diff --git a/Doc/library/importlib.metadata.rst b/Doc/library/importlib.metadata.rst index 5039bc6e85ee20..674ce5807fdf11 100644 --- a/Doc/library/importlib.metadata.rst +++ b/Doc/library/importlib.metadata.rst @@ -406,6 +406,84 @@ metadata in locations other than the file system, subclass a custom finder, return instances of this derived ``Distribution`` in the ``find_distributions()`` method. +Example +------- + +Consider for example a custom finder that loads Python +modules from a database:: + + class DatabaseImporter(importlib.abc.MetaPathFinder): + def __init__(self, db): + self.db = db + + def find_spec(self, fullname, target=None) -> ModuleSpec: + return self.db.spec_from_name(fullname) + + sys.meta_path.append(DatabaseImporter(connect_db(...))) + +That importer now presumably provides importable modules from a +database, but it provides no metadata or entry points. For this +custom importer to provide metadata, it would also need to implement +``DistributionFinder``:: + + from importlib.metadata import DistributionFinder + + class DatabaseImporter(DistributionFinder): + ... + + def find_distributions(self, context=DistributionFinder.Context()): + query = dict(name=context.name) if context.name else {} + for dist_record in self.db.query_distributions(query): + yield DatabaseDistribution(dist_record) + +In this way, ``query_distributions`` would return records for +each distribution served by the database matching the query. For +example, if ``requests-1.0`` is in the database, ``find_distributions`` +would yield a ``DatabaseDistribution`` for ``Context(name='requests')`` +or ``Context(name=None)``. + +For the sake of simplicity, this example ignores ``context.path``\. The +``path`` attribute defaults to ``sys.path`` and is the set of import paths to +be considered in the search. A ``DatabaseImporter`` could potentially function +without any concern for a search path. Assuming the importer does no +partitioning, the "path" would be irrelevant. In order to illustrate the +purpose of ``path``, the example would need to illustrate a more complex +``DatabaseImporter`` whose behavior varied depending on +``sys.path``/``PYTHONPATH``. In that case, the ``find_distributions`` should +honor the ``context.path`` and only yield ``Distribution``\ s pertinent to that +path. + +``DatabaseDistribution``, then, would look something like:: + + class DatabaseDistribution(importlib.metadata.Distributon): + def __init__(self, record): + self.record = record + + def read_text(self, filename): + """ + Read a file like "METADATA" for the current distribution. + """ + if filename == "METADATA": + return f"""Name: {self.record.name} + Version: {self.record.version} + """ + if filename == "entry_points.txt": + return "\n".join( + f"""[{ep.group}]\n{ep.name}={ep.value}""" + for ep in self.record.entry_points) + + def locate_file(self, path): + raise RuntimeError("This distribution has no file system") + +This basic implementation should provide metadata and entry points for +packages served by the ``DatabaseImporter``, assuming that the +``record`` supplies suitable ``.name``, ``.version``, and +``.entry_points`` attributes. + +The ``DatabaseDistribution`` may also provide other metadata files, like +``RECORD`` (required for ``Distribution.files``) or override the +implementation of ``Distribution.files``. See the source for more inspiration. + .. _`entry point API`: https://setuptools.readthedocs.io/en/latest/pkg_resources.html#entry-points .. _`metadata API`: https://setuptools.readthedocs.io/en/latest/pkg_resources.html#metadata-api