lazy-static

lazy module constants -- think lazy imports but for assignments

roughly named after the (deprecated) rust crate lazy_static

installation

pip install lazy-static

why ?

python 3.15 adds lazy imports which are neat but are a little too easy to accidentally make eager.

take this small example for a little wrapper around pyyaml:

import functools

lazy import yaml

loads = functools.partial(yaml.load, Loader=yaml.CSafeLoader)

did you catch the bug? yeah that import isn't lazy due to accessing attributes!

it would be great if I could do:

lazy loads = functools.partial(yaml.load, Loader=yaml.CSafeLoader)

but there's no such thing...¹ unless...

Usage

lazy-static adds a single api: @lazy_static.lazy

taking a simple example:

import argparse
from lazy_static import lazy

@lazy
def computed_once() -> int:
    print('computing!')
    return 5 * 5 * 5

def main() -> int:
    parser = argparse.ArgumentParser()
    parser.parse_args()  # just for `--help`
    print(f'got {computed_once}')
    return 0

if __name__ == '__main__':
    raise SystemExit(main())

the lazy attribute is deferred when not accessed:

$ python3 t.py --help
usage: t.py [-h]

options:
  -h, --help  show this help message and exit

but acts like a normal variable otherwise!

$ python3 t.py
computing!
got 125

it even works with static typing!

$ diff -u t.py.bak t.py
--- t.py.bak    2026-07-24 17:47:14.278681851 -0400
+++ t.py    2026-07-24 17:47:22.759961815 -0400
@@ -10,6 +10,7 @@
     parser = argparse.ArgumentParser()
     parser.parse_args()  # just for `--help`
     print(f'got {computed_once}')
+    reveal_type(computed_once)
     return 0

 if __name__ == '__main__':

$ mypy t.py
t.py:13: note: Revealed type is "int"
Success: no issues found in 1 source file

if we take the earlier example we can define our loads partial lazily while preserving the lazy import!

import functools

from lazy_static import lazy
lazy import yaml

@lazy
def loads():
    return functools.partial(yaml.loads, Loader=yaml.CSafeLoader)