[Python-Dev] Final call for PEP 488: eliminating PYO files

[Guido van Rossum]

I am willing to be the BDFL for this PEP. I have tried to skim the recent
discussion (only python-dev) and I don't see much remaining controversy.
HOWEVER... The PEP is not clear (or at least too subtle) about the actual
name for optimization level 0. If I have foo.py, and I compile it three
times with three different optimization levels (no optimization; -O; -OO),
and then I look in __pycache__, would I see this:

# (1)
foo.cpython-35.pyc
foo.cpython-35.opt-1.pyc
foo.cpython-35.opt-2.pyc

Or would I see this?

# (2)
foo.cpython-35.opt-0.pyc
foo.cpython-35.opt-1.pyc
foo.cpython-35.opt-2.pyc

Your lead-in ("I have decided to have the default case of no optimization
levels mean that the .pyc file name will have *no* optimization level
specified in the name and thus be just as it is today.") makes me think I
should expect (1), but I can't actually pinpoint where the language of the
PEP says this.


On Fri, Mar 20, 2015 at 11:34 AM, Brett Cannon <bcannon at gmail.com> wrote:

> I have decided to have the default case of no optimization levels mean
> that the .pyc file name will have *no* optimization level specified in
> the name and thus be just as it is today. I made this decision due to
> potential backwards-compatibility issues -- although I expect them to be
> minutes -- and to not force other implementations like PyPy to have some
> bogus value set since they don't have .pyo files to begin with (PyPy
> actually uses bytecode for -O and don't bother with -OO since PyPy already
> uses a bunch of memory when running).
>
> Since this closes out the last open issue, I need either a BDFL decision
> or a BDFAP to be assigned to make a decision. Guido?
>
> ======================================
>
> PEP: 488
> Title: Elimination of PYO files
> Version: $Revision$
> Last-Modified: $Date$
> Author: Brett Cannon <brett at python.org>
> Status: Draft
> Type: Standards Track
> Content-Type: text/x-rst
> Created: 20-Feb-2015
> Post-History:
>     2015-03-06
>     2015-03-13
>     2015-03-20
>
> Abstract
> ========
>
> This PEP proposes eliminating the concept of PYO files from Python.
> To continue the support of the separation of bytecode files based on
> their optimization level, this PEP proposes extending the PYC file
> name to include the optimization level in the bytecode repository
> directory when it's called for (i.e., the ``__pycache__`` directory).
>
>
> Rationale
> =========
>
> As of today, bytecode files come in two flavours: PYC and PYO. A PYC
> file is the bytecode file generated and read from when no
> optimization level is specified at interpreter startup (i.e., ``-O``
> is not specified). A PYO file represents the bytecode file that is
> read/written when **any** optimization level is specified (i.e., when
> ``-O`` **or** ``-OO`` is specified). This means that while PYC
> files clearly delineate the optimization level used when they were
> generated -- namely no optimizations beyond the peepholer -- the same
> is not true for PYO files. To put this in terms of optimization
> levels and the file extension:
>
>   - 0: ``.pyc``
>   - 1 (``-O``): ``.pyo``
>   - 2 (``-OO``): ``.pyo``
>
> The reuse of the ``.pyo`` file extension for both level 1 and 2
> optimizations means that there is no clear way to tell what
> optimization level was used to generate the bytecode file. In terms
> of reading PYO files, this can lead to an interpreter using a mixture
> of optimization levels with its code if the user was not careful to
> make sure all PYO files were generated using the same optimization
> level (typically done by blindly deleting all PYO files and then
> using the `compileall` module to compile all-new PYO files [1]_).
> This issue is only compounded when people optimize Python code beyond
> what the interpreter natively supports, e.g., using the astoptimizer
> project [2]_.
>
> In terms of writing PYO files, the need to delete all PYO files
> every time one either changes the optimization level they want to use
> or are unsure of what optimization was used the last time PYO files
> were generated leads to unnecessary file churn. The change proposed
> by this PEP also allows for **all** optimization levels to be
> pre-compiled for bytecode files ahead of time, something that is
> currently impossible thanks to the reuse of the ``.pyo`` file
> extension for multiple optimization levels.
>
> As for distributing bytecode-only modules, having to distribute both
> ``.pyc`` and ``.pyo`` files is unnecessary for the common use-case
> of code obfuscation and smaller file deployments. This means that
> bytecode-only modules will only load from their non-optimized
> ``.pyc`` file name.
>
>
> Proposal
> ========
>
> To eliminate the ambiguity that PYO files present, this PEP proposes
> eliminating the concept of PYO files and their accompanying ``.pyo``
> file extension. To allow for the optimization level to be unambiguous
> as well as to avoid having to regenerate optimized bytecode files
> needlessly in the `__pycache__` directory, the optimization level
> used to generate the bytecode file will be incorporated into the
> bytecode file name. When no optimization level is specified, the
> pre-PEP ``.pyc`` file name will be used (i.e., no change in file name
> semantics). This increases backwards-compatibility while also being
> more understanding of Python implementations which have no use for
> optimization levels (e.g., PyPy[10]_).
>
> Currently bytecode file names are created by
> ``importlib.util.cache_from_source()``, approximately using the
> following expression defined by PEP 3147 [3]_, [4]_, [5]_::
>
>     '{name}.{cache_tag}.pyc'.format(name=module_name,
>                                     cache_tag=sys.implementation.cache_tag)
>
> This PEP proposes to change the expression when an optimization
> level is specified to::
>
>     '{name}.{cache_tag}.opt-{optimization}.pyc'.format(
>             name=module_name,
>             cache_tag=sys.implementation.cache_tag,
>             optimization=str(sys.flags.optimize))
>
> The "opt-" prefix was chosen so as to provide a visual separator
> from the cache tag. The placement of the optimization level after
> the cache tag was chosen to preserve lexicographic sort order of
> bytecode file names based on module name and cache tag which will
> not vary for a single interpreter. The "opt-" prefix was chosen over
> "o" so as to be somewhat self-documenting. The "opt-" prefix was
> chosen over "O" so as to not have any confusion in case "0" was the
> leading prefix of the optimization level.
>
> A period was chosen over a hyphen as a separator so as to distinguish
> clearly that the optimization level is not part of the interpreter
> version as specified by the cache tag. It also lends to the use of
> the period in the file name to delineate semantically different
> concepts.
>
> For example, if ``-OO`` had been passed to the interpreter then instead
> of ``importlib.cpython-35.pyo`` the file name would be
> ``importlib.cpython-35.opt-2.pyc``.
>
> It should be noted that this change in no way affects the performance
> of import. Since the import system looks for a single bytecode file
> based on the optimization level of the interpreter already and
> generates a new bytecode file if it doesn't exist, the introduction
> of potentially more bytecode files in the ``__pycache__`` directory
> has no effect in terms of stat calls. The interpreter will continue
> to look for only a single bytecode file based on the optimization
> level and thus no increase in stat calls will occur.
>
> The only potentially negative result of this PEP is the probable
> increase in the number of ``.pyc`` files and thus increase in storage
> use. But for platforms where this is an issue,
> ``sys.dont_write_bytecode`` exists to turn off bytecode generation so
> that it can be controlled offline.
>
>
> Implementation
> ==============
>
> importlib
> ---------
>
> As ``importlib.util.cache_from_source()`` is the API that exposes
> bytecode file paths as well as being directly used by importlib, it
> requires the most critical change. As of Python 3.4, the function's
> signature is::
>
>   importlib.util.cache_from_source(path, debug_override=None)
>
> This PEP proposes changing the signature in Python 3.5 to::
>
>   importlib.util.cache_from_source(path, debug_override=None, *,
> optimization=None)
>
> The introduced ``optimization`` keyword-only parameter will control
> what optimization level is specified in the file name. If the
> argument is ``None`` then the current optimization level of the
> interpreter will be assumed (including no optimization). Any argument
> given for ``optimization`` will be passed to ``str()`` and must have
> ``str.isalnum()`` be true, else ``ValueError`` will be raised (this
> prevents invalid characters being used in the file name). If the
> empty string is passed in for ``optimization`` then the addition of
> the optimization will be suppressed, reverting to the file name
> format which predates this PEP.
>
> It is expected that beyond Python's own two optimization levels,
> third-party code will use a hash of optimization names to specify the
> optimization level, e.g.
> ``hashlib.sha256(','.join(['no dead code', 'const
> folding'])).hexdigest()``.
> While this might lead to long file names, it is assumed that most
> users never look at the contents of the __pycache__ directory and so
> this won't be an issue.
>
> The ``debug_override`` parameter will be deprecated. As the parameter
> expects a boolean, the integer value of the boolean will be used as
> if it had been provided as the argument to ``optimization`` (a
> ``None`` argument will mean the same as for ``optimization``). A
> deprecation warning will be raised when ``debug_override`` is given a
> value other than ``None``, but there are no plans for the complete
> removal of the parameter at this time (but removal will be no later
> than Python 4).
>
> The various module attributes for importlib.machinery which relate to
> bytecode file suffixes will be updated [7]_. The
> ``DEBUG_BYTECODE_SUFFIXES`` and ``OPTIMIZED_BYTECODE_SUFFIXES`` will
> both be documented as deprecated and set to the same value as
> ``BYTECODE_SUFFIXES`` (removal of ``DEBUG_BYTECODE_SUFFIXES`` and
> ``OPTIMIZED_BYTECODE_SUFFIXES`` is not currently planned, but will be
> not later than Python 4).
>
> All various finders and loaders will also be updated as necessary,
> but updating the previous mentioned parts of importlib should be all
> that is required.
>
>
> Rest of the standard library
> ----------------------------
>
> The various functions exposed by the ``py_compile`` and
> ``compileall`` functions will be updated as necessary to make sure
> they follow the new bytecode file name semantics [6]_, [1]_. The CLI
> for the ``compileall`` module will not be directly affected (the
> ``-b`` flag will be implicit as it will no longer generate ``.pyo``
> files when ``-O`` is specified).
>
>
> Compatibility Considerations
> ============================
>
> Any code directly manipulating bytecode files from Python 3.2 on
> will need to consider the impact of this change on their code (prior
> to Python 3.2 -- including all of Python 2 -- there was no
> __pycache__ which already necessitates bifurcating bytecode file
> handling support). If code was setting the ``debug_override``
> argument to ``importlib.util.cache_from_source()`` then care will be
> needed if they want the path to a bytecode file with an optimization
> level of 2. Otherwise only code **not** using
> ``importlib.util.cache_from_source()`` will need updating.
>
> As for people who distribute bytecode-only modules (i.e., use a
> bytecode file instead of a source file), they will have to choose
> which optimization level they want their bytecode files to be since
> distributing a ``.pyo`` file with a ``.pyc`` file will no longer be
> of any use. Since people typically only distribute bytecode files for
> code obfuscation purposes or smaller distribution size then only
> having to distribute a single ``.pyc`` should actually be beneficial
> to these use-cases. And since the magic number for bytecode files
> changed in Python 3.5 to support PEP 465 there is no need to support
> pre-existing ``.pyo`` files [8]_.
>
>
> Rejected Ideas
> ==============
>
> Completely dropping optimization levels from CPython
> ----------------------------------------------------
>
> Some have suggested that instead of accommodating the various
> optimization levels in CPython, we should instead drop them
> entirely. The argument is that significant performance gains would
> occur from runtime optimizations through something like a JIT and not
> through pre-execution bytecode optimizations.
>
> This idea is rejected for this PEP as that ignores the fact that
> there are people who do find the pre-existing optimization levels for
> CPython useful. It also assumes that no other Python interpreter
> would find what this PEP proposes useful.
>
>
> Alternative formatting of the optimization level in the file name
> -----------------------------------------------------------------
>
> Using the "opt-" prefix and placing the optimization level between
> the cache tag and file extension is not critical. All options which
> have been considered are:
>
> * ``importlib.cpython-35.opt-1.pyc``
> * ``importlib.cpython-35.opt1.pyc``
> * ``importlib.cpython-35.o1.pyc``
> * ``importlib.cpython-35.O1.pyc``
> * ``importlib.cpython-35.1.pyc``
> * ``importlib.cpython-35-O1.pyc``
> * ``importlib.O1.cpython-35.pyc``
> * ``importlib.o1.cpython-35.pyc``
> * ``importlib.1.cpython-35.pyc``
>
> These were initially rejected either because they would change the
> sort order of bytecode files, possible ambiguity with the cache tag,
> or were not self-documenting enough. An informal poll was taken and
> people clearly preferred the formatting proposed by the PEP [9]_.
> Since this topic is non-technical and of personal choice, the issue
> is considered solved.
>
>
> Embedding the optimization level in the bytecode metadata
> ---------------------------------------------------------
>
> Some have suggested that rather than embedding the optimization level
> of bytecode in the file name that it be included in the file's
> metadata instead. This would mean every interpreter had a single copy
> of bytecode at any time. Changing the optimization level would thus
> require rewriting the bytecode, but there would also only be a single
> file to care about.
>
> This has been rejected due to the fact that Python is often installed
> as a root-level application and thus modifying the bytecode file for
> modules in the standard library are always possible. In this
> situation integrators would need to guess at what a reasonable
> optimization level was for users for any/all situations. By
> allowing multiple optimization levels to co-exist simultaneously it
> frees integrators from having to guess what users want and allows
> users to utilize the optimization level they want.
>
>
> References
> ==========
>
> .. [1] The compileall module
>    (https://docs.python.org/3/library/compileall.html#module-compileall)
>
> .. [2] The astoptimizer project
>    (https://pypi.python.org/pypi/astoptimizer)
>
> .. [3] ``importlib.util.cache_from_source()``
>    (
> https://docs.python.org/3.5/library/importlib.html#importlib.util.cache_from_source
> )
>
> .. [4] Implementation of ``importlib.util.cache_from_source()`` from
> CPython 3.4.3rc1
>    (
> https://hg.python.org/cpython/file/038297948389/Lib/importlib/_bootstrap.py#l437
> )
>
> .. [5] PEP 3147, PYC Repository Directories, Warsaw
>    (http://www.python.org/dev/peps/pep-3147)
>
> .. [6] The py_compile module
>    (https://docs.python.org/3/library/compileall.html#module-compileall)
>
> .. [7] The importlib.machinery module
>    (
> https://docs.python.org/3/library/importlib.html#module-importlib.machinery
> )
>
> .. [8] ``importlib.util.MAGIC_NUMBER``
>    (
> https://docs.python.org/3/library/importlib.html#importlib.util.MAGIC_NUMBER
> )
>
> .. [9] Informal poll of file name format options on Google+
>    (https://plus.google.com/u/0/+BrettCannon/posts/fZynLNwHWGm)
>
> .. [10] The PyPy Project
>    (http://pypy.org/)
>
>
> Copyright
> =========
>
> This document has been placed in the public domain.
>
>
> ..
>    Local Variables:
>    mode: indented-text
>    indent-tabs-mode: nil
>    sentence-end-double-space: t
>    fill-column: 70
>    coding: utf-8
>    End:
>
>
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-- 
--Guido van Rossum (python.org/~guido)
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