File Format

We wanted the file format to be easy to read as a diff when auditing a change to the lock file. As such, and thanks to PEP 518 and pyproject.toml, we decided to go with the TOML file format.

Secure by Design

Viewing the requirements file format as the closest we have to a lock file standard, there are a few issues with the file format when it comes to security. First is that the file format simply does not require you to specify the exact version of a package. This is why tools like pip-tools exist to help manage that users of requirements files.

Second, you must opt into specifying what files are acceptable to be installed by using the --hash argument for a specific dependency. This is also optional with pip-tools as it requires specifying the --generate-hashes CLI argument. This requires --require-hashes for pip to make sure no dependencies lack a hash to check.

Third, even when you control what files may be installed, it does not prevent other packages from being installed. If a dependency is not listed in the requirements file, pip will happily go searching for a file to meet that need. You must specify --no-deps as an argument to pip to prevent unintended dependency resolution outside of the requirements file.

Fourth, the format allows for installing a source distribution file (aka “sdist”). By its very nature, installing an sdist requires executing arbitrary Python code, meaning that there is no control over what files may be installed. Only by specifying --only-binary :all: can you guarantee pip to only use a wheel file for each package.

To recap, in order for a requirements file to be as secure as what is being proposed, a user should always do the following steps:

1. Use pip-tools and its command pip-compile --generate-hashes
2. Install the requirements file using pip install --require-hashes --no-deps --only-binary :all:

Critically, all of those flags, and both the specificity and exhaustion of what to install that pip-tools provides, are optional for requirements files.

As such, the proposal raised in this PEP is secure by design which combats some supply chain attacks. Hashes for files which would be used to install from are required. You can only install from wheels to unambiguously define what files will be placed in the file system. Installers must lead to an deterministic installation from a lock file for a given platform. All of this leads to a reproducible installation which you can deem trustworthy (when you have audited the lock file and what it lists).

Cross-Platform

Various projects which already have a lock file, like PDM and Poetry, provide a lock file which is cross-platform. This allows for a single lock file to work on multiple platforms while still leading to the exact same top-level requirements to be installed everywhere with the installation being consistent/unambiguous on each platform.

As to why this is useful, let’s use an example involving PyWeek (a week-long game development competition). Assume you are developing on Linux, while someone you choose to partner with is using macOS. Now assume the judges are using Windows. How do you make sure everyone is using the same top-level dependencies, while allowing for any platform-specific requirements (e.g. a package requires a helper package under Windows)?

With a cross-platform lock file, you can make sure that the key requirements are met consistently across all platforms. You can then also make sure that all users on the same platform get the same reproducible installation.

Simple Installer

The separation of concerns between a locker and an installer allows for an installer to have a much simpler operation to perform. As such, it not only allows for installers to be easier to write, but facilitates in making sure installers create unambiguous, reproducible installations correctly.

The installer can also expend less computation/energy in creating the installation. This is beneficial not only for faster installs, but also from an energy consumption perspective, as installers are expected to be run more often than lockers.

This has led to a design where the locker must do more work upfront to the benefit installers. It also means the complexity of package dependencies is simpler and easier to comprehend in a lock files to avoid ambiguity.

Details

Lock files MUST use the TOML file format. This not only prevents the need to have another file format in the Python packaging ecosystem thanks to its adoption by PEP 518 for pyproject.toml, but also assists in making lock files more human-readable.

Lock files MUST end their file names with .pylock.toml. The .toml part unambiguously distinguishes the format of the file, and helps tools like code editors support the file appropriately. The .pylock part distinguishes the file from other TOML files the user has, to make the logic easier for tools to create functionality specific to Python lock files, instead of TOML files in general.

The following sections are the top-level keys of the TOML file data format. Any field not listed as required is considered optional.

Example

version = "1.0"
created-at = 2021-10-19T22:33:45.520739+00:00

[tool]
# Tool-specific table.

[metadata]
requires = ["mousebender", "coveragepy[toml]"]
marker = "sys_platform == 'linux'"  # As an example for coverage.
requires-python = ">=3.7"

[[package.attrs."21.2.0"]]
filename = "attrs-21.2.0-py2.py3-none-any.whl"
hashes.sha256 = "149e90d6d8ac20db7a955ad60cf0e6881a3f20d37096140088356da6c716b0b1"
url = "https://files.pythonhosted.org/packages/20/a9/ba6f1cd1a1517ff022b35acd6a7e4246371dfab08b8e42b829b6d07913cc/attrs-21.2.0-py2.py3-none-any.whl"
requires-python = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*"

[[package.attrs."21.2.0"]]
# If attrs had another wheel file (e.g. that was platform-specific),
# it could be listed here.

[[package."coveragepy[toml]"."6.2.0"]]
filename = "coverage-6.2-cp310-cp310-manylinux_2_5_x86_64.manylinux1_x86_64.manylinux_2_12_x86_64.manylinux2010_x86_64.whl"
hashes.sha256 = "c7912d1526299cb04c88288e148c6c87c0df600eca76efd99d84396cfe00ef1d"
url = "https://files.pythonhosted.org/packages/da/64/468ca923e837285bd0b0a60bd9a287945d6b68e325705b66b368c07518b1/coverage-6.2-cp310-cp310-manylinux_2_5_x86_64.manylinux1_x86_64.manylinux_2_12_x86_64.manylinux2010_x86_64.whl"
requires-python = ">=3.6"
requires = ["tomli"]

[[package."coveragepy[toml]"."6.2.0"]]
filename = "coverage-6.2-cp310-cp310-musllinux_1_1_x86_64.whl "
hashes.sha256 = "276651978c94a8c5672ea60a2656e95a3cce2a3f31e9fb2d5ebd4c215d095840"
url = "https://files.pythonhosted.org/packages/17/d6/a29f2cccacf2315150c31d8685b4842a6e7609279939a478725219794355/coverage-6.2-cp310-cp310-musllinux_1_1_x86_64.whl"
requires-python = ">=3.6"
requires = ["tomli"]

# More wheel files for `coverage` could be listed for more
# extensive support (i.e. all Linux-based wheels).

[[package.mousebender."2.0.0"]]
filename = "mousebender-2.0.0-py3-none-any.whl"
hashes.sha256 = "a6f9adfbd17bfb0e6bb5de9a27083e01dfb86ed9c3861e04143d9fd6db373f7c"
url = "https://files.pythonhosted.org/packages/f4/b3/f6fdbff6395e9b77b5619160180489410fb2f42f41272994353e7ecf5bdf/mousebender-2.0.0-py3-none-any.whl"
requires-python = ">=3.6"
requires = ["attrs", "packaging"]

[[package.packaging."20.9"]]
filename = "packaging-20.9-py2.py3-none-any.whl"
hashes.blake-256 = "3e897ea760b4daa42653ece2380531c90f64788d979110a2ab51049d92f408af"
hashes.sha256 = "67714da7f7bc052e064859c05c595155bd1ee9f69f76557e21f051443c20947a"
url = "https://files.pythonhosted.org/packages/3e/89/7ea760b4daa42653ece2380531c90f64788d979110a2ab51049d92f408af/packaging-20.9-py2.py3-none-any.whl"
requires-python = ">=3.6"
requires = ["pyparsing"]

[[package.pyparsing."2.4.7"]]
filename = "pyparsing-2.4.7-py2.py3-none-any.whl"
hashes.sha256 = "ef9d7589ef3c200abe66653d3f1ab1033c3c419ae9b9bdb1240a85b024efc88b"
url = "https://files.pythonhosted.org/packages/8a/bb/488841f56197b13700afd5658fc279a2025a39e22449b7cf29864669b15d/pyparsing-2.4.7-py2.py3-none-any.whl"
direct = true  # For demonstration purposes.
requires-python = ">=2.6, !=3.0.*, !=3.1.*, !=3.2.*"

[[package.tomli."2.0.0"]]
filename = "tomli-2.0.0-py3-none-any.whl"
hashes.sha256 = "b5bde28da1fed24b9bd1d4d2b8cba62300bfb4ec9a6187a957e8ddb9434c5224"
url = "https://files.pythonhosted.org/packages/e2/9f/5e1557a57a7282f066351086e78f87289a3446c47b2cb5b8b2f614d8fe99/tomli-2.0.0-py3-none-any.whl"
requires-python = ">=3.7"

Expectations for Lockers

Lockers MUST create lock files for which a topological sort of the packages which qualify for installation on the specified platform results in a graph for which only a single version of any package qualifies for installation and there is at least one compatible file to install for each package. This leads to a lock file for any supported platform where the only decision an installer can make is what the “best-fitting” wheel is to install (which is discussed below).

Lockers are expected to utilize metadata.marker, metadata.tag, and metadata.requires-python as appropriate as well as environment markers specified via requires and Python version requirements via requires-python to enforce this result for installers. Put another way, the information used in the lock file is not expected to be pristine/raw from the locker’s input and instead is to be changed as necessary to the benefit of the locker’s goals.

Expectations for Installers

The expected algorithm for resolving what to install is:

1. Construct a dependency graph based on the data in the lock file with metadata.requires as the starting/root point.
2. Eliminate all files that are unsupported by the specified platform.
3. Eliminate all irrelevant edges between packages based on marker evaluation for requires.
4. Raise an error if a package version is still reachable from the root of the dependency graph but lacks any compatible file.
5. Verify that all packages left only have one version to install, raising an error otherwise.
6. Install the best-fitting wheel file for each package which remains.

Installers MUST follow a deterministic algorithm determine what the “best-fitting wheel file” is. A simple solution for this is to rely upon the packaging project and its packaging.tags module to determine wheel file precedence.

Installers MUST support installing into an empty environment. Installers MAY support installing into an environment that already contains installed packages (and whatever that would entail to be supported).

Usability

First, a pip freeze equivalent tool could be developed which creates a lock file. While installed packages do not by themselves provide enough information to statically create a lock file, a user could provide local directories and index URLs to construct one. This would then lead to lock files that are stricter than a requirements file by limiting the lock file to the current platform. This would also allow people to see whether their environment would be reproducible.

Second, a stand-alone installer should be developed. As the requirements on an installer are much simpler than what pip provides, it should be reasonable to have an installer that is independently developed.

Third, a tool to convert a pinned requirements file as emitted by pip-tools could be developed. Much like the pip freeze equivalent outlined above, some input from the user may be needed. But this tool could act as a transitioning step for anyone who has an appropriate requirements file. This could also act as a test before potentially having pip-tools grow some --lockfile flag to use this PEP.

All of this could be required before the PEP transitions from conditional acceptance to full acceptance (and give the community a chance to test if this PEP is potentially useful).

Interoperability

At this point, the goal would be to increase interoperability between tools.

First, pip would become an installer. By having the most widely used installer support the format, people can innovate on the locker side while knowing people will have the tools necessary to actually consume a lock file.

Second, pip becomes a locker. Once again, pip’s reach would make the format accessible for the vast majority of Python users very quickly.

Third, a project with a pre-existing lock file format supports at least exporting to the lock file format (e.g. PDM or Pyflow). This would show that the format meets the needs of other projects.

Acceptance

With the tooling available throughout the community, acceptance would be shown via those not exclusively tied to the Python community supporting the file format based on what they believe their users want.

First, tools that operate on requirements files like code editors having equivalent support for lock files.

Second, consumers of requirements files like cloud providers would also accept lock files.

At this point the PEP would have permeated out far enough to be on par with requirements files in terms of general accpetance and potentially more if projects had dropped their own lock files for this PEP.

File Formats Other Than TOML

JSON was briefly considered, but due to:

1. TOML already being used for pyproject.toml
2. TOML being more human-readable
3. TOML leading to better diffs

the decision was made to go with TOML. There was some concern over Python’s standard library lacking a TOML parser, but most packaging tools already use a TOML parser thanks to pyproject.toml so this issue did not seem to be a showstopper. Some have also argued against this concern in the past by the fact that if packaging tools abhor installing dependencies and feel they can’t vendor a package then the packaging ecosystem has much bigger issues to rectify than the need to depend on a third-party TOML parser.

Alternative Naming Schemes

Specifying a directory to install file to was considered, but ultimately rejected due to people’s distaste for the idea.

It was also suggested to not have a special file name suffix, but it was decided that hurt discoverability by tools too much.

Supporting a Single Lock File

At one point the idea of only supporting single lock file which contained all possible lock information was considered. But it quickly became apparent that trying to devise a data format which could encompass both a lock file format which could support multiple environments as well as strict lock outcomes for reproducible builds would become quite complex and cumbersome.

The idea of supporting a directory of lock files as well as a single lock file named pyproject-lock.toml was also considered. But any possible simplicity from skipping the directory in the case of a single lock file seemed unnecessary. Trying to define appropriate logic for what should be the pyproject-lock.toml file and what should go into pyproject-lock.d seemed unnecessarily complicated.

Using a Flat List Instead of a Dependency Graph

The first version of this PEP proposed that the lock file have no concept of a dependency graph. Instead, the lock file would list exactly what should be installed for a specific platform such that installers did not have to make any decisions about what to install, only validating that the lock file would work for the target platform.

This idea was eventually rejected due to the number of combinations of potential PEP 508 environment markers. The decision was made that trying to have lockers generate all possible combinations as individual lock files when a project wants to be cross-platform would be too much.

Use Wheel Tags in the File Name

Instead of having the metadata.tag field there was a suggestion of encoding the tags into the file name. But due to the addition of the metadata.marker field and what to do when no tags were needed, the idea was dropped.

Alternative Names for requires

Some other names for what became requires were installs, needs, and dependencies. Initially this PEP chose needs after asking a Python beginner which term they preferred. But based on feedback on an earlier draft of this PEP, requires was chosen as the term.

Accepting PEP 650

PEP 650 was an earlier attempt at trying to tackle this problem by specifying an API for installers instead of standardizing on a lock file format (ala PEP 517). The initial response to PEP 650 could be considered mild/lukewarm. People seemed to be consistently confused over which tools should provide what functionality to implement the PEP. It also potentially incurred more overhead as it would require executing Python APIs to perform any actions involving packaging.

This PEP chooses to standardize around an artifact instead of an API (ala PEP 621). This would allow for more tool integrations as it removes the need to specifically use Python to do things such as create a lock file, update it, or even install packages listed in a lock file. It also allows for easier introspection by forcing dependency graph details to be written in a human-readable format. It also allows for easier sharing of knowledge by standardizing what people need to know more (e.g. tutorials become more portable between tools when it comes to understanding the artifact they produce). It’s also simply the approach other language communities have taken and seem to be happy with.

Acceptance of this PEP would mean PEP 650 gets rejected.

Specifying Requirements per Package Instead of per File

An earlier draft of this PEP specified dependencies at the package level instead of per file. While this has traditionally been how packaging systems work, it actually did not reflect accurately how things are specified. As such, this PEP was subsequently updated to reflect the granularity that dependencies can truly be specified at.

Specify Where Lockers Gather Input

This PEP does not specify how a locker gets its input. An initial suggestion was to partially reuse PEP 621, but due to disagreements on how flexible the potential input should be in terms of specifying things such as indexes, etc., it was decided this would best be left to a separate PEP.

Allowing Source Distributions and Source Trees to be an Opt-In, Supported File Format

After extensive discussion, it was decided that this PEP would not support source distributions (aka sdists) or source trees as an acceptable format for code. Introducing sdists and source trees to this PEP would immediately undo the reproducibility and security goals due to needing to execute code to build the sdist or source tree. It would also greatly increase the complexity for (at least) installers as the dynamic build nature of sdists and source trees means the installer would need to handle fully resolving whatever requirements the sdists produced dynamically, both from a building and installation perspective.

Due to all of this, it was decided it was best to have a separate discussion about what supporting sdists and source trees after this PEP is accepted/rejected. As the proposed file format is versioned, introducing sdists and source tree support in a later PEP is doable.

It should be noted, though, that this PEP is not stop an out-of-band solution from being developed to be used in conjunction with this PEP. Building wheel files from sdists and shipping them with code upon deployment so they can be included in the lock file is one option. Another is to use a requirements file just for sdists and source trees, then relying on a lock file for all wheels.