Locking environments

Locking is to take a dependency, e.g., ruff, and write an exact version to use to a file. When working with many dependencies, it is useful to lock the exact versions so the environment can be reproduced. Without locking, the versions of dependencies could change over time, when using a different tool, or across platforms.

Locking requirements

uv allows dependencies to be locked in the requirements.txt format. It is recommended to use the standard pyproject.toml to define dependencies, but other dependency formats are supported as well. See the documentation on declaring dependencies for more details on how to define dependencies.

To lock dependencies declared in a pyproject.toml:

$ uv pip compile pyproject.toml -o requirements.txt

Note by default the uv pip compile output is just displayed and --output-file / -o argument is needed to write to a file.

To lock dependencies declared in a requirements.in:

$ uv pip compile requirements.in -o requirements.txt

To lock dependencies declared in multiple files:

$ uv pip compile pyproject.toml requirements-dev.in -o requirements-dev.txt

uv also supports legacy setup.py and setup.cfg formats. To lock dependencies declared in a setup.py:

$ uv pip compile setup.py -o requirements.txt

To lock dependencies from stdin, use -:

$ echo "ruff" | uv pip compile -

To lock with optional dependencies enabled, e.g., the "foo" extra:

$ uv pip compile pyproject.toml --extra foo

To lock with all optional dependencies enabled:

$ uv pip compile pyproject.toml --all-extras

Note extras are not supported with the requirements.in format.

Upgrading requirements

When using an output file, uv will consider the versions pinned in an existing output file. If a dependency is pinned it will not be upgraded on a subsequent compile run. For example:

$ echo "ruff==0.3.0" > requirements.txt
$ echo "ruff" | uv pip compile - -o requirements.txt
# This file was autogenerated by uv via the following command:
#    uv pip compile - -o requirements.txt
ruff==0.3.0

To upgrade a dependency, use the --upgrade-package flag:

$ uv pip compile - -o requirements.txt --upgrade-package ruff

To upgrade all dependencies, there is an --upgrade flag.

Syncing an environment

Dependencies can be installed directly from their definition files or from compiled requirements.txt files with uv pip install. See the documentation on installing packages from files for more details.

When installing with uv pip install, packages that are already installed will not be removed unless they conflict with the lockfile. This means that the environment can have dependencies that aren't declared in the lockfile, which isn't great for reproducibility. To ensure the environment exactly matches the lockfile, use uv pip sync instead.

To sync an environment with a requirements.txt file:

$ uv pip sync requirements.txt

To sync an environment with a pyproject.toml file:

$ uv pip sync pyproject.toml

Adding constraints

Constraints files are requirements.txt-like files that only control the version of a requirement that's installed. However, including a package in a constraints file will not trigger the installation of that package. Constraints can be used to add bounds to dependencies that are not dependencies of the current project.

To define a constraint, define a bound for a package:

constraints.txt

pydantic<2.0

To use a constraints file:

$ uv pip compile requirements.in --constraint constraints.txt

Note that multiple constraints can be defined in each file and multiple files can be used.

Overriding dependency versions

Overrides files are requirements.txt-like files that force a specific version of a requirement to be installed, regardless of the requirements declared by any constituent package, and regardless of whether this would be considered an invalid resolution.

While constraints are additive, in that they're combined with the requirements of the constituent packages, overrides are absolute, in that they completely replace the requirements of the constituent packages.

Overrides are most often used to remove upper bounds from a transitive dependency. For example, if a requires c>=1.0,<2.0 and b requires c>=2.0 and the current project requires a and b then the dependencies cannot be resolved.

To define an override, define the new requirement for the problematic package:

overrides.txt

c>=2.0

To use an overrides file:

$ uv pip compile requirements.in --override overrides.txt

Now, resolution can succeed. However, note that if a is correct that it does not support c>=2.0 then a runtime error will likely be encountered when using the packages.

Note that multiple overrides can be defined in each file and multiple files can be used.