On Fri, Mar 20, 2015 at 4:41 PM Guido van Rossum <gu...@python.org> wrote:
> 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
>
#1
>
> 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.
>
It was meant to be explained by "When no optimization level is specified,
the pre-PEP ``.pyc`` file name will be used (i.e., no change in file name
semantics)", but obviously it's a bit too subtle. I just updated the PEP
with an explicit list of bytecode file name examples based on no -O, -O,
and -OO.
-Brett
>
>
> On Fri, Mar 20, 2015 at 11:34 AM, Brett Cannon <bcan...@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 <br...@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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