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Registering Extensions with Entry Points

Often, third party packages will have a user-facing API as well as define extensions to the Numba compiler. In those situations, the new types and overloads can registered with Numba when the package is imported by the user. However, there are situations where a Numba extension would not normally be imported directly by the user, but must still be registered with the Numba compiler. An example of this is the numba-scipy package, which adds support for some SciPy functions to Numba. The end user does not need to import numba_scipy to enable compiler support for SciPy, the extension only needs to be installed in the Python environment.

Numba discovers extensions using the entry points feature of setuptools. This allows a Python package to register an initializer function that will be called before numba compiles for the first time. The delay ensures that the cost of importing extensions is deferred until it is necessary.

Adding Support for the “Init” Entry Point

A package can register an initialization function with Numba by adding the entry_points argument to the setup() function call in setup.py:

        "numba_extensions": [
            "init = numba_scipy:_init_extension",

Numba currently only looks for the init entry point in the numba_extensions group. The entry point should be a function (any name, as long as it matches what is listed in setup.py) that takes no arguments, and the return value is ignored. This function should register types, overloads, or call other Numba extension APIs. The order of initialization of extensions is undefined.

Testing your Entry Point

Numba loads all entry points when the first function is compiled. To test your entry point, it is not sufficient to just import numba; you have to define and run a small function, like this:

import numba; numba.njit(lambda x: x + 1)(123)

It is not necessary to import your module: entry points are identified by the entry_points.txt file in your library’s *.egg-info directory.

The setup.py build command does not create eggs, but setup.py sdist (for testing in a local directory) and setup.py install do. All entry points registered in eggs that are on the Python path are loaded. Be sure to check for stale entry_points.txt when debugging.