Title: A file format to list Python dependencies for reproducibility of an application
Author: Brett Cannon firstname.lastname@example.org,
Pradyun Gedam email@example.com,
Tzu-ping Chung firstname.lastname@example.org
PEP-Delegate: Paul Moore email@example.com
Discussions-To: PEP 665: Specifying Installation Requirements for Python Projects
Type: Standards Track
Post-History: 29-Jul-2021, 03-Nov-2021, 25-Nov-2021
This PEP specifies a file format to specify the list of Python package
installation requirements for an application, and the relation between
the specified requirements. The list of requirements is considered
exhaustive for the installation target, and thus not requiring any
information beyond the platform being installed for, and the file
itself. The file format is flexible enough to allow installing the
requirements across different platforms, which allows for
reproducibility on multiple platforms from the same file.
There are several terms whose definition must be agreed upon in order
to facilitate a discussion on the topic of this PEP.
A package is something you install as a dependency and use via the
import system. The packages on PyPI are an example of this.
An application or app is an end product that other external code
does not directly rely on via the import system (i.e. they are
standalone). Desktop applications, command-line tools, etc. are
examples of applications.
A lock file records the packages that are to be installed for an
app. Traditionally, the exact version of the package to be installed
is specified by a lock file, but specified packages are not always
installed on a given platform (according a filtering logic described
in a later section), which enables the lock file to describe
reproducibility across multiple platforms. Examples of this are
package-lock.json from npm_,
Poetry.lock from Poetry_, etc.
Locking is the act of taking the input of the packages an app
depends on and producting a lock file from that.
A locker is a tool which produces a lock file.
An installer consumes a lock file to install what the lock file
Applications want reproducible installs for a few reasons (we are not
worrying about package development, integration into larger systems
that would handle locking dependencies external to the Python
application, or other situations where flexible installation
requirements are desired over strict, reproducible installations).
One, reproducibility eases development. When you and your fellow
developers all end up with the same files on a specific platform, you
make sure you are all developing towards the same experience for the
application. You also want your users to install the same files as
you expect to guarantee the experience is the same as you developed
Two, you want to be able to reproduce what gets installed across
multiple platforms. Thanks to Python’s portability across operating
systems, CPUs, etc., it is very easy and often desirable to create
applications that are not restricted to a single platform. Thus, you
want to be flexible enough to allow for differences in your package
dependencies between platforms, while still having consistency
and reproducibility on any one specific platform.
Three, reproducibility is more secure. When you control exactly what
files are installed, you can make sure no malicious actor is
attempting to slip nefarious code into your application (i.e. some
supply chain attacks). By using a lock file which always leads to
reproducible installs, we can avoid certain risks entirely.
Four, relying on the
wheel file_ format provides reproducibility
without requiring build tools to support reproducibility themselves.
Thanks to wheels being static and not executing code as part of
installation, wheels always lead to a reproducible result. Compare
this to source distributions (aka sdists) or source trees which only
lead to a reproducible install if their build tool supports
reproducibility due to inherent code execution. Unfortunately the vast
majority of build tools do not support reproducible builds, so this
PEP helps alleviate that issue by only supporting wheels as a package
This PEP proposes a standard for a lock file, as the current solutions
don’t meet the outlined goals. Today, the closest we come to a lock
file standard is the
requirements file format_ from pip.
Unfortunately, that format does not lead to inherently reproducible
installs (it requires optional features both in the requirements file
and the installer itself, to be discussed later).
The community itself has also shown a need for lock files based on the
fact that multiple tools have independently created their own lock
Unfortunately, those tools all use differing lock file formats. This
means tooling around these tools much be unique. This impacts tooling
such as code editors and hosting providers, which want to be as
flexible as possible when it comes to accepting a user’s application
code, but also have a limit as to how much development resources they
can spend to add support for yet another lock file format. A
standardized format would allow tools to focus their work on a single
target, and make sure that workflow decisions made by developers
outside of the lock file format are of no concern to e.g. hosting
Other programming language communities have also shown the usefulness
of lock files by developing their own solution to this problem. Some
of those communities include:
The trend in programming languages in the past decade seems to have
been toward providing a lock file solution.
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.
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
pip-tools_ exist to help manage that users of
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.
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
--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:
#. Use pip-tools and its command
#. 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).
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
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
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
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
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
.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.
This field is required.
The version of the lock file being used. The key MUST be a string
consisting of a number that follows the same formatting as the
Metadata-Version key in the
core metadata spec_.
The value MUST be set to
"1.0" until a future PEP allows for a
different value. The introduction of a new optional key to the file
format SHOULD increase the minor version. The introduction of a new
required key or changing the format MUST increase the major version.
How to handle other scenarios is left as a per-PEP decision.
Installers MUST warn the user if the lock file specifies a version
whose major version is supported but whose minor version is
unsupported/unrecognized (e.g. the installer supports
the lock file specifies
Installers MUST raise an error if the lock file specifies a major
version which is unsupported (e.g. the installer supports
but the lock file specifies
This field is required.
The timestamp for when the lock file was generated (using TOML’s
native timestamp type). It MUST be recorded using the UTC time zone to
If the SOURCE_DATE_EPOCH_ environment variable is set, it MUST be used
as the timestamp by the locker. This facilitates reproducibility of
the lock file itself.
Tools may create their own sub-tables under the
tool table. The
rules for this table match those for
pyproject.toml and its
[tool] table from the
build system declaration spec_.
This table is required.
A table containing data applying to the overall lock file.
A key storing a string containing an environment marker as
specified in the
dependency specifier spec_.
The locker MAY specify an environment marker which specifies any
restrictions the lock file was generated under.
If the installer is installing for an environment which does not
satisfy the specified environment marker, the installer MUST raise an
error as the lock file does not support the target installation
A key storing a string specifying
platform compatibility tags_
(i.e. wheel tags). The tag MAY be a compressed tag set.
If the installer is installing for an environment which does not
satisfy the specified tag (set), the installer MUST raise an error
as the lock file does not support the targeted installation
This field is required.
An array of strings following the
dependency specifier spec_. This
array represents the top-level package dependencies of the lock file
and thus the root of the dependency graph.
A string specifying the supported version(s) of Python for this lock
file. It follows the same format as that specified for the
Requires-Python field in the
core metadata spec_.
This array is required.
An array per package and version containing entries for the potential
(wheel) files to install (as represented by
Lockers MUST normalize a project’s name according to the
simple repository API_. If extras are specified as part of the
project to install, the extras are to be included in the key name and
are to be sorted in lexicographic order.
Within the file, the tables for the projects SHOULD be sorted by:
#. Project/key name in lexicographic order
#. Package version, newest/highest to older/lowest according to the
version specifiers spec_
#. Optional dependencies (extras) via lexicographic order
#. File name based on the
filename field (discussed
These recommendations are to help minimize diff changes between tool
This field is required.
A string representing the name of the file as represented by an entry
in the array. This field is required to simplify installers as the
file name is required to resolve wheel tags derived from the file
name. It also guarantees that the association of the array entry to
the file it is meant for is always clear.
This table is required.
A table with keys specifying a hash algorithm and values as the hash
for the file represented by this entry in the
Lockers SHOULD list hashes in lexicographic order. This is to help
minimize diff sizes and the potential to overlook hash value changes.
An installer MUST only install a file which matches one of the
A string representing a URL where to get the file.
The installer MAY support any schemes it wants for URLs
file: as well as
An installer MAY choose to not use the URL to retrieve a file
if a file matching the specified hash can be found using alternative
means (e.g. on the file system in a cache directory).
A boolean representing whether an installer should consider the
project installed “directly” as specified by the
direct URL origin of installed distributions spec_.
If the key is true, then the installer MUST follow the
direct URL origin of installed distributions spec_ for recording
the installation as “direct”.
A string specifying the support version(s) of Python for this file. It
follows the same format as that specified for the
Requires-Python field in the
core metadata spec_.
An array of strings following the
dependency specifier spec_ which
represent the dependencies of this file.
version = "1.0" created-at = 2021-10-19T22:33:45.520739+00:00 [tool] # Tool-specific table. [metadata] requires = ["mousebender"] requires-python = ">=3.6" [[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.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" required-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.*"
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
Lockers are expected to utilize
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.
The expected algorithm for resolving what to install is:
#. Construct a dependency graph based on the data in the lock file
metadata.requires as the starting/root point.
#. Eliminate all files that are unsupported by the specified platform.
#. Eliminate all irrelevant edges between packages based on marker
#. Raise an error if a package version is still reachable from the
root of the dependency graph but lacks any compatible file.
#. Verify that all packages left only have one version to install,
raising an error otherwise.
#. Install the best-fitting wheel file for each package which
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 <https://pypi.org/p/packaging/>__
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
The pip_ team has
they are interested in supporting this PEP if accepted. The current
proposal for pip may even
supplant the need <https://github.com/jazzband/pip-tools/issues/1526#issuecomment-961883367>__
PDM_ has also said they would
support the PEP <https://github.com/pdm-project/pdm/issues/718>__
Pyflow_ has said they
"like the idea" <https://github.com/David-OConnor/pyflow/issues/153#issuecomment-962482058>__
of the PEP.
Poetry_ has said they would not support the PEP as-is because
"Poetry supports sdists files, directory and VCS dependencies which are not supported" <https://github.com/python-poetry/poetry/issues/4710#issuecomment-973946104>.
Recording requirements at the file level, which is on purpose to
better reflect what can occur when it comes to dependencies,
"is contradictory to the design of Poetry" <https://github.com/python-poetry/poetry/issues/4710#issuecomment-973946104>.
This also excludes export support to a this PEP’s lock file as
"Poetry exports the information present in the poetry.lock file into another format" <https://github.com/python-poetry/poetry/issues/4710#issuecomment-974551351>__
and sdists and source trees are included in
Thus it is not a clean translation from Poetry’s lock file to this
PEP’s lock file format.
As there is no pre-existing specification regarding lock files, there
are no explicit backwards compatibility concerns.
As for pre-existing tools that have their own lock file, some updating
will be required. Most document the lock file name, but not its
contents. For projects which do not commit their lock file to
version control, they will need to update the equivalent of their
.gitignore file. For projects that do commit their lock file to
version control, what file(s) get committed will need an update.
For projects which do document their lock file format like pipenv_,
they will very likely need a major version release which changes the
lock file format.
A lock file should not introduce security issues but instead help
solve them. By requiring the recording of hashes for files, a lock
file is able to help prevent tampering with code since the hash
details were recorded. Relying on only wheel files means what files
will be installed can be known ahead of time and is reproducible. A
lock file also helps prevent unexpected package updates being
installed which may in turn be malicious.
Teaching of this PEP will very much be dependent on the lockers and
installers being used for day-to-day use. Conceptually, though, users
could be taught that a lock file specifies what should be installed
for a project to work. The benefits of consistency and security should
be emphasized to help users realize why they should care about lock
No proof-of-concept or reference implementation currently exists. An
example locker and installer will be provided before this PEP is
fully accepted (although this is not a necessarily a requirement for
JSON_ was briefly considered, but due to:
#. TOML already being used for
#. TOML being more human-readable
#. 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.
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.
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.
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.
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
metadata.marker field and what to do when no tags were needed,
the idea was dropped.
Some other names for what became
dependencies. Initially this PEP chose
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.
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 <https://discuss.python.org/t/pep-650-specifying-installer-requirements-for-python-projects/6657/>__
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.
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.
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.
extensive discussion <https://discuss.python.org/t/supporting-sdists-and-source-trees-in-pep-665/11869/>__,
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
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.
Thanks to Frost Ming of PDM_ and Sébastien Eustace of Poetry_ for
providing input around dynamic install-time resolution of PEP 508
Thanks to Kushal Das for making sure reproducible builds stayed a
concern for this PEP.
Thanks to Andrea McInnes for initially settling the bikeshedding and
choosing the paint colour of
needs (at which point people ralled
requires colour instead).
This document is placed in the public domain or under the
CC0-1.0-Universal license, whichever is more permissive.
… _build system declaration spec: Declaring build system dependencies — Python Packaging User Guide
… _core metadata spec: Core metadata specifications — Python Packaging User Guide
… _Dart: https://dart.dev/
… _dependency specifier spec: Dependency specifiers — Python Packaging User Guide
… _direct URL origin of installed distributions spec: Recording the Direct URL Origin of installed distributions — Python Packaging User Guide
… _Git: https://git-scm.com/
… _Go: https://go.dev/
… _JSON: https://www.json.org/
… _npm: https://www.npmjs.com/
… _PDM: pdm · PyPI
… _pip: https://pip.pypa.io/
… _pip-tools: pip-tools · PyPI
… _Pipenv: pipenv · PyPI
… _platform compatibility tags: Platform compatibility tags — Python Packaging User Guide
… _Poetry: poetry · PyPI
… _Pyflow: pyflow · PyPI
… _PyWeek: https://pyweek.org/
… _requirements file format: Requirements File Format - pip documentation v22.0.dev0
… _Rust: https://www.rust-lang.org/
… _SecureDrop: https://securedrop.org/
… _simple repository API: Simple repository API — Python Packaging User Guide
… _source distribution file: Source distribution format — Python Packaging User Guide
… _SOURCE_DATE_EPOCH: SOURCE_DATE_EPOCH specification
… _TOML: https://toml.io
… _version specifiers spec: Version specifiers — Python Packaging User Guide
… _wheel file: Binary distribution format — Python Packaging User Guide