---
id: promise
version: "2.3"
license: MIT
license_treatment: permissive
maintenance: dormant
---

# promise — Promises/A+ implementation for Python

License: permissive · Maintenance: dormant · Downloads: 3.3M/mo

## What it is and what it does

Promise is a Python implementation of the Promises/A+ specification, a standard for handling asynchronous operations through chainable callbacks. It lets you wrap async work in a Promise object, resolve or reject it with a value, and attach handlers via .then() and .catch() that execute when the promise settles. The library supports composing multiple promises (Promise.all, Promise.for_dict) and wrapping existing futures.

The package has no runtime dependencies and works with Python 2.7 through 3.8 (and PyPy). It is designed to be a readable, performant alternative to raw callbacks for async patterns in Python, though modern Python now has native async/await syntax and asyncio, which may be preferable for new code.

Use it for:

- Wrapping callback-based async operations (e.g., I/O) in a chainable Promise interface for cleaner control flow
- Composing multiple async tasks with Promise.all() to wait for all to complete before proceeding
- Converting Python Futures into Promises for interoperability with promise-based libraries or frameworks
- Building promise-based libraries or frameworks that need a standard async abstraction layer
- Handling rejection and error propagation through .catch() in async workflows without explicit try/except nesting

## Worth the install?

AI-flagged interpretation of the facts above — verify before relying.

Implements Promises/A+ specification for Python, enabling asynchronous value resolution and rejection with chainable then/catch handlers.

Yes-with-conditions. The package is stable (MIT, no known vulnerabilities, 367 GitHub stars) but dormant since 2019. Install only if you are maintaining legacy code that already uses promise or integrating with a framework that requires it. For new projects, use Python's native async/await and asyncio instead.

## Install

pip install promise
uv add promise
poetry add promise

## Installing promise

Before you install: High install friction due to source-only distribution (promise-2.3.tar.gz). Maintenance is dormant—last release was 2019-12-18 and last commit 2024-07-09, with no active development signals.

License in practice: MIT license (permissive) places no restrictions on use, modification, or distribution. Safe for commercial and open-source projects.

Quickstart:

from promise import Promise

promise = Promise(lambda resolve, reject: resolve('RESOLVED!'))
result = promise.then(lambda value: print(value))

Verify before relying:

- Whether promise.get() method exists and how it blocks/retrieves resolved values (shown in docs but not formally specified)
- Python 3.9+ compatibility—classifiers list only up to 3.8; current support unclear
- Whether this package is maintained or if a modern alternative is recommended for new projects

## Package facts

- License: MIT (permissive)
- Python support: unspecified
- Install friction: high
- Maintenance: dormant
- Downloads: 3.3M/month (top 5,000 on PyPI)
- Known vulnerabilities: none known

## Tags

promises async pattern python, promises/a+ implementation, deferred value resolution, chainable async callbacks, promise then catch, future-like promises, concurrent value handling, async-pattern, legacy-maintained

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