uv/docs/preview/toolchains.md

Warning: This documentation refers to experimental features that may change.

Toolchains

A Python toolchain is composed of a Python interpreter (i.e. the python executable), the standard library, and other supporting files. It is common for an operating system to come with a Python toolchain installed and there are many tools to help manage Python toolchains.

Requesting a toolchain

uv will automatically download a toolchain if it cannot be found.

In stable commands, this behavior requires enabling preview mode. For example, when creating a virtual environment:

uv venv --preview --python 3.11.6

uv will ensure that Python 3.11.6 is available — downloading and installing it if necessary — then create the virtual environment with it.

For commands that are in preview, like uv sync, preview behavior is always on.

uv sync --python 3.12.3

Many toolchain request formats are supported:

• <version> e.g. 3, 3.12, 3.12.3
• <version-specifier> e.g. >=3.12,<3.13
• <implementation> e.g. cpython or cp
• <implementation>@<version> e.g. cpython@3.12
• <implementation><version> e.g. cpython3.12 or cp312
• <implementation><version-specifier> e.g. cpython>=3.12,<3.13
• <implementation>-<version>-<os>-<arch>-<libc> e.g. cpython-3.12.3-macos-aarch64-none

At this time, only CPython downloads are supported. However, PyPy support is planned.

Installing a toolchain

Sometimes it is preferable to install the toolchains before they are needed.

To install a toolchain at a specific version:

uv python install 3.12.3

To install the latest patch version:

uv python install 3.12

To install a version that satisfies constraints:

uv python install '>=3.8,<3.10'

To install multiple versions:

uv python install 3.9 3.10 3.11

Installing project toolchains

By default uv python install will verify that a managed toolchain is installed or install the latest version.

However, a project may define a .python-version file specifying the default Python toolchain to be used. If present, uv will install the toolchain listed in the file.

Alternatively, a project that requires multiple Python versions may also define a .python-versions file. If present, uv will install all of the toolchains listed in the file. This file takes precedence over the .python-version file.

uv will also respect Python requirements defined in a pyproject.toml file during project command invocations.

Viewing available toolchains

To list installed and available toolchains:

uv python list

By default, downloads for other platforms and old patch versions are hidden.

To view all versions:

uv python list --all-versions

To view toolchains for other platforms:

uv python list --all-platforms

To exclude downloads and only show installed toolchains:

uv python list --only-installed

Adjusting toolchain preferences

By default, uv will attempt to use Python toolchains found on the system and only download managed interpreters when necessary. However, It's possible to adjust uv's toolchain selection preference with the toolchain-preference option.

• only-managed: Only use managed toolchains, never use system toolchains.
• prefer-installed-managed: Prefer installed managed toolchains, but use system toolchains if not found. If neither can be found, download a managed interpreter.
• prefer-managed: Prefer managed toolchains, even if one needs to be downloaded, but use system toolchains if found.
• prefer-system: Prefer system toolchains, only use managed toolchains if no system interpreter is found.
• only-system: Only use system toolchains, never use managed toolchains.

These options allow disabling uv's managed toolchains entirely or always using them and ignoring any existing system installations.

Discovery order

When searching for a toolchain, the following locations are checked:

• Managed toolchains in the UV_PYTHON_INSTALL_DIR.
• A Python interpreter on the PATH as python3 on macOS and Linux, or python.exe on Windows.
• On Windows, the Python interpreter returned by py --list-paths that matches the requested version.

If a specific Python version is requested, e.g. --python 3.7, additional executable names are included in the search:

• A Python interpreter on the PATH as, e.g., python3.7 on macOS and Linux.