loky
A robust implementation of concurrent.futures.ProcessPoolExecutor
Decision gist · record as of 2026-08-14
Yes. Loky is actively maintained, has no known vulnerabilities, and solves real problems in the standard library's process pool implementation—particularly safer spawning behavior and reusability. The single dependency (cloudpickle) is lightweight. Install it if you need robust parallel task execution, especially on macOS or with third-party C libraries, or if you want to avoid repeated executor spawn overhead.AI-flagged interpretation of the facts on this page — verify before relying
Before you install
- Requires Python 3.9 or later.
- Low friction: pure Python wheel with a single runtime dependency (cloudpickle).
- Actively maintained with recent commits; last release was 352 days ago and the repository shows ongoing activity.
License · maintenance · safety
permissive license (permissive) — BSD 3-Clause License permits commercial and private use with minimal restrictions; you must retain copyright notices and disclaimers in source and binary distributions.
last release 2025-08-27 (352 days) · last repo commit 2026-08-11 · 625 stars
0 known vulnerabilities (OSV.dev, 2026-08-14) · 1,977,414 downloads/mo, #3,391 on PyPI
Alternatives
Verify before relying
pip install loky
from loky import get_reusable_executor
def task(x):
return x * 2
executor = get_reusable_executor(max_workers=4)
result = executor.submit(task, 5).result()
print(result)- Whether psutil is truly optional or required for production use despite being listed as optional in the description.
- Performance characteristics and overhead compared to standard concurrent.futures.ProcessPoolExecutor in Python 3.7+.
What it is and what it does
Loky is a robust implementation of Python's ProcessPoolExecutor that addresses known limitations in the standard library's multiprocessing and concurrent.futures modules. It uses fork + exec on POSIX systems instead of fork alone, which prevents crashes when third-party libraries (like OpenMP or macOS Accelerate) are involved. The package integrates cloudpickle transparently, allowing you to submit interactively defined functions and lambdas to worker processes without the typical pickling constraints.
The core feature is the reusable executor pattern: instead of spawning and tearing down a new executor for each batch of tasks, you can maintain a singleton instance that persists across calls, with optional automatic worker shutdown after idle timeouts. This avoids repeated spawning overhead. Loky also eliminates the need for `if __name__ == "__main__":` guards on Windows by leveraging cloudpickle to serialize functions from the `__main__` module, and it implements deadlock-free handling of worker process crashes with meaningful error reporting.
Use it for
- Running CPU-bound tasks in parallel across multiple cores without fork-only multiprocessing hazards.
- Submitting lambda functions and locally defined functions to worker processes in interactive environments.
- Maintaining a persistent pool of workers across multiple independent parallel workloads to reduce spawn overhead.
- Executing tasks on macOS or with OpenMP-linked libraries where standard multiprocessing.Pool causes crashes.
- Batch processing workflows that need automatic worker cleanup after periods of inactivity.
Worth the install?
AI-flagged interpretation of the facts on this page. Verify before relying on it.
Yes.
Loky is actively maintained, has no known vulnerabilities, and solves real problems in the standard library's process pool implementation—particularly safer spawning behavior and reusability. The single dependency (cloudpickle) is lightweight. Install it if you need robust parallel task execution, especially on macOS or with third-party C libraries, or if you want to avoid repeated executor spawn overhead.
Install
loky on PyPI
Before you install
Low friction: pure Python wheel with a single runtime dependency (cloudpickle). Actively maintained with recent commits; last release was 352 days ago and the repository shows ongoing activity.
Requires Python 3.9 or later.
License in practice
BSD 3-Clause License permits commercial and private use with minimal restrictions; you must retain copyright notices and disclaimers in source and binary distributions.
Quickstart
pip install loky
from loky import get_reusable_executor
def task(x):
return x * 2
executor = get_reusable_executor(max_workers=4)
result = executor.submit(task, 5).result()
print(result)Verify before relying
- Whether psutil is truly optional or required for production use despite being listed as optional in the description.
- Performance characteristics and overhead compared to standard concurrent.futures.ProcessPoolExecutor in Python 3.7+.
Package facts
| License | permissive license permissive |
| Python support | Supports the current Python release >=3.9 |
| Install friction | Low. Pure-Python wheel |
| Runtime dependencies | 1 packagecloudpickle |
| Maintenance | Actively maintained 352 days since the last release |
| Last repo commit | |
| First released | |
| Downloads | 1,977,414 / month, #3,391 on PyPI 30-day window, as of 2026-08-14 |
| Known vulnerabilities | None known OSV.dev, checked 2026-08-14 |
| Classifiers | Development Status :: 5 - Production/StableEnvironment :: ConsoleIntended Audience :: DevelopersIntended Audience :: Science/ResearchLicense :: OSI Approved :: BSD LicenseOperating System :: OS IndependentProgramming Language :: Python :: 3Programming Language :: Python :: 3.10Programming Language :: Python :: 3.11Programming Language :: Python :: 3.12Programming Language :: Python :: 3.13Programming Language :: Python :: 3.9Topic :: Scientific/EngineeringTopic :: Software Development :: LibrariesTopic :: Utilities |
Evidence: loky-3.5.6-py3-none-any.whl
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