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uv/docs/concepts/python-versions.md
John Mumm e9d5780369
Support transparent Python patch version upgrades (#13954) 
> NOTE: The PRs that were merged into this feature branch have all been
independently reviewed. But it's also useful to see all of the changes
in their final form. I've added comments to significant changes
throughout the PR to aid discussion.

This PR introduces transparent Python version upgrades to uv, allowing
for a smoother experience when upgrading to new patch versions.
Previously, upgrading Python patch versions required manual updates to
each virtual environment. Now, virtual environments can transparently
upgrade to newer patch versions.

Due to significant changes in how uv installs and executes managed
Python executables, this functionality is initially available behind a
`--preview` flag. Once an installation has been made upgradeable through
`--preview`, subsequent operations (like `uv venv -p 3.10` or patch
upgrades) will work without requiring the flag again. This is
accomplished by checking for the existence of a minor version symlink
directory (or junction on Windows).

### Features

* New `uv python upgrade` command to upgrade installed Python versions
to the latest available patch release:
``` 
# Upgrade specific minor version 
uv python upgrade 3.12 --preview
# Upgrade all installed minor versions
uv python upgrade --preview
```
* Transparent upgrades also occur when installing newer patch versions: 
```
uv python install 3.10.8 --preview
# Automatically upgrades existing 3.10 environments
uv python install 3.10.18
```
* Support for transparently upgradeable Python `bin` installations via
`--preview` flag
```
uv python install 3.13 --preview
# Automatically upgrades the `bin` installation if there is a newer patch version available
uv python upgrade 3.13 --preview
```
* Virtual environments can still be tied to a patch version if desired
(ignoring patch upgrades):
```
uv venv -p 3.10.8
```

### Implementation

Transparent upgrades are implemented using:
* Minor version symlink directories (Unix) or junctions (Windows)
* On Windows, trampolines simulate paths with junctions
* Symlink directory naming follows Python build standalone format: e.g.,
`cpython-3.10-macos-aarch64-none`
* Upgrades are scoped to the minor version key (as represented in the
naming format: implementation-minor version+variant-os-arch-libc)
* If the context does not provide a patch version request and the
interpreter is from a managed CPython installation, the `Interpreter`
used by `uv python run` will use the full symlink directory executable
path when available, enabling transparently upgradeable environments
created with the `venv` module (`uv run python -m venv`)

New types:
* `PythonMinorVersionLink`: in a sense, the core type for this PR, this
is a representation of a minor version symlink directory (or junction on
Windows) that points to the highest installed managed CPython patch
version for a minor version key.
* `PythonInstallationMinorVersionKey`: provides a view into a
`PythonInstallationKey` that excludes the patch and prerelease. This is
used for grouping installations by minor version key (e.g., to find the
highest available patch installation for that minor version key) and for
minor version directory naming.

### Compatibility

* Supports virtual environments created with:
  * `uv venv`
* `uv run python -m venv` (using managed Python that was installed or
upgraded with `--preview`)
  * Virtual environments created within these environments
* Existing virtual environments from before these changes continue to
work but aren't transparently upgradeable without being recreated
* Supports both standard Python (`python3.10`) and freethreaded Python
(`python3.10t`)
* Support for transparently upgrades is currently only available for
managed CPython installations

Closes #7287
Closes #7325
Closes #7892
Closes #9031
Closes #12977

---------

Co-authored-by: Zanie Blue <contact@zanie.dev>
2025-06-20 16:17:13 +02:00

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# 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).
### 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`](../reference/cli.md/#uv-python-pin) command.
A global `.python-version` file can be created in the user configuration directory with the
[`uv python pin --global`](../reference/cli.md/#uv-python-pin) 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:
```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 the [Python version request](#requesting-a-version) 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:
```console
$ 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:
```console
$ 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:
```console
$ 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:
```console
$ uv python upgrade 3.12
```
To upgrade all installed Python versions:
```console
$ 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:
```console
$ 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:
```console
$ uv python list
```
To filter the Python versions, provide a request, e.g., to show all Python 3.13 interpreters:
```console
$ uv python list 3.13
```
Or, to show all PyPy interpreters:
```console
$ uv python list pypy
```
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
```
See the [`uv python list`](../reference/cli.md#uv-python-list) reference for more details.
## 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 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](../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.
### 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`](../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.
## 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:
```console
$ uv python list --managed-python
```
Similarly, to ignore managed Python versions and only use system Python versions, use the
`--no-managed-python` flag:
```console
$ 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).
## Adjusting Python version preferences
The [`python-preference`](../reference/settings.md#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](#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
As Python does not publish official distributable CPython binaries, uv instead uses pre-built
distributions from the Astral
[`python-build-standalone`](https://github.com/astral-sh/python-build-standalone) project.
`python-build-standalone` is also is used in many other Python projects, like
[Rye](https://github.com/astral-sh/rye), [Mise](https://mise.jdx.dev/lang/python.html), and
[bazelbuild/rules_python](https://github.com/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](https://gregoryszorc.com/docs/python-build-standalone/main/quirks.html)
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.
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