Explain Python entry points?

Question

I've read the documentation on egg entry points in Pylons and on the Peak pages, and I still don't really understand. Could someone explain them to me?

Answer 1

EntryPoints provide a persistent, filesystem-based object name registration and name-based direct object import mechanism (implemented by the setuptools package).

They associate names of Python objects with free-form identifiers. So any other code using the same Python installation and knowing the identifier can access an object with the associated name, no matter where the object is defined. The associated names can be any names existing in a Python module; for example name of a class, function or variable. The entry point mechanism does not care what the name refers to, as long as it is importable.

As an example, let's use (the name of) a function, and an imaginary python module with a fully-qualified name 'myns.mypkg.mymodule':

def the_function():
   "function whose name is 'the_function', in 'mymodule' module"
   print "hello from the_function"

Entry points are registered via an entry points declaration in setup.py. To register the_function under entrypoint called 'my_ep_func':

    entry_points = {
        'my_ep_group_id': [
            'my_ep_func = myns.mypkg.mymodule:the_function'
        ]
    },

As the example shows, entry points are grouped; there's corresponding API to look up all entry points belonging to a group (example below).

Upon a package installation (ie. running 'python setup.py install'), the above declaration is parsed by setuptools. It then writes the parsed information in special file. After that, the pkg_resources API (part of setuptools) can be used to look up the entry point and access the object(s) with the associated name(s):

import pkg_resources

named_objects = {}
for ep in pkg_resources.iter_entry_points(group='my_ep_group_id'):
   named_objects.update({ep.name: ep.load()})

Here, setuptools read the entry point information that was written in special files. It found the entry point, imported the module (myns.mypkg.mymodule), and retrieved the_function defined there, upon call to pkg_resources.load().

Assuming there were no other entry point registrations for the same group id, calling the_function would then be simple:

>>> named_objects['my_ep_func']()
hello from the_function

Thus, while perhaps a bit difficult to grasp at first, the entry point mechanism is actually quite simple to use. It provides an useful tool for pluggable Python software development.