without arguments)

  • ravel() (no order argument; ‘C’ order only)

  • repeat() (no axis argument)

  • reshape() (only the 1-argument form)

  • sort() (without arguments)

  • sum() (with or without the axis and/or dtype arguments.)

    • axis only supports integer values.

    • If the axis argument is a compile-time constant, all valid values are supported. An out-of-range value will result in a LoweringError at compile-time.

    • If the axis argument is not a compile-time constant, only values from 0 to 3 are supported. An out-of-range value will result in a runtime exception.

    • All numeric dtypes are supported in the dtype parameter. timedelta arrays can be used as input arrays but timedelta is not supported as dtype parameter.

    • When a dtype is given, it determines the type of the internal accumulator. When it is not, the selection is made automatically based on the input array’s dtype, mostly following the same rules as NumPy. However, on 64-bit Windows, Numba uses a 64-bit accumulator for integer inputs (int64 for int32 inputs and uint64 for uint32 inputs), while NumPy would use a 32-bit accumulator in those cases.

  • transpose()

  • view() (only the 1-argument form)

  • __contains__()

Warning

Sorting may be slightly slower than Numpy’s implementation.

Functions

Linear algebra

Basic linear algebra is supported on 1-D and 2-D contiguous arrays of floating-point and complex numbers:

Note

The implementation of these functions needs SciPy to be installed.

Reductions

The following reduction functions are supported:

Other functions

The following top-level functions are supported:

The following constructors are supported, both with a numeric input (to construct a scalar) or a sequence (to construct an array):

  • numpy.bool_

  • numpy.complex64

  • numpy.complex128

  • numpy.float32

  • numpy.float64

  • numpy.int8

  • numpy.int16

  • numpy.int32

  • numpy.int64

  • numpy.intc

  • numpy.intp

  • numpy.uint8

  • numpy.uint16

  • numpy.uint32

  • numpy.uint64

  • numpy.uintc

  • numpy.uintp

The following machine parameter classes are supported, with all purely numerical attributes:

Literal arrays

Neither Python nor Numba has actual array literals, but you can construct arbitrary arrays by calling numpy.array() on a nested tuple:

a = numpy.array(((a, b, c), (d, e, f)))

(nested lists are not yet supported by Numba)

Modules

random

Numba supports top-level functions from the numpy.random module, but does not allow you to create individual RandomState instances. The same algorithms are used as for the standard random module (and therefore the same notes apply), but with an independent internal state: seeding or drawing numbers from one generator won’t affect the other.

The following functions are supported.

Initialization

Permutations

stride_tricks

The following function from the numpy.lib.stride_tricks module is supported:

  • as_strided() (the strides argument is mandatory, the subok argument is not supported)

Standard ufuncs

One objective of Numba is having all the standard ufuncs in NumPy understood by Numba. When a supported ufunc is found when compiling a function, Numba maps the ufunc to equivalent native code. This allows the use of those ufuncs in Numba code that gets compiled in nopython mode.

Limitations

Right now, only a selection of the standard ufuncs work in nopython mode. Following is a list of the different standard ufuncs that Numba is aware of, sorted in the same way as in the NumPy documentation.

Math operations

UFUNC

MODE

name

object mode

nopython mode

add

Yes

Yes

subtract

Yes

Yes

multiply

Yes

Yes

divide

Yes

Yes

logaddexp

Yes

Yes

logaddexp2

Yes

Yes

true_divide

Yes

Yes

floor_divide

Yes

Yes

negative

Yes

Yes

power

Yes

Yes

remainder

Yes

Yes

mod

Yes

Yes

fmod

Yes

Yes

divmod (*)

Yes

Yes

abs

Yes

Yes

absolute

Yes

Yes

fabs

Yes

Yes

rint

Yes

Yes

sign

Yes

Yes

conj

Yes

Yes

exp

Yes

Yes

exp2

Yes

Yes

log

Yes

Yes

log2

Yes

Yes

log10

Yes

Yes

expm1

Yes

Yes

log1p

Yes

Yes

sqrt

Yes

Yes

square

Yes

Yes

reciprocal

Yes

Yes

conjugate

Yes

Yes

gcd

Yes

Yes

lcm

Yes

Yes

(*) not supported on timedelta types

Trigonometric functions

UFUNC

MODE

name

object mode

nopython mode

sin

Yes

Yes

cos

Yes

Yes

tan

Yes

Yes

arcsin

Yes

Yes

arccos

Yes

Yes

arctan

Yes

Yes

arctan2

Yes

Yes

hypot

Yes

Yes

sinh

Yes

Yes

cosh

Yes

Yes

tanh

Yes

Yes

arcsinh

Yes

Yes

arccosh

Yes

Yes

arctanh

Yes

Yes

deg2rad

Yes

Yes

rad2deg

Yes

Yes

degrees

Yes

Yes

radians

Yes

Yes

Bit-twiddling functions

UFUNC

MODE

name

object mode

nopython mode

bitwise_and

Yes

Yes

bitwise_or

Yes

Yes

bitwise_xor

Yes

Yes

bitwise_not

Yes

Yes

invert

Yes

Yes

left_shift

Yes

Yes

right_shift

Yes

Yes

Comparison functions

UFUNC

MODE

name

object mode

nopython mode

greater

Yes

Yes

greater_equal

Yes

Yes

less

Yes

Yes

less_equal

Yes

Yes

not_equal

Yes

Yes

equal

Yes

Yes

logical_and

Yes

Yes

logical_or

Yes

Yes

logical_xor

Yes

Yes

logical_not

Yes

Yes

maximum

Yes

Yes

minimum

Yes

Yes

fmax

Yes

Yes

fmin

Yes

Yes

Floating functions

UFUNC

MODE

name

object mode

nopython mode

isfinite

Yes

Yes

isinf

Yes

Yes

isnan

Yes

Yes

signbit

Yes

Yes

copysign

Yes

Yes

nextafter

Yes

Yes

modf

Yes

No

ldexp

Yes (*)

Yes

frexp

Yes

No

floor

Yes

Yes

ceil

Yes

Yes

trunc

Yes

Yes

spacing

Yes

Yes

(*) not supported on windows 32 bit

Datetime functions

UFUNC

MODE

name

object mode

nopython mode

isnat

Yes

Yes