Expand the 'Extending' docs with an example. (#113187)

* Expand the 'Extending' docs to provide a minimal example. Closes python/importlib_metadata#427.

Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>
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Jason R. Coombs 2024-05-01 13:04:02 -04:00 committed by GitHub
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@ -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