from numba import jit
def f(x, y):
# A somewhat trivial example
return x + y
In this mode, compilation will be deferred until the first function
execution. Numba will infer the argument types at call time, and generate
optimized code based on this information. Numba will also be able to
compile separate specializations depending on the input types. For example,
calling the f() function above with integer or complex numbers will
generate different code paths:
>>> f(1, 2)
>>> f(1j, 2)
(2+1j)
Eager compilation
You can also tell Numba the function signature you are expecting. The
function f() would now look like:
from numba import jit, int32
@jit(int32(int32, int32))
def f(x, y):
# A somewhat trivial example
return x + y
int32(int32, int32) is the function’s signature. In this case, the
corresponding specialization will be compiled by the @jit decorator,
and no other specialization will be allowed. This is useful if you want
fine-grained control over types chosen by the compiler (for example,
to use single-precision floats).
If you omit the return type, e.g. by writing (int32, int32) instead of
int32(int32, int32), Numba will try to infer it for you. Function
signatures can also be strings, and you can pass several of them as a list;
see the numba.jit() documentation for more details.
Of course, the compiled function gives the expected results:
>>> f(1,2)
and if we specified int32 as return type, the higher-order bits get
discarded:
>>> f(2**31, 2**31 + 1)
Calling and inlining other functions
Numba-compiled functions can call other compiled functions. The function
calls may even be inlined in the native code, depending on optimizer
heuristics. For example:
def square(x):
return x ** 2
def hypot(x, y):
return math.sqrt(square(x) + square(y))
The @jit decorator must be added to any such library function,
otherwise Numba may generate much slower code.
Signature specifications
Explicit @jit signatures can use a number of types. Here are some
common ones:
void is the return type of functions returning nothing (which
actually return None when called from Python)
intp and uintp are pointer-sized integers (signed and unsigned,
respectively)
intc and uintc are equivalent to C int and unsigned int
integer types
int8, uint8, int16, uint16, int32, uint32,
int64, uint64 are fixed-width integers of the corresponding bit
width (signed and unsigned)
float32 and float64 are single- and double-precision floating-point
numbers, respectively
complex64 and complex128 are single- and double-precision complex
numbers, respectively
array types can be specified by indexing any numeric type, e.g. float32[:]
for a one-dimensional single-precision array or int8[:,:] for a
two-dimensional array of 8-bit integers.
Compilation options
A number of keyword-only arguments can be passed to the @jit decorator.
nopython
Numba has two compilation modes: nopython mode and
object mode. The former produces much faster code, but has
limitations that can force Numba to fall back to the latter. To prevent
Numba from falling back, and instead raise an error, pass nopython=True.
@jit(nopython=True)
def f(x, y):
return x + y
See also
Troubleshooting and tips
nogil
Whenever Numba optimizes Python code to native code that only works on
native types and variables (rather than Python objects), it is not necessary
anymore to hold Python’s global interpreter lock (GIL).
Numba will release the GIL when entering such a compiled function if you
passed nogil=True.
@jit(nogil=True)
def f(x, y):
return x + y
Code running with the GIL released runs concurrently with other
threads executing Python or Numba code (either the same compiled function,
or another one), allowing you to take advantage of multi-core systems.
This will not be possible if the function is compiled in object mode.
When using nogil=True, you’ll have to be wary of the usual pitfalls
of multi-threaded programming (consistency, synchronization, race conditions,
etc.).
cache
To avoid compilation times each time you invoke a Python program,
you can instruct Numba to write the result of function compilation into
a file-based cache. This is done by passing cache=True:
@jit(cache=True)
def f(x, y):
return x + y
parallel
Enables automatic parallelization (and related optimizations) for those
operations in the function known to have parallel semantics. For a list of
supported operations, see Automatic parallelization with @jit. This feature is enabled by
passing parallel=True and must be used in conjunction with
nopython=True:
