nipreps / acres

Resource-loading utility for Python

Acres: Access resources on your terms

This module provides simple, consistent access to package resources.

from acres import Loader

loader = Loader(somepkg)

res_text = loader.readable('data/some_resource.txt').read_text()
res_bytes = loader.readable('data/another_resource.bin').read_bytes()

with loader.as_path('data') as data_dir:
    # data_dir is a pathlib.Path until the "with" scope closes

resource_path = loader.cached('data/another_resource.bin')
# The path pointed to by resource_path will exist until interpreter exit

Module loader

The primary use case for acres.Loader is to allow a directory module to provide access to its resources.

Suppose you have a module structure:

src/
  mypkg/
    data/
      resourceDir/
        ...
      __init__.py
      resource.ext
    __init__.py
    ...

In src/mypkg/data/__init__.py, add:

'''Data package

.. autofunction:: load_resource

.. automethod:: load_resource.readable

.. automethod:: load_resource.as_path

.. automethod:: load_resource.cached
'''

from acres import Loader

load_resource = Loader(__spec__.name)

mypkg.data.load_resource() is now a function that will return a Path to a resource that is guaranteed to exist until interpreter exit:

from mypkg.data import load_resource

resource_file: Path = load_resource('resource.ext')

For additional control, you can use load_resource.readable() to return a Path-like object that implements .read_text() and .read_bytes():

resource_contents: bytes = load_resource.readable('resource.ext').read_bytes()

Or a context manager with a limited lifetime:

with load_resource.as_path('resourceDir') as resource_dir:
    # Work with the contents of `resource_dir` as a `Path`

# Outside the `with` block, `resource_dir` may no longer point to an existing path.

Note that load_resource() is a shorthand for load_resource.cached(), whose explicitness might be more to your taste.

The __spec__.name anchor

Previous versions recommended using Loader(__package__). Before Python 3.10, __package__ might be None during a [zipimport][], and __package__ has been deprecated in Python 3.13, to be removed in 3.15.

__spec__.parent is an exact equivalent for __package__, but for __init__.py files, __spec__.name is equivalent. __spec__.name is also guaranteed to be a string and not None, which lets it play nicely with type checkers.

Interpreter-scoped resources, locally scoped loaders

Loader.cached uses a global cache. This ensures that cached files do not get unloaded if a Loader() instance is garbage collected, and it also ensures that instances created cheaply and garbage collected when out of scope.

Why acres?

importlib.resources provides a simple, composable interface, using the files() and as_file() functions:

from importlib.resources import files, as_file

with as_file(files(my_module) / 'data' / 'resource.ext') as resource_path:
    # Interact with resource_path as a pathlib.Path

# resource_path *may* no longer exist

files() returns a Traversable object, which is similar to a pathlib.Path, except that the object may not actually exist, so os functions may not work correctly on it. as_files() turns a Traversable into a Path that exists on the filesystem for the duration of the with block.

To make matters more complicated, if a package is unpacked on the filesystem, files() returns an actual Path object. It is therefore easy to miss bugs where a true Path is always needed but a Traversable may be returned when the package is zipped.

Finally, the scoping of as_files() is frequently inconvenient. If you pass a Path to an object inside an as_files() context, but the resource access is deferred, the Path may point to a nonexistent file or directory. Again, this bug will only exhibit when the package is zipped.

acres.Loader aims to clearly delineate the scopes and capabilities of the accessed resources, including providing an interpreter-lifetime scope.