PEP: 552
Title: Deterministic pycs
Version: $Revision$
Last-Modified: $Date$
Author: Benjamin Peterson <benjamin@python.org>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 2017-09-04
Python-Version: 3.7


Abstract
========

This PEP proposes to an extension to the pyc format to make it more
deterministic.


Rationale
=========

A `reproducible build`_ is one where the same byte-for-byte output is generated
every time the same sources are built—even across different machines (naturally
subject to the requirement that they have rather similar environments
setup). Reproducibility is important for security. It is also a key concept in
content-based build systems such as Bazel_, which are most effective when the
output files’ contents are a deterministic function of the input files’
contents.

The current Python pyc format is the marshaled code object of the module
prefixed by a `magic number`_, the source timestamp, and the source file
size. The presence of a source timestamp means that a pyc is not a deterministic
function of the input file’s contents—it also depends on the volatile metadata,
mtime, of the source. Thus, the pycs are a barrier to proper reproducibility.

Distributors of Python code are currently stuck with the options of

1. not distributing pycs and losing the caching advantages

2. distributing pycs and losing reproducibility

3. carefully giving all Python source files a deterministic timestamp
   (see https://github.com/python/cpython/pull/296)

4. doing a complicated mixture of 1. and 2. like generating pycs at installation
   time

None of these options are very attractive. This PEP proposes a better solution.

(Note there are other problems [#frozensets]_ [#interning]_ we do not
address here that can make pycs non-deterministic.)


Specification
=============

Python will begin to recognize two magic number variants for every pyc
version. One magic number will correspond to the current pyc format and the
other to "hash-based" pycs introduced by this PEP.

In hash-based pycs, the second field in the pyc header (currently the
"timestamp" field) will become a bitset of flags. We define the lowest flag in
this bitset called ``check_source``. Following the bitset is a 64-bit hash of
the source file. We will use a SipHash_ (with a hardcoded key) of the contents
of the source file. Another a fast hash like MD5 or BLAKE2_ would also work. We
choose SipHash because Python already has a builtin implementation of it from
:pep:`456`, although an interface that allows picking the SipHash key must be
exposed to Python.

When Python encounters a hash-based pyc, its behavior depends on the setting of
the ``check_source`` flag. If the ``check_source`` flag is set, Python will
determine the validity of the pyc by hashing the source file and comparing the
hash with the expected hash in the pyc. If the pyc needs to be regenerated, it
will be regenerated as a hash-based pyc again with the ``check_source`` flag
set.

For hash-based pycs with the ``check_source`` unset, Python will simply load the
pyc without checking the hash of the source file. The expectation in this case
is that some external system (e.g., the local Linux distribution’s package
manager) is responsible for keeping pycs up to date, so Python itself doesn’t
have to check. Even when validation is disabled, the hash field should be set
correctly, so out-of-band consistency checkers can verify the up-to-dateness of
the pyc. Note also that the :pep:`3147` edict that pycs without corresponding
source files not be loaded will still be enforced for hash-based pycs.

The ``py_compile`` and ``compileall`` tools will be extended with new options to
generate hash-based pycs with and without the ``check_source`` bit set. Their
programmatic APIs will also gain similar functionality.


References
==========

.. _reproducible build: https://reproducible-builds.org/
.. _Bazel: https://bazel.build/
.. _BLAKE2: https://blake2.net/
.. _SipHash: https://131002.net/siphash/
.. [#frozensets] http://benno.id.au/blog/2013/01/15/python-determinism
.. [#interning] http://bugzilla.opensuse.org/show_bug.cgi?id=1049186
.. _magic number: https://docs.python.org/3/library/importlib.html#importlib.util.MAGIC_NUMBER


Credits
=======

The author would like to thank Gregory P. Smith for useful conversations on the
topic of this PEP.


Copyright
=========

This document has been placed in the public domain.



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