uv/docs/concepts/python-versions.md

Python versions

A Python version is composed of a Python interpreter (i.e. the python executable), the standard library, and other supporting files.

Managed and system Python installations

Since it is common for a system to have an existing Python installation, uv supports discovering Python versions. However, uv also supports installing Python versions itself. To distinguish between these two types of Python installations, uv refers to Python versions it installs as managed Python installations and all other Python installations as system Python installations.

!!! note

uv does not distinguish between Python versions installed by the operating system vs those
installed and managed by other tools. For example, if a Python installation is managed with
`pyenv`, it would still be considered a _system_ Python version in uv.

Requesting a version

A specific Python version can be requested with the --python flag in most uv commands. For example, when creating a virtual environment:

$ uv venv --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.

The following Python version 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)

Additionally, a specific system Python interpreter can be requested with:

• <executable-path> (e.g., /opt/homebrew/bin/python3)
• <executable-name> (e.g., mypython3)
• <install-dir> (e.g., /some/environment/)

By default, uv will automatically download Python versions if they cannot be found on the system. This behavior can be disabled with the python-downloads option.

Python version files

The .python-version file can be used to create a default Python version request. uv searches for a .python-version file in the working directory and each of its parents. If none is found, uv will check the user-level configuration directory. Any of the request formats described above can be used, though use of a version number is recommended for interoperability with other tools.

A .python-version file can be created in the current directory with the uv python pin command.

A global .python-version file can be created in the user configuration directory with the uv python pin --global command.

Discovery of .python-version files can be disabled with --no-config.

uv will not search for .python-version files beyond project or workspace boundaries (except the user configuration directory).

Installing a Python version

uv bundles a list of downloadable CPython and PyPy distributions for macOS, Linux, and Windows.

!!! tip

By default, Python versions are automatically downloaded as needed without using
`uv python install`.

To install a Python version 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

To install a specific implementation:

$ uv python install pypy

All the Python version request formats are supported except those that are used for requesting local interpreters such as a file path.

By default uv python install will verify that a managed Python version is installed or install the latest version. If a .python-version file is present, uv will install the Python version listed in the file. A project that requires multiple Python versions may define a .python-versions file. If present, uv will install all the Python versions listed in the file.

!!! important

The available Python versions are frozen for each uv release. To install new Python versions,
you may need upgrade uv.

Installing Python executables

!!! important

Support for installing Python executables is in _preview_. This means the behavior is experimental
and subject to change.

To install Python executables into your PATH, provide the --preview option:

$ uv python install 3.12 --preview

This will install a Python executable for the requested version into ~/.local/bin, e.g., as python3.12.

!!! tip

If `~/.local/bin` is not in your `PATH`, you can add it with `uv tool update-shell`.

To install python and python3 executables, include the --default option:

$ uv python install 3.12 --default --preview

When installing Python executables, uv will only overwrite an existing executable if it is managed by uv — e.g., if ~/.local/bin/python3.12 exists already uv will not overwrite it without the --force flag.

uv will update executables that it manages. However, it will prefer the latest patch version of each Python minor version by default. For example:

$ uv python install 3.12.7 --preview  # Adds `python3.12` to `~/.local/bin`
$ uv python install 3.12.6 --preview  # Does not update `python3.12`
$ uv python install 3.12.8 --preview  # Updates `python3.12` to point to 3.12.8

Upgrading Python versions

!!! important

Support for upgrading Python versions is in _preview_. This means the behavior is experimental
and subject to change.

Upgrades are only supported for uv-managed Python versions.

Upgrades are not currently supported for PyPy and GraalPy.

uv allows transparently upgrading Python versions to the latest patch release, e.g., 3.13.4 to 3.13.5. uv does not allow transparently upgrading across minor Python versions, e.g., 3.12 to 3.13, because changing minor versions can affect dependency resolution.

uv-managed Python versions can be upgraded to the latest supported patch release with the python upgrade command:

To upgrade a Python version to the latest supported patch release:

$ uv python upgrade 3.12

To upgrade all installed Python versions:

$ uv python upgrade

After an upgrade, uv will prefer the new version, but will retain the existing version as it may still be used by virtual environments.

If the Python version was installed with preview enabled, e.g., uv python install 3.12 --preview, virtual environments using the Python version will be automatically upgraded to the new patch version.

!!! note

If the virtual environment was created _before_ opting in to the preview mode, it will not be
included in the automatic upgrades.

If a virtual environment was created with an explicitly requested patch version, e.g., uv venv -p 3.10.8, it will not be transparently upgraded to a new version.

Minor version directories

Automatic upgrades for virtual environments are implemented using a directory with the Python minor version, e.g.:

~/.local/share/uv/python/cpython-3.12-macos-aarch64-none

which is a symbolic link (on Unix) or junction (on Windows) pointing to a specific patch version:

$ readlink ~/.local/share/uv/python/cpython-3.12-macos-aarch64-none
~/.local/share/uv/python/cpython-3.12.11-macos-aarch64-none

If this link is resolved by another tool, e.g., by canonicalizing the Python interpreter path, and used to create a virtual environment, it will not be automatically upgraded.

Project Python versions

uv will respect Python requirements defined in requires-python in the pyproject.toml file during project command invocations. The first Python version that is compatible with the requirement will be used, unless a version is otherwise requested, e.g., via a .python-version file or the --python flag.

Viewing available Python versions

To list installed and available Python versions:

$ uv python list

To filter the Python versions, provide a request, e.g., to show all Python 3.13 interpreters:

$ uv python list 3.13

Or, to show all PyPy interpreters:

$ uv python list pypy

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

To view all versions:

$ uv python list --all-versions

To view Python versions for other platforms:

$ uv python list --all-platforms

To exclude downloads and only show installed Python versions:

$ uv python list --only-installed

See the uv python list reference for more details.

Finding a Python executable

To find a Python executable, use the uv python find command:

$ uv python find

By default, this will display the path to the first available Python executable. See the discovery rules for details about how executables are discovered.

This interface also supports many request formats, e.g., to find a Python executable that has a version of 3.11 or newer:

$ uv python find '>=3.11'

By default, uv python find will include Python versions from virtual environments. If a .venv directory is found in the working directory or any of the parent directories or the VIRTUAL_ENV environment variable is set, it will take precedence over any Python executables on the PATH.

To ignore virtual environments, use the --system flag:

$ uv python find --system

Discovery of Python versions

When searching for a Python version, the following locations are checked:

• Managed Python installations in the UV_PYTHON_INSTALL_DIR.
• A Python interpreter on the PATH as python, python3, or python3.x on macOS and Linux, or python.exe on Windows.
• On Windows, the Python interpreters in the Windows registry and Microsoft Store Python interpreters (see py --list-paths) that match the requested version.

In some cases, uv allows using a Python version from a virtual environment. In this case, the virtual environment's interpreter will be checked for compatibility with the request before searching for an installation as described above. See the pip-compatible virtual environment discovery documentation for details.

When performing discovery, non-executable files will be ignored. Each discovered executable is queried for metadata to ensure it meets the requested Python version. If the query fails, the executable will be skipped. If the executable satisfies the request, it is used without inspecting additional executables.

When searching for a managed Python version, uv will prefer newer versions first. When searching for a system Python version, uv will use the first compatible version — not the newest version.

If a Python version cannot be found on the system, uv will check for a compatible managed Python version download.

Python pre-releases

Python pre-releases will not be selected by default. Python pre-releases will be used if there is no other available installation matching the request. For example, if only a pre-release version is available it will be used but otherwise a stable release version will be used. Similarly, if the path to a pre-release Python executable is provided then no other Python version matches the request and the pre-release version will be used.

If a pre-release Python version is available and matches the request, uv will not download a stable Python version instead.

Disabling automatic Python downloads

By default, uv will automatically download Python versions when needed.

The python-downloads option can be used to disable this behavior. By default, it is set to automatic; set to manual to only allow Python downloads during uv python install.

!!! tip

The `python-downloads` setting can be set in a
[persistent configuration file](./configuration-files.md) to change the default behavior, or
the `--no-python-downloads` flag can be passed to any uv command.

Requiring or disabling managed Python versions

By default, uv will attempt to use Python versions found on the system and only download managed Python versions when necessary. To ignore system Python versions, and only use managed Python versions, use the --managed-python flag:

$ uv python list --managed-python

Similarly, to ignore managed Python versions and only use system Python versions, use the --no-managed-python flag:

$ uv python list --no-managed-python

To change uv's default behavior in a configuration file, use the python-preference setting.

Adjusting Python version preferences

The python-preference setting determines whether to prefer using Python installations that are already present on the system, or those that are downloaded and installed by uv.

By default, the python-preference is set to managed which prefers managed Python installations over system Python installations. However, system Python installations are still preferred over downloading a managed Python version.

The following alternative options are available:

• only-managed: Only use managed Python installations; never use system Python installations. Equivalent to --managed-python.
• system: Prefer system Python installations over managed Python installations.
• only-system: Only use system Python installations; never use managed Python installations. Equivalent to --no-managed-python.

!!! note

Automatic Python version downloads can be [disabled](#disabling-automatic-python-downloads)
without changing the preference.

Python implementation support

uv supports the CPython, PyPy, and GraalPy Python implementations. If a Python implementation is not supported, uv will fail to discover its interpreter.

The implementations may be requested with either the long or short name:

• CPython: cpython, cp
• PyPy: pypy, pp
• GraalPy: graalpy, gp

Implementation name requests are not case-sensitive.

See the Python version request documentation for more details on the supported formats.

Managed Python distributions

uv supports downloading and installing CPython and PyPy distributions.

CPython distributions

As Python does not publish official distributable CPython binaries, uv instead uses pre-built distributions from the Astral python-build-standalone project. python-build-standalone is also is used in many other Python projects, like Rye, Mise, and bazelbuild/rules_python.

The uv Python distributions are self-contained, highly-portable, and performant. While Python can be built from source, as in tools like pyenv, doing so requires preinstalled system dependencies, and creating optimized, performant builds (e.g., with PGO and LTO enabled) is very slow.

These distributions have some behavior quirks, generally as a consequence of portability; see the python-build-standalone quirks documentation for details. Additionally, some platforms may not be supported (e.g., distributions are not yet available for musl Linux on ARM).

PyPy distributions

PyPy distributions are provided by the PyPy project.