Package Discovery and Namespace Package

Note

a full specification for the keyword supplied to setup.cfg or setup.py can be found at keywords reference

Note

the examples provided here are only to demonstrate the functionality introduced. More metadata and options arguments need to be supplied if you want to replicate them on your system. If you are completely new to setuptools, the quickstart section is a good place to start.

Setuptools provide powerful tools to handle package discovery, including support for namespace package. Normally, you would specify the package to be included manually in the following manner:

[options]
#...
packages =
    mypkg1
    mypkg2
setup(
    #...
    packages = ['mypkg1', 'mypkg2']
)

This can get tiresome reallly quickly. To speed things up, we introduce two functions provided by setuptools:

[options]
packages = find:
#or
packages = find_namespace:
from setuptools import find_packages
#or
from setuptools import find_namespace_packages

Using find: or find_packages

Let’s start with the first tool. find: (find_packages) takes a source directory and two lists of package name patterns to exclude and include, and then return a list of str representing the packages it could find. To use it, consider the following directory

mypkg/
    src/
        pkg1/__init__.py
        pkg2/__init__.py
        additional/__init__.py

    setup.cfg #or setup.py

To have your setup.cfg or setup.py to automatically include packages found in src that starts with the name pkg and not additional:

[options]
packages = find:
package_dir =
    =src

[options.packages.find]
where = src
include = pkg*
exclude = additional
setup(
    #...
    packages = find_packages(
        where = 'src',
        include = ['pkg*',],
        exclude = ['additional',]
    ),
    package_dir = {"":"src"}
    #...
)

Using find_namespace: or find_namespace_packages

setuptools provides the find_namespace: (find_namespace_packages) which behaves similarly to find: but works with namespace package. Before diving in, it is important to have a good understanding of what namespace packages are. Here is a quick recap:

Suppose you have two packages named as follows:

/Users/Desktop/timmins/foo/__init__.py
/Library/timmins/bar/__init__.py

If both Desktop and Library are on your PYTHONPATH, then a namespace package called timmins will be created automatically for you when you invoke the import mechanism, allowing you to accomplish the following

>>> import timmins.foo
>>> import timmins.bar

as if there is only one timmins on your system. The two packages can then be distributed separately and installed individually without affecting the other one. Suppose you are packaging the foo part:

foo/
    src/
        timmins/foo/__init__.py
    setup.cfg # or setup.py

and you want the foo to be automatically included, find: won’t work because timmins doesn’t contain __init__.py directly, instead, you have to use find_namespace::

[options]
package_dir =
    =src
packages = find_namespace:

[options.packages.find_namespace]
where = src

When you install the zipped distribution, timmins.foo would become available to your interpreter.

You can think of find_namespace: as identical to find: except it would count a directory as a package even if it doesn’t contain __init__.py file directly. As a result, this creates an interesting side effect. If you organize your package like this:

foo/
    timmins/
        foo/__init__.py
    setup.cfg # or setup.py
    tests/
        test_foo/__init__.py

a naive find_namespace: would include tests as part of your package to be installed. A simple way to fix it is to adopt the aforementioned src layout.

Legacy Namespace Packages

The fact you can create namespace package so effortlessly above is credited to PEP 420. It use to be more cumbersome to accomplish the same result. Historically, there were two methods to create namespace packages. One is the pkg_resources style supported by setuptools and the other one being pkgutils style offered by pkgutils module in Python. Both are now considered deprecated despite the fact they still linger in many existing packages. These two differ in many subtle yet significant aspects and you can find out more on Python packaging user guide

pkg_resource style namespace package

This is the method setuptools directly supports. Starting with the same layout, there are two pieces you need to add to it. First, an __init__.py file directly under your namespace package directory that contains the following:

__import__("pkg_resources").declare_namespace(__name__)

And the namespace_packages keyword in your setup.cfg or setup.py:

[options]
namespace_packages = timmins
setup(
    # ...
    namespace_packages = ['timmins']
)

And your directory should look like this

/foo/
    src/
        timmins/
            __init__.py
            foo/__init__.py
    setup.cfg #or setup.py

Repeat the same for other packages and you can achieve the same result as the previous section.

pkgutil style namespace package

This method is almost identical to the pkg_resource except that the namespace_packages declaration is omitted and the __init__.py file contains the following:

__path__ = __import__('pkgutil').extend_path(__path__, __name__)

The project layout remains the same and setup.cfg remains the same.