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Object-like Entrypoint Access

Project description

The setuptools package provides a handy feature known as “entrypoints”. An entrypoint is a named reference to a Python object, such as a function, class, or value, which other packages can reference using the entrypoint group and name. These are useful for creating extendable Python projects, as one project can create entrypoints that will be used as hooks or plugins by another project. However, making use of entrypoints is rather awkward; setuptools provides the pkg_resources.iter_entry_points() function, which iterates through the defined entrypoints, but the caller must still load the entrypoint, which could possibly result in some errors which must be caught so the next entrypoint with the given name can be tried.

This package provides an alternative interface to entrypoints which eliminates this boilerplate, is easier to use, and also speeds up repetitive access to specific entrypoints. This is done through the creation of dictionary-like and list-like objects to represent entrypoint groups and lists of loaded entrypoints. A special object, eps, is also provided that allows reference to an entrypoint group via simple attribute access.

Looking Up an Entrypoint

We’ll begin with an example. Let’s say your application looks for an entrypoint named “hook” in the entrypoint group “example_app.example_group”. This can be as simple as:

hook = entrypointer.eps.example_app.example_group['hook']

If the named entrypoint doesn’t exist, this will raise a KeyError, but you can use the get() method to return something else instead:

hook = entrypointer.eps.example_app.example_group.get('hook')

For a hook method, your application may actually prefer to call all defined hooks. This can be accomplished like so:

for hook in entrypointer.eps.example_app.example_group.get_all('hook', []):
    hook('calling your hook')

Some applications may wish to use entrypoint group names that happen to not be valid Python identifiers, e.g., “Example App.Example Group”. This can be accomplished with a simple getattr() call on entrypoint.eps:

group = getattr(entrypointer.eps, 'Example App.Example Group')

The only restriction on a group name is that no component can start with a leading underscore (‘_’).

Using Entrypoints without entrypointer.eps

The entrypointer.eps object is provided for convenience; it is not required to be used. To obtain a dictionary-like object for an entrypoint group, simply instantiate the entrypointer.EntrypointDict class:

group = entrypointer.EntrypointDict('example_app.example_group')
hooks = group.get_all('hook')

This class provides all of the basic dict methods, such as keys() and values(). It also provides variants, such as items_all(), which yield list-like objects of entrypoints. This allows use of entrypoints for a wide variety of effects, such as the “hook” pattern demonstrated above, or for extending commands your application makes available, or for a variety of other uses.

Obtaining a List of Entrypoints

Although it is recommended to use the entrypointer.EntrypointDict class for accessing entrypoints, it is possible to use the list-like object, entrypointer.EntrypointList, directly. It is instantiated with the entrypoint group name and the name of the entrypoint itself, e.g.:

hooks = entrypointer.EntrypointList('example_app.example_group', 'hook')

Efficiency Considerations

The pkg_resources.iter_entry_points() function is somewhat slow. The entrypointer.EntrypointList and entrypointer.EntrypointDict classes are designed to call it as few times as possible and cache the results for future use. Further, they are optimized for the common case of using the first entrypoint with a given name; they stop loading entrypoints after the first one that loads successfully. Finally, these classes do their work lazily, calling pkg_resources.iter_entry_points() only when necessary to implement the method being called. This causes the expense of using entrypoints to be amortized over the lifetime of the application, while still maintaining a high speed due to the caching. However, one side effect of this caching is that changes to the installed packages that would be picked up by calling pkg_resources.iter_entry_points() will not be detected when using entrypointer; if this is important to your application, you may want to call the entrypointer.EntrypointDict and entrypoint.EntrypointList classes directly and discard them when you are done; note that this will reduce the efficiency, as they will need to call pkg_resources.iter_entry_points() more frequently.

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