lazy-loader 0.1rc1

lazy-loader makes it easy to load subpackages and functions on demand.

Motivation

1. Allow subpackages to be made visible to users without incurring import costs.
2. Allow external libraries to be imported only when used, improving import times.

For a more detailed discussion, see the SPEC.

Installation

pip install -U lazy-loader

Usage

Lazily load subpackages

Consider the __init__.py from scikit-image:

subpackages = [
    ...,
    'filters',
    ...
]

import lazy_loader as lazy
__getattr__, __dir__, _ = lazy.attach(__name__, subpackages)

You can now do:

import skimage as ski
ski.filters.gaussian(...)

The filters subpackages will only be loaded once accessed.

Lazily load subpackages and functions

Consider skimage/filters/__init__.py:

from ..util import lazy

__getattr__, __dir__, __all__ = lazy.attach(
    __name__,
    submodules=['rank']
    submod_attrs={
        '_gaussian': ['gaussian', 'difference_of_gaussians'],
        'edges': ['sobel', 'scharr', 'prewitt', 'roberts',
                  'laplace', 'farid']
    }
)

The above is equivalent to:

from . import rank
from ._gaussian import gaussian, difference_of_gaussians
from .edges import (sobel, scharr, prewitt, roberts,
                    laplace, farid)

Except that all subpackages (such as rank) and functions (such as sobel) are loaded upon access.

Early failure

With lazy loading, missing imports no longer fail upon loading the library. During development and testing, you can set the EAGER_IMPORT environment variable to disable lazy loading.

External libraries

The lazy.attach function discussed above is used to set up package internal imports.

Use lazy.load to lazily import external libraries:

linalg = lazy.load('scipy.linalg')  # `linalg` will only be loaded when accessed

You can also ask lazy.load to raise import errors as soon as it is called:

linalg = lazy.load('scipy.linalg', error_on_import=True)