prybar: Create temporary pkg_resources entry points at runtime

Use prybar to temporarily define pkg_resources entry points at runtime. The primary use case is testing code which works with entry points.

Entry points?

Entry points are Python’s way of advertising and consuming plugins. Python packages can advertise an object (function, class, data, etc) which can be discovered and loaded by another package.

Entry points are normally statically defined in package metadata (e.g. via The static nature of entry points makes thoroughly testing code that loads entry points awkward.

prybar allows entry points to be created and removed on the fly, making it easy to test your plugin loading code.


prybar is available from PyPi as prybar:

$ pip install prybar

prybar requires Python 3.6 or greater.

Using prybar

prybar provides dynamic_entrypoint() — a context manager which creates an entry point when entered and removes it when left. It can also be used as a function decorator, or via explicit .start() and .stop() calls.

Brief Example

pkg_resources.iter_entry_points is the normal way to discover entry points. We’ll define a function to load the first named entry point in a group (category).

>>> from pkg_resources import iter_entry_points
>>> def load_entrypoint(group, name):
...     return next((ep.load() for ep in
...                 iter_entry_points(group, name=name)), None)

Initially no entry point will exist:

>>> load_entrypoint('example.hash_types', 'sha256') is None

We can create it at runtime with prybar:

>>> from prybar import dynamic_entrypoint
>>> with dynamic_entrypoint('example.hash_types',
...                         name='sha256', module='hashlib'):
...     hash = load_entrypoint('example.hash_types', 'sha256')
...     hash(b'foo').hexdigest()[:6]

It’s gone again after leaving the with block:

>>> load_entrypoint('example.hash_types', 'sha256') is None

More examples

Multiple entry points can be registered by nesting with blocks, or using contextlib.ExitStack:

>>> from contextlib import ExitStack
>>> epoints = [dynamic_entrypoint('example.types',
...                               name=name, module='builtins')
...            for name in ['int', 'float', 'str', 'bool']]
>>> with ExitStack() as stack:
...     for ep in epoints:
...         stack.enter_context(ep)
...     for ep in iter_entry_points('example.types'):
...         t = ep.load()
...         t('12')

dynamic_entrypoint can also be used as a decorator:

>>> @dynamic_entrypoint('example.hash_types',
...                     name='sha256', module='hashlib')
... def example_function():
...     hash = load_entrypoint('example.hash_types', 'sha256')
...     return hash(b'foo').hexdigest()[:6]
>>> load_entrypoint('example.hash_types', 'sha256') is None
>>> example_function()

And via start() and stop() methods:

>>> sha256_dep = dynamic_entrypoint(
...     'example.hash_types', name='sha256', module='hashlib')
>>> load_entrypoint('example.hash_types', 'sha256') is None
>>> sha256_dep.start()
>>> hash = load_entrypoint('example.hash_types', 'sha256')
>>> hash(b'foo').hexdigest()[:6]
>>> sha256_dep.stop()
>>> load_entrypoint('example.hash_types', 'sha256') is None

The entry point can be specified in several ways in addition to the name and module seen above.

attribute can be used if the desired entry point name differs from the target’s name in the module:

>>> from collections import Counter
>>> with dynamic_entrypoint('example', name='thing',
...                         module='collections',
...                         attribute='Counter'):
...     thing = load_entrypoint('example', 'thing')
...     thing is Counter

A function or class can be passed as entrypoint, from which the name, module and attribute are inferred:

>>> with dynamic_entrypoint('example', entrypoint=Counter):
...     thing = load_entrypoint('example', 'Counter')
...     thing is Counter

The name attribute can be used to override the entry point’s name:

>>> with dynamic_entrypoint('example', name='foo',
...                         entrypoint=Counter):
...     thing = load_entrypoint('example', 'foo')
...     thing == Counter

Nested functions can be specified:

>>> with dynamic_entrypoint('example', module='collections',
...                         name='fromkeys',
...                         attribute=('Counter', 'fromkeys')):
...     thing = load_entrypoint('example', 'fromkeys')
...     thing == Counter.fromkeys
>>> with dynamic_entrypoint('example', entrypoint=Counter.fromkeys):
...     thing = load_entrypoint('example', 'fromkeys')
...     thing == Counter.fromkeys
>>> with dynamic_entrypoint('example', name='foo',
...                         entrypoint=Counter.fromkeys):
...     thing = load_entrypoint('example', 'foo')
...     thing == Counter.fromkeys

A string using the entry point syntax from can be used:

>>> with dynamic_entrypoint(
...         'example',
...         entrypoint='thing = collections:Counter.fromkeys'):
...     thing = load_entrypoint('example', 'thing')
...     thing == Counter.fromkeys

Or a pkg_resources.EntryPoint object can be passed:

>>> from pkg_resources import EntryPoint
>>> ep = EntryPoint('thing', 'collections', attrs=('Counter', 'fromkeys'))
>>> with dynamic_entrypoint('example', entrypoint=ep):
...     thing = load_entrypoint('example', 'thing')
...     thing == Counter.fromkeys

API Reference


prybar.dynamic_entrypoint(group: str, entrypoint: Union[Callable, Type[object], str, pkg_resources.EntryPoint, None] = None, *, name: Optional[str] = None, module: Optional[str] = None, attribute: Optional[str] = None, scope: Optional[str] = None, working_set: Optional[pkg_resources.WorkingSet] = None) → prybar.DynamicEntrypoint[source]

prybar.dynamic_entrypoint() registers and de-registers pkg_resources entry points at runtime.

It acts as a context manager and function decorator. The entrypoint is registered within the with statement, or while the decorated function is running. It can also be registered and de-registered manually using its start() and stop() methods.

The context manager/decorator is re-entrant, i.e. a single instance can be used in multiple with statements, or decorate multiple functions, and each usage site can be entered multiple times while the same or other usage sites are still in use. The entry point is registered once when the first with block or decorated function is entered, and remains registered until all with blocks/decorated functions have been left.

In contrast, the start()/stop() API immediatley registers and deregisters the entry point. Calling start multiple times has no effect after the first, neither does calling stop.

Use of the start()/stop() API is incompatible with the context manager/decorator API. An error will be raised if an attempt is made to start()/stop() while a with block or decorated function is active, and vice versa.

The group must always be provided as a string, but he entrypoint can be specified in several ways:

  • By providing a name and module. The target in the module can be specified with attribute if it differs from name.
  • By passing a function or class as entrypoint. The name, module and attribute are then inferred automatically. The name can be overriden.
  • By passing a string to be parsed as entrypoint. The format is the same as used in, e.g. "my_name = my_module.submodule:my_func".
  • By passing a pre-created pkg_resources.EntryPoint object as entrypoint.
  • group – The name of the entrypoint group to register the entrypoint under. For example, myproject.plugins.
  • name – The name of the entrypoint.
  • module – The dotted path of the module the entrypoint references.
  • attribute – The name of the object within the module the entrypoint references (defaults to name).
  • entrypoint – Either a function (or other object), or an entrypoint string to parse, or a pre-created pkg_resources.Entrypoint object
  • scope

    A name to scope your entrypoints within. group, name pairs must be unique within a scope, but multiple entrypoints with the same group and name can be created in different scopes. The scope defaults to prybar.scope.default if not specified.

    Note: internally this defines the name of the pkg_resources.Distribution that the entrypoint is registered under. If you specify a scope you should use your package’s name as the prefix to avoid conflicts with entry points from other packages.

  • working_set – The pkg_resources.WorkingSet to register entrypoints in. Defaults to the default pkg_resources.working_set.

The context manager/decorator — a prybar.DynamicEntrypoint, which also supports start() and stop() methods.