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](#discovery-of-python-versions) Python versions. However, uv also supports
[installing Python versions](#installing-a-python-version) 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:

```console
$ 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](#disabling-automatic-python-downloads).

## 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:

```console
$ uv python install 3.12.3
```

To install the latest patch version:

```console
$ uv python install 3.12
```

To install a version that satisfies constraints:

```console
$ uv python install '>=3.8,<3.10'
```

To install multiple versions:

```console
$ uv python install 3.9 3.10 3.11
```

To install a specific implementation:

```console
$ uv python install pypy
```

All of the [Python version request](#requesting-a-version) formats are supported except those that
are used for requesting local interpreters such as a file path.

## Project Python versions

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

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

To list installed and available Python versions:

```console
$ uv python list
```

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

To view all versions:

```console
$ uv python list --all-versions
```

To view Python versions for other platforms:

```console
$ uv python list --all-platforms
```

To exclude downloads and only show installed Python versions:

```console
$ uv python list --only-installed
```

## Finding a Python executable

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

```console
$ uv python find
```

By default, this will display the path to the first available Python executable. See the
[discovery rules](#discovery-of-python-versions) for details about how executables are discovered.

This interface also supports many [request formats](#requesting-a-version), e.g., to find a Python
executable that has a version of 3.11 or newer:

```console
$ 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:

```console
$ 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 interpreter returned by `py --list-paths` that matches 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](../pip/environments.md#discovery-of-python-environments)
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](#requesting-a-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.

## Disabling automatic Python downloads

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

The [`python-downloads`](../reference/settings.md#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.

## Adjusting Python version preferences

By default, uv will attempt to use Python versions found on the system and only download managed
interpreters when necessary.

The [`python-preference`](../reference/settings.md#python-preference) option can be used to adjust
this behavior. By default, it 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
- `system`: Prefer system Python installations over managed Python installations
- `only-system`: Only use system Python installations; never use managed Python installations

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

!!! 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](#requesting-a-version) documentation for more details on the
supported formats.

## Managed Python distributions

uv supports downloading and installing CPython and PyPy distributions.

### CPython distributions

Python does not publish official distributable CPython binaries, uv uses third-party standalone
distributions from the
[`python-build-standalone`](https://github.com/indygreg/python-build-standalone) project. The
project is partially maintained by the uv maintainers and is used by many other Python projects.

The uv Python distributions are self-contained, highly-portable, and performant. While Python can be
built from source, as in tools like `pyenv`, it requires preinstalled system dependencies and
creating optimized, performant builds is very slow.

These distributions have some behavior quirks, generally as a consequence of portability. See the
[`python-build-standalone` quirks](https://gregoryszorc.com/docs/python-build-standalone/main/quirks.html)
documentation for details.

### PyPy distributions

PyPy distributions are provided by the PyPy project.