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https://github.com/marrow/package

Plugin discovery and management, dependency graphing, and object import path canonicalization.
https://github.com/marrow/package

canonicalization cpython plugin-manager plugin-system pypi pypy python reference-parsing

Last synced: about 2 months ago
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Plugin discovery and management, dependency graphing, and object import path canonicalization.

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README 

        
==============

Marrow Package

==============



    © 2014-2023 Alice Bevan-McGregor and contributors.



..



    https://github.com/marrow/package



..



    |latestversion| |masterstatus| |mastercover| |issuecount|



1. What is Marrow Package?

==========================



This package is a combination of utilities for handling object lookup, resolving object names, and managing simple to

complex plugin architectures.  Notably it includes a dependency graph system for extensions and helper for looking up

qualified object names.



This library is fully unit tested where possible.



2. Installation

===============



Installing ``marrow.package`` is easy, just execute the following in a terminal::



    pip install marrow.package



**Note:** We *strongly* recommend always using a container, virtualization, or sandboxing environment of some kind when

developing using Python; installing things system-wide is yucky (for a variety of reasons) nine times out of ten.  We

prefer light-weight `virtualenv `_, others prefer solutions as

robust as `Vagrant `_.



If you add ``marrow.package`` to the ``install_requires`` argument of the call to ``setup()`` in your application's

``setup.py`` file, Marrow Package will be automatically installed and made available when your own application or

library is installed.  We recommend using "less than" version numbers to ensure there are no unintentional

side-effects when updating.  Use ``marrow.package<2.2`` to get all bugfixes for the current release, and

``marrow.package<3.0`` to get bugfixes and feature updates while ensuring that large breaking changes are not installed.



2.1. Development Version

------------------------



    |developstatus| |developcover|



Development takes place on `GitHub `_ in the

`marrow.package `_ project.  Issue tracking, documentation, and downloads

are provided there.



Installing the current development version requires `Git `_, a distributed source code management

system.  If you have Git you can run the following to download and *link* the development version into your Python

runtime::



    git clone https://github.com/marrow/package.git

    (cd package; python setup.py develop)



You can then upgrade to the latest version at any time::



    (cd package; git pull; python setup.py develop)



If you would like to make changes and contribute them back to the project, fork the GitHub project, make your changes,

and submit a pull request.  This process is beyond the scope of this documentation; for more information see

`GitHub's documentation `_.



3. Getting Object References

============================



Object references describe the module and attribute path needed to resolve the object.  For example, ``foo:bar`` is a

reference that describes importing "foo" prior to retrieving an object named "bar" from the module.  On Python 3.3+ a

useful shortcut is provided, ``__qualname__`` which speeds up this lookup.



For example, let's define a class and get a reference to it::



    from marrow.package.canonical import name

    

    class Example(object):

        pass

    

    assert name(Example) == '__main__:Example'



You can, depending on platform, retrieve a reference to any of the following types of objects:



* Module level:

	* class

	* class instance

	* class method

	* class staticmethod

	* function

	* instance classmethod

	* instance method

	* instance staticmethod

* nested classes and methods

* closures



3.1. Resolving Plugin References

================================



The ``load`` utility can optionally be provided a plugin namespace to search. If the target object is found within the

namespace, the name of the plugin entry will be returned. By default, if a named plugin can **not** be found, a

``LookupError`` will be raised. If a direct reference is acceptable, the boolean ``direct`` argument (third positional)

can be made truthy to permit direct references.



    from marrow.package.canonical import name



    assert name(name, 'marrow.package.sample') == 'name'



4. Resolving Object References

==============================



Two utilities are provided which allow you resolve string path references to objects.  The first is quite simple::



    from marrow.package.loader import traverse

    

    assert traverse({'foo': {'bar': 27}}, 'foo.bar') == 27



This will search the dictionary described for a "foo" element, then "bar" element.



The ``traverse`` function takes some additional optional arguments.  If ``executable`` is ``True`` any executable

function encountered will be executed without arguments. Traversal will continue on the result of that call.  You can

change the separator as desired, i.e. to a '/' using the ``separator`` argument.



By default attributes (but not array elements) prefixed with an underscore are taboo.  They will not resolve, raising

a LookupError.  You can allow these by setting ``protect`` to ``False``.



Certain allowances are made: if a 'path segment' is numerical, it's treated as an array index. If attribute lookup

fails, it will re-try on that object using array notation and continue from there.  This makes lookup very flexible.



4.1. Resolving Import References

--------------------------------



The more complete API for name resolution uses the ``load`` function, which takes the same optional keyword arguments

as ``traverse``.  Additionally, this function accepts an optional ``namespace`` to search for plugins within.  For

example::



    from marrow.package.loader import load

    from pip import main

    

    # Load class Foo from example.objects

    load('example.objects:Foo')

        

    # Load the result of the class method ``new`` of the Foo object

    load('example.objects:Foo.new', executable=True)

    

    # Load the "pip" command-line interface.

    assert load('pip', 'console_scripts') is main



Providing a namespace does not prevent explicit object lookup (dot-colon notation) from working.



4.2. Caching Import References

------------------------------



An attribute-access dictionary is provided that acts as an import cache::



    from marrow.package.cache import PackageCache

    from pip import main

    

    cache = PackageCache('console_scripts')

    

    assert cache.pip is main

    assert cache['pip'] is main

    assert len(cache) == 1

    assert 'pip' in cache



4.3. Lazy Import Reference Attributes

-------------------------------------



You can lazily load and cache an object reference upon dereferencing from an instance using the ``lazyload`` utility

from the ``marrow.package.lazy`` module.  Assign the result of calling this function with either an object reference

passed in positionally::



    class MyClass:

        debug = lazyload('logging:debug')



Or the attribute path to traverse (using ``marrow.package.loader:traverse``) prefixed by a period::



    class AnotherClass:

        target = 'logging:info'

        log = lazyload('.target')



Any additional arguments are passed to the eventual call to `load()`.  This utility builds on a simpler one that is

also offered for fully-tested re-use, ``lazy``, a decorator like ``@property`` which will cache the result, with

thread-safe locking to ensure only one call will ever be made to the decorated function, per instance.



5. Managing Plugins

===================



This package provides two main methods of dealing with plugins and extensions, the first is simple, the second

provides full dependency graphing of the extensions.



5.1. Plugin Manager

-------------------



The ``PluginManager`` class takes two arguments: the first is the entry point ``namespace`` to search, the second is

an optional iterable of folders to add to the Python search path for installed packages, allowing your application to

have a dedicated plugins folder.



It provides a ``register`` method which take a name and the object to use as the plugin and registers it internally,

supporting both attribute and array-like notation for retrieval, as well as iteration of plugins (includes all entry

point plugins found and any custom registered ones).



5.2. Extension Manager

----------------------



At a higher level is a ``PluginManager`` subclass called ``ExtensionManager`` which additionally exposes a ``sort``

method capable of resolving dependency order for extensions which follow a simple protocol: have an attribute or array

element matching the following, all optional:



* ``provides`` — declare tags describing the features offered by the plugin

* ``needs`` — declare the tags that must be present for this extension to function

* ``uses`` — declare the tags that must be evaluated prior to this extension, but aren't hard requirements

* ``first`` — declare that this extension is a dependency of all other non-first extensions

* ``last`` — declare that this extension depends on all other non-last extensions

* ``excludes`` — declare tags that must not be present in other plugins for this one to be usable



6. Version History

==================



Version 1.0

-----------



* **Initial release.**  Combination of utilities from other Marrow projects.



Version 1.0.1

-------------



* **Extended decorator support.**  New code paths and tests added to cover canonicalization of decorated functions.



Version 1.0.2

-------------



* **Diagnostic information.**  Removed extraneous diagnostic information.



Version 1.1

-----------



* **Added lazy evaluation.**  There are two new helpers for caching of ``@property``-style attributes and lazy lookup

  of object references.



Version 1.2

-----------



* **Deprecated Python 2.6 and 3.3.** While no particular backwards incompatible change was made; as setuptools no

  longer supports these versions, these versions are now hard/impossible to test.

* **Allow extensions to declare exclusions.** Flags that must not be defined for the extension to be usable.



Version 2.0

-----------



* **Updated minimum Python version.** Marrow Package now requires Python 3.5 or later.

* **Removed Python 2 support and version specific code.** The project has been updated to modern Python packaging

  standards, including modern namespace use. Modern namespaces are wholly incompatible with the previous namespacing

  mechanism; this project can not be simultaneously installed with any Marrow project that is Python 2 compatible.

* **Extensive type annotation and in-development validation.** When run without optimizations (`-O` argument to Python

  or `PYTHONOPTIMIZE` environment variable) type annotations will be validated.

* **Reduced test fragility.** Previously the tests utilized the `console_scripts` namespace, this was fragile to the

  presence of other installed libraries, e.g. `numpy` broke the tests on Travis.



Version 2.1

-----------



* **Migrated from Travis-CI to GitHub Actions for test automation.**

* **Implement package-relative path lookup.** The `load` utility function can now resolve the path to a file relative

  to a package. This is particularly useful for looking up the path to template files or on-disk static assets.

* **Protected attribute access now fails.** Underscore-prefixed attributes are assumed to be "protected", with the

  technical note that Python adds new internal "double underscore" attributes which must not spontaneously exist, or

  generate errors other than `AttributeError`.

* **Tests are now independent of third-party plugin registration.**



Version 2.1.1

-------------



* **Update type hinting validation.** The ``typeguard`` package has removed a functional utility; decoration now used.

* **Canonical plugin name resolution.** The ``name()`` utility can now resolve the plugin name if given a plugin

  namespace to check.



7. License

==========



Marrow Package has been released under the MIT Open Source license.



7.1. The MIT License

--------------------



Copyright © 2014-2023 Alice Bevan-McGregor and contributors.



Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated

documentation files (the “Software”), to deal in the Software without restriction, including without limitation the

rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit

persons to whom the Software is furnished to do so, subject to the following conditions:



The above copyright notice and this permission notice shall be included in all copies or substantial portions of the

Software.



THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE

WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NON-INFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR

COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR

OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.



.. |masterstatus| image:: http://img.shields.io/travis/marrow/package/master.svg?style=flat

    :target: https://travis-ci.org/marrow/package

    :alt: Release Build Status



.. |developstatus| image:: http://img.shields.io/travis/marrow/package/develop.svg?style=flat

    :target: https://travis-ci.org/marrow/package

    :alt: Development Build Status



.. |latestversion| image:: http://img.shields.io/pypi/v/marrow.package.svg?style=flat

    :target: https://pypi.python.org/pypi/marrow.package

    :alt: Latest Version



.. |mastercover| image:: http://img.shields.io/codecov/c/github/marrow/package/master.svg?style=flat

    :target: https://codecov.io/github/marrow/package?branch=master

    :alt: Release Test Coverage



.. |developcover| image:: http://img.shields.io/codecov/c/github/marrow/package/develop.svg?style=flat

    :target: https://codecov.io/github/marrow/package?branch=develop

    :alt: Development Test Coverage



.. |issuecount| image:: http://img.shields.io/github/issues/marrow/package.svg?style=flat

    :target: https://github.com/marrow/package/issues

    :alt: Github Issues



.. |cake| image:: http://img.shields.io/badge/cake-lie-1b87fb.svg?style=flat