# What is PEP 751?

[Python Enhancement Proposal 751 (PEP 751)](https://peps.python.org/pep-0751/) introduces a standardized file format for specifying dependencies to enable reproducible installation in Python environments. This proposal, authored by Brett Cannon and accepted in March 2025, establishes a formal specification for lock files in the Python ecosystem.

[Lock files serve a critical purpose in modern software development](https://pydevtools.com/handbook/explanation/what-is-a-lock-file.md): they record the exact versions of dependencies needed to reproduce a consistent environment. Until now, the Python ecosystem lacked a standard format for lock files, with tools like [PDM](https://pydevtools.com/handbook/reference/pdm.md), [pip](https://pydevtools.com/handbook/reference/pip.md) freeze, [pip-tools](https://pydevtools.com/handbook/reference/pip-tools.md), [Poetry](https://pydevtools.com/handbook/reference/poetry.md), and [uv](https://pydevtools.com/handbook/reference/uv.md) each using their own approaches.

## Why Lock Files Matter

Lock files solve several key problems in dependency management:

1. Reproducibility: They ensure the same packages are installed regardless of when or where installation occurs.
2. Consistency: Everyone working on a project installs the exact same dependencies.
3. Security: By pinning specific versions and including file hashes, they prevent supply chain attacks.

Without a lock file standard, tooling has been fragmented, creating issues with portability and vendor lock-in. Different tools couldn't easily consume each other's lock files, forcing developers to choose a single ecosystem or manage complex conversions.

## Key Features of PEP 751

The PEP 751 lock file format (`pylock.toml`) offers several important features:

* Human-readable TOML format: Makes auditing and debugging easier
* No resolver needed at install time: Simplifies and speeds up installation
* Security by default: Includes file hashes to verify package integrity
* Compatibility with multiple use cases: Supports both single-use and multi-use lock files
* Environment markers support: Handles platform-specific dependencies
* Support for package attestations: Enhances supply chain security

## Lock File Structure

The standard defines a TOML-based file with a clear structure:

```toml
lock-version = "1.0"
environments = ["sys_platform == 'linux'", "sys_platform == 'win32'"]
requires-python = ">=3.12"
created-by = "tool-name"

[[packages]]
name = "package-name"
version = "1.2.3"
wheels = [
  {name = "package-1.2.3-py3-none-any.whl", hashes = {sha256 = "..."}}
]
```

This format captures all necessary information to install dependencies consistently across environments while maintaining human readability.

## Single-Use vs. Multi-Use Lock Files

PEP 751 supports two approaches to lock files:

1. Single-use lock files: Similar to `requirements.txt`, serving a single purpose (like development or production dependencies)
2. Multi-use lock files: Support multiple use cases through extras and dependency groups within a single file, reducing duplication and coordination challenges

## Adoption by Python Packaging Tools

Several tools have shipped PEP 751 support since the spec was accepted in March 2025:

- [pip](https://pydevtools.com/handbook/reference/pip.md) added an experimental `pip lock` command in 25.1 (April 2025) that writes `pylock.toml` files, and added experimental `pip install -r pylock.toml` in 26.1 (April 2026) for installing from one. Both sides are explicitly experimental: the generated lockfile is only valid for the Python version and platform that produced it, and the install side does not yet support extras or dependency groups. A `pip sync` command is being discussed ([pypa/pip#13737](https://github.com/pypa/pip/issues/13737)) but has not shipped.
- [uv](https://pydevtools.com/handbook/reference/uv.md) supports reading and exporting PEP 751 lockfiles via `uv export --format pylock.toml`, while keeping its own cross-platform `uv.lock` as the primary format.
- [PDM](https://pydevtools.com/handbook/reference/pdm.md) supports exporting to PEP 751 alongside its native lockfile.
- [Poetry](https://pydevtools.com/handbook/reference/poetry.md) tracks PEP 751 adoption in an open issue and has not shipped support as of April 2026.

Tools that maintain their own native lockfile often treat `pylock.toml` as an export target rather than a replacement, because per-tool lockfiles can capture information the PEP 751 format does not yet standardize (such as uv's cross-platform resolution).

## Learn More

* [How to create a pylock.toml lockfile](https://pydevtools.com/handbook/how-to/how-to-create-a-pylock-toml-lockfile.md)
* [How to install from a pylock.toml lockfile with pip](https://pydevtools.com/handbook/how-to/how-to-install-from-a-pylock-toml-lockfile-with-pip.md)
* [How to use a uv lockfile for reproducible Python environments](https://pydevtools.com/handbook/how-to/how-to-use-a-uv-lockfile-for-reproducible-python-environments.md)
* [What is a Lockfile?](https://pydevtools.com/handbook/explanation/what-is-a-lock-file.md)
* [Why pylock.toml Includes Digital Attestations](https://pydevtools.com/handbook/explanation/why-pylock-toml-includes-digital-attestations.md)
* [PEP 751](https://peps.python.org/pep-0751/)