According to the python documentation:
Packages are a way of structuring Python’s module namespace by using “dotted module names”. For example the module name A.B designates a submodule named B in a package named A. [...]
The way that this behaviour is achieved in practice is by creating a folder structure like the following:
package/ Top-level package (import package) __init__.py Marks the folder as a package and contains initialization moduleA.py module (from package import moduleA) subpackage/ subpackage (from package import subpackage) __init__.py moduleB.py module (from package.subpackage import moduleB) ...
The page linked on top of this section contains a more detailed examples as well as more information on how to write and use packages.
The python community widely uses the term ‘package’ for both a package in the sense of a collection of subpackages and modules, as well as all the additional files necessary for building and installing a package. More about distributing packages here.
In this documentation we will refer to the latter as a distribution when we wish to distinguish between the two concepts.
distribution/ Distribution enclosing package and additional files package/ The package that the distribution installs __init__.py ... MANIFEST.in (optional) lists non-python files to be installed README.rst (optional) description to be used by github etc and PyPI setup.py (required) contains requirements, metainformation, etc
Incidentally a distribution can contain and install more than one package at a time.
The most user-friendly way to distribute a package is to create such a distribution and uploading it to PyPI. Users then can simply install the package(s) by running
Installing a Package¶
What happens when
pip is used to install a package is explained in detail in the python packaging user guide. However it is worth summarizing some points here.
- the dependencies on other python packages as specified in
setup.pyare automatically resolved and installed;
- a folder
<distribution-name>.egg-info/is created, containing metadata about the distribution;
- if the
-eoption is given, a symbolic link is put into the python package search path, pointing to the distribution top level directory. This is where the
.egg-infofolder gets created. Changes to the source code will be picked up by python without reinstalling, however changes to the distribution metadata will not.
setuptools package to which
pip is a frontend has a feature called entry points. When a distribution which registers entry points is installed, the entry point specifications are written to a file inside the distribution’s
setuptools provides a package
pkg_resources which can find these entry points by distribution, group and/or name and load the data structure to which it points. This is the way AiiDA finds and loads classes provided by plugins.
There is a list of entry point groups defined by AiiDA on the page AiiDA Entry Points.