# 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`:

    setup(
        ...,
        entry_points={
            "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.