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`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](https://scientific-python.org/specs/spec-0001/).

## Installation

```
pip install -U lazy_loader
```

We recommend using `lazy_loader` with Python >= 3.11.
If using Python 3.11, please upgrade to 3.11.9 or later.
If using Python 3.12, please upgrade to 3.12.3 or later.
These versions [avoid](https://github.com/python/cpython/pull/114781) a [known race condition](https://github.com/python/cpython/issues/114763).

## Usage

### Lazily load subpackages

Consider the `__init__.py` from [scikit-image](https://scikit-image.org):

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

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

You can now do:

```python
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`:

```python
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:

```python
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.

### Type checkers

Static type checkers and IDEs cannot infer type information from
lazily loaded imports. As a workaround you can load [type
stubs](https://mypy.readthedocs.io/en/stable/stubs.html) (`.pyi`
files) with `lazy.attach_stub`:

```python
import lazy_loader as lazy
__getattr__, __dir__, _ = lazy.attach_stub(__name__, "subpackages.pyi")
```

Note that, since imports are now defined in `.pyi` files, those
are not only necessary for type checking but also at runtime.

The SPEC [describes this workaround in more
detail](https://scientific-python.org/specs/spec-0001/#type-checkers).

### 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:

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

_Note that lazily importing *sub*packages,
i.e. `load('scipy.linalg')` will cause the package containing the
subpackage to be imported immediately; thus, this usage is
discouraged._

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

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

#### Optional requirements

One use for lazy loading is for loading optional dependencies, with
`ImportErrors` only arising when optional functionality is accessed. If optional
functionality depends on a specific version, a version requirement can
be set:

```python
np = lazy.load("numpy", require="numpy >=1.24")
```

In this case, if `numpy` is installed, but the version is less than 1.24,
the `np` module returned will raise an error on attribute access. Using
this feature is not all-or-nothing: One module may rely on one version of
numpy, while another module may not set any requirement.

_Note that the requirement must use the package [distribution name][] instead
of the module [import name][]. For example, the `pyyaml` distribution provides
the `yaml` module for import._

[distribution name]: https://packaging.python.org/en/latest/glossary/#term-Distribution-Package
[import name]: https://packaging.python.org/en/latest/glossary/#term-Import-Package
