1.11. Frequently Asked Questions

1.11.1. Programming Can I pass a function as an argument to a jitted function?

You can’t, but in many cases you can use a closure to emulate it. For example, this example:

def f(g, x):
    return g(x) + g(-x)

result = f(my_g_function, 1)

could be rewritten using a factory function:

def make_f(g):
    # Note: a new f() is compiled each time make_f() is called!
    def f(x):
        return g(x) + g(-x)
    return f

f = make_f(my_g_function)
result = f(1) Numba doesn’t seem to care when I modify a global variable

Numba considers global variables as compile-time constants. If you want your jitted function to update itself when you have modified a global variable’s value, one solution is to recompile it using the recompile() method. This is a relatively slow operation, though, so you may instead decide to rearchitect your code and turn the global variable into a function argument. Can I debug a jitted function?

Calling into pdb or other such high-level facilities is currently not supported from Numba-compiled code. However, you can temporarily disable compilation by setting the NUMBA_DISABLE_JIT environment variable. How can I create a Fortran-ordered array?

Numba currently doesn’t support the order argument to most Numpy functions such as numpy.empty() (because of limitations in the type inference algorithm). You can work around this issue by creating a C-ordered array and then transposing it. For example:

a = np.empty((3, 5), order='F')
b = np.zeros(some_shape, order='F')

can be rewritten as:

a = np.empty((5, 3)).T
b = np.zeros(some_shape[::-1]).T How can I increase integer width?

By default, Numba will generally use machine integer width for integer variables. On a 32-bit machine, you may sometimes need the magnitude of 64-bit integers instead. You can simply initialize relevant variables as np.int64 (for example np.int64(0) instead of 0). It will propagate to all computations involving those variables. How can I tell if parallel=True worked?

Set the environment variable NUMBA_WARNINGS to non-zero and if the parallel=True transformations failed for a function decorated as such, a warning will be displayed.

1.11.2. Performance Does Numba inline functions?

Numba gives enough information to LLVM so that functions short enough can be inlined. This only works in nopython mode. Does Numba vectorize array computations (SIMD)?

Numba doesn’t implement such optimizations by itself, but it lets LLVM apply them. Does Numba automatically parallelize code?

It can, in some cases:

  • Ufuncs and gufuncs with the target="parallel" option will run on multiple threads.
  • The experimental parallel=True option to @jit will attempt to optimize array operations and run them in parallel. It also adds support for prange() to explicitly parallelize a loop.

You can also manually run computations on multiple threads yourself and use the nogil=True option (see releasing the GIL). Numba can also target parallel execution on GPU architectures using its CUDA and HSA backends. Can Numba speed up short-running functions?

Not significantly. New users sometimes expect to JIT-compile such functions:

def f(x, y):
    return x + y

and get a significant speedup over the Python interpreter. But there isn’t much Numba can improve here: most of the time is probably spent in CPython’s function call mechanism, rather than the function itself. As a rule of thumb, if a function takes less than 10 µs to execute: leave it.

The exception is that you should JIT-compile that function if it is called from another jitted function. There is a delay when JIT-compiling a complicated function, how can I improve it?

Try to pass cache=True to the @jit decorator. It will keep the compiled version on disk for later use.

A more radical alternative is ahead-of-time compilation.

1.11.3. Integration with other utilities Can I “freeze” an application which uses Numba?

If you’re using PyInstaller or a similar utility to freeze an application, you may encounter issues with llvmlite. llvmlite needs a non-Python DLL for its working, but it won’t be automatically detected by freezing utilities. You have to inform the freezing utility of the DLL’s location: it will usually be named llvmlite/binding/libllvmlite.so or llvmlite/binding/llvmlite.dll, depending on your system. I get errors when running a script twice under Spyder

When you run a script in a console under Spyder, Spyder first tries to reload existing modules. This doesn’t work well with Numba, and can produce errors like TypeError: No matching definition for argument type(s).

There is a fix in the Spyder preferences. Open the “Preferences” window, select “Console”, then “Advanced Settings”, click the “Set UMR excluded modules” button, and add numba inside the text box that pops up.

To see the setting take effect, be sure to restart the IPython console or kernel. Why does Numba complain about the current locale?

If you get an error message such as the following:

RuntimeError: Failed at nopython (nopython mode backend)
LLVM will produce incorrect floating-point code in the current locale

it means you have hit a LLVM bug which causes incorrect handling of floating-point constants. This is known to happen with certain third-party libraries such as the Qt backend to matplotlib.

To work around the bug, you need to force back the locale to its default value, for example:

import locale
locale.setlocale(locale.LC_NUMERIC, 'C')

1.11.4. Miscellaneous How do I reference/cite/acknowledge Numba in other work?

For academic use, the best option is to cite our ACM Proceedings: Numba: a LLVM-based Python JIT compiler.