[Python-checkins] r78453 - in peps/trunk: pep-3147-1.dia pep-3147-1.png pep-3147.txt
barry.warsaw
python-checkins at python.org
Thu Feb 25 21:39:11 CET 2010
Author: barry.warsaw
Date: Thu Feb 25 21:39:11 2010
New Revision: 78453
Log:
Updates to PYC Repository Directories, reflecting current thinking on the
approach.
Modified:
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peps/trunk/pep-3147-1.png
peps/trunk/pep-3147.txt
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--- peps/trunk/pep-3147.txt (original)
+++ peps/trunk/pep-3147.txt Thu Feb 25 21:39:11 2010
@@ -8,7 +8,7 @@
Content-Type: text/x-rst
Created: 2009-12-16
Python-Version: 3.2
-Post-History:
+Post-History: 2010-01-30, 2010-02-25
Abstract
@@ -17,49 +17,75 @@
This PEP describes an extension to Python's import mechanism which
improves sharing of Python source code files among multiple installed
different versions of the Python interpreter. It does this by
-allowing many different byte compilation files (.pyc files) to be
+allowing more than one byte compilation file (.pyc files) to be
co-located with the Python source file (.py file). The extension
described here can also be used to support different Python
compilation caches, such as JIT output that may be produced by an
Unladen Swallow [1]_ enabled C Python.
+Background
+==========
+
+CPython compiles its source code into "byte code", and for performance
+reasons, it caches this byte code on the file system whenever the
+source file has changes. This makes loading of Python modules much
+faster because the compilation phase can be bypassed. When your
+source file is `foo.py`, CPython caches the byte code in a `foo.pyc`
+file right next to the source.
+
+Byte code files contain two 32-bit numbers followed by the marshaled
+[2]_ code object. The 32-bit numbers represent a magic number and a
+timestamp. The magic number changes whenever Python changes the byte
+code format, e.g. by adding new byte codes to its virtual machine.
+This ensures that pyc files built for previous versions of the VM
+won't cause problems. The timestamp is used to make sure that the pyc
+file is not older than the py file that was used to create it. When
+either the magic number or timestamp do not match, the py file is
+recompiled and a new pyc file is written.
+
+In practice, it is well known that pyc files are not compatible across
+Python major releases. A reading of import.c [3]_ in the Python
+source code proves that within recent memory, every new CPython major
+release has bumped the pyc magic number.
+
+
Rationale
=========
-Linux distributions such as Ubuntu [2]_ and Debian [3]_ provide more
+Linux distributions such as Ubuntu [4]_ and Debian [5]_ provide more
than one Python version at the same time to their users. For example,
-Ubuntu 9.10 Karmic Koala can install Python 2.5, 2.6, and 3.1, with
-Python 2.6 being the default.
+Ubuntu 9.10 Karmic Koala users can install Python 2.5, 2.6, and 3.1,
+with Python 2.6 being the default.
-In order to ease the burden on operating system packagers for these
-distributions, the distribution packages do not contain Python version
-numbers [4]_; they are shared across all Python versions installed on
-the system. Putting Python version numbers in the packages would be a
-maintenance nightmare, since all the packages - *and their
-dependencies* - would have to be updated every time a new Python
-release was added or removed from the distribution. Because of the
-sheer number of packages available, this amount of work is infeasible.
-
-For pure Python modules, sharing is possible because upstream
-maintainers typically support multiple versions of Python in a source
-compatible way. In practice though, it is well known that pyc files
-are not compatible across Python major releases. A reading of
-import.c [5]_ in the Python source code proves that within recent
-memory, every new CPython major release has bumped the pyc magic
-number.
+This causes a conflict for Python source files installed by the
+system (including third party packages), because you cannot compile a
+single Python source file for more than one Python version at a time.
+Thus if your system wanted to install a `/usr/share/python/foo.py`, it
+could not create a `/usr/share/python/foo.pyc` file usable across all
+installed Python versions.
+
+Furthermore, in order to ease the burden on operating system packagers
+for these distributions, the distribution packages do not contain
+Python version numbers [6]_; they are shared across all Python
+versions installed on the system. Putting Python version numbers in
+the packages would be a maintenance nightmare, since all the packages
+- *and their dependencies* - would have to be updated every time a new
+Python release was added or removed from the distribution. Because of
+the sheer number of packages available, this amount of work is
+infeasible.
Even C extensions can be source compatible across multiple versions of
Python. Compiled extension modules are usually not compatible though,
-and PEP 384 [6]_ has been proposed to address this by defining a
+and PEP 384 [7]_ has been proposed to address this by defining a
stable ABI for extension modules.
-Because the distributions cannot share pyc files, elaborate mechanisms
-have been developed to put the resulting pyc files in non-shared
-locations while the source code is still shared. Examples include the
-symlink-based Debian regimes python-support [7]_ and python-central
-[8]_. These approaches make for much more complicated, fragile,
-inscrutable, and fragmented policies for delivering Python
+Because these distributions cannot share pyc files, elaborate
+mechanisms have been developed to put the resulting pyc files in
+non-shared locations while the source code is still shared. Examples
+include the symlink-based Debian regimes python-support [8]_ and
+python-central [9]_. These approaches make for much more complicated,
+fragile, inscrutable, and fragmented policies for delivering Python
applications to a wide range of users. Arguably more users get Python
from their operating system vendor than from upstream tarballs. Thus,
solving this pyc sharing problem for CPython is a high priority for
@@ -71,29 +97,106 @@
Proposal
========
-Python's import machinery is extended to search for byte code cache
-files in a directory co-located with the source file, but with an
-extension 'pyr'. The pyr directory contains individual files with the
-cached byte compilation of the source code, identical to current pyc
-and pyo files. The files inside the pyr directory retain their file
-extensions, but the base name is replaced by the hexlified [10]_ magic
-number of the Python version the byte code is compatible with.
-
-The file extension pyr was chosen because 'r' is a mnemonic for
-'repository', and there appears to be no prior uses of the extension
-[9]_.
-
-For example, a module `foo` with source code in `foo.py` and byte
-compiled with Python 2.5, Python 2.6, Python 2.6 `-O`, Python 2.6
-`-U`, and Python 3.1 would have the following file system layout::
-
- foo.py
- foo.pyr/
- f2b30a0d.pyc # Python 2.5
- f2d10a0d.pyc # Python 2.6
- f2d10a0d.pyo # Python 2.6 -O
- f2d20a0d.pyc # Python 2.6 -U
- 0c4f0a0d.pyc # Python 3.1
+Python's import machinery is extended to write and search for byte
+code cache files in a single directory inside every Python package
+directory. This directory will be called `__pycache__`.
+
+Further, pyc files will contain a magic string that
+differentiates the Python version they were compiled for. This allows
+multiple byte compiled cache files to co-exist for a single Python
+source file.
+
+This scheme has the added benefit of reducing the clutter in a Python
+package directory.
+
+What would this look like in practice?
+
+Let's say we have a Python package named `alpha` which contains a
+sub-package name `beta`. The source directory layout might look like
+this::
+
+ alpha/
+ __init__.py
+ one.py
+ two.py
+ beta/
+ __init__.py
+ three.py
+ four.py
+
+After byte compiling this package with Python 3.2, you would see the
+following layout::
+
+ alpha/
+ __pycache__
+ __init__.cpython-32.pyc
+ one.cpython-32.pyc
+ two.cpython-32.pyc
+ __init__.py
+ one.py
+ two.py
+ beta/
+ __pycache__
+ __init__.cpython-32.pyc
+ three.cpython-32.pyc
+ four.cpython-32.pyc
+ __init__.py
+ three.py
+ four.py
+
+Let's say that two new versions of Python are installed, one is Python
+3.3 and another is Unladen Swallow. After byte compilation, the file
+system would look like this::
+
+ alpha/
+ __pycache__
+ __init__.cpython-32.pyc
+ __init__.cpython-33.pyc
+ __init__.unladen-10.pyc
+ one.cpython-32.pyc
+ one.cpython-33.pyc
+ one.unladen-10.pyc
+ two.cpython-32.pyc
+ two.cpython-33.pyc
+ two.unladen-10.pyc
+ __init__.py
+ one.py
+ two.py
+ beta/
+ __pycache__
+ __init__.cpython-32.pyc
+ __init__.cpython-33.pyc
+ __init__.unladen-10.pyc
+ three.cpython-32.pyc
+ three.cpython-33.pyc
+ three.unladen-10.pyc
+ four.cpython-32.pyc
+ four.cpython-33.pyc
+ four.unladen-10.pyc
+ __init__.py
+ three.py
+ four.py
+
+As you can see, as long as the Python version identifier string is
+unique, any number of pyc files can co-exist. These identifier
+strings are described in more detail below.
+
+A nice property of this layout is that the `__pycache__` directories
+can generally be ignored, such that a normal directory listing would
+show something like this::
+
+ alpha/
+ __pycache__
+ __init__.py
+ one.py
+ two.py
+ beta/
+ __pycache__
+ __init__.py
+ three.py
+ four.py
+
+This is much less cluttered than even today's Python.
Python behavior
@@ -105,56 +208,105 @@
interpreter's magic number, and the source file is not newer than the
`pyc` file.
-When Python finds a `foo.py` file for which no `foo.pyc` file or
-`foo.pyr` directory exists, Python will by default load the `foo.py`
-file and write a `foo.pyc` file next to the source file. This is
-unchanged from current behavior.
-
-When the Python executable is given a `-R` flag, or the environment
-variable `$PYTHONPYR` is set, then Python will create a `foo.pyr`
-directory and write a `pyc` file to that directory with the hexlified
-magic number as the base name.
-
-If during import, Python finds an existing `pyc` file but no `pyr`
-directory, and the `$PYTHONPYR` environment variable is not set, then
-the `pyc` file is loaded as normal and no `pyr` directory is created.
-
-If during import, Python finds a `pyr` directory with a matching `pyc`
-file, *regardless of whether `$PYTHONPYR` is set or not*, then
-`foo.pyr/<magic>.pyc` is loaded and import completes successfully.
-Thus a matching `pyc` file inside a `pyr` directory always takes
-precedence over a sibling `pyc` file.
-
-If during import, Python finds a `pyr` directory that does not contain
-a matching `pyc` file, and no sibling `foo.pyc` file exists, Python
-will load the source file and write a sibling `foo.pyc` file, unless
-the `-R` flag is given in which case a `foo.pyr/<magic>.pyc` file will
-be written.
-Here is a flowchart illustrating the rules.
+Case 1: The first import
+------------------------
+
+When Python is asked to import module `foo`, it searches for a
+`foo.py` file (or `foo` package, but that's not important for this
+discussion) along its `sys.path`. When Python locates the `foo.py`
+file it will look for a `__pycache__` directory in the directory where
+it found the `foo.py`. If the `__pycache__` directory is missing,
+Python will create it. Then it will parse and byte compile the
+`foo.py` file and save the byte code in `__pycache__/foo.<magic>.pyc`,
+where <magic> is defined by the Python implementation, but will be a
+human readable string such as `cpython-32`.
+
+
+Case 2: The second import
+-------------------------
+
+When Python is asked to import module `foo` a second time (in a
+different process of course), it will again search for the `foo.py`
+file along its `sys.path`. When Python locates the `foo.py` file, it
+looks for a matching `__pycache__/foo.<magic>.pyc` and finding this,
+it reads the byte code and continues as usual.
+
+
+Case 3: __pycache__/foo.pyc with no source
+------------------------------------------
+
+It's possible that the `foo.py` file somehow got removed, while
+leaving the cached pyc file still on the file system. If the
+`__pycache__/foo.pyc` file exists, but the `foo.py` file used to
+create it does not, Python will raise an `ImportError` when asked to
+import foo. In other words, by default, Python will not support
+importing a module unless the source file exists.
+
+Python users who want to deploy sourceless imports are instructed to
+create a custom importer that supports this behavior. Options include
+importing pycs from a zip file, or locating pyc files where the py
+source file would have existed.
+
+
+Case 4: legacy pyc files
+------------------------
+
+Python will ignore all legacy pyc files. In other words, if a
+`foo.pyc` file exists next to the `foo.py` file, it will be ignored in
+all cases, including sourceless deployments. Python users wishing to
+support this use case can create a custom importer.
+
+
+Flow chart
+==========
+
+Here is a flow chart describing how modules are loaded:
.. image:: pep-3147-1.png
:scale: 75
-Effects on non-conforming Python versions
-=========================================
+Magic identifiers
+=================
-Python implementations which don't know anything about `pyr`
-directories will ignore them. This means that they will read and
-write `pyc` files as usual. A conforming implementation will still
-prefer any existing `foo.pyr/<magic>.pyc` file over an existing
-sibling `pyc` file.
-
-The one possible conflicting state is where a sibling `pyc` file
-exists, but its magic number does not match.
-
-In the default case, when Python finds a `pyc` file with a
-non-matching magic number, it simply overwrites the `pyc` file with
-the new byte code and magic number. In the absence of the `-R` flag,
-this remains unchanged. When the `-R` flag was given, the
-non-matching sibling `pyc` file is ignored - it is neither removed nor
-overwritten - and a `foo.pyr/<magic>.pyc` file is written instead.
+pyc files inside of the `__pycache__` directories contain a magic
+identifier in their file names. These are mnemonic tags for the
+actual magic numbers used by the importer. For example, for Python
+3.2, we could use the hexlified [10]_ magic number as a unique
+identifier::
+
+ >>> from binascii import hexlify
+ >>> from imp import get_magic
+ >>> 'foo.{}.pyc'.format(hexlify(get_magic()).decode('ascii'))
+ 'foo.580c0d0a.pyc'
+
+This isn't particularly human friendly though. Instead, this PEP
+proposes to add a mapping between internal magic numbers and a
+user-friendly tag. Newer versions of Python can add to this mapping
+so that all later Pythons know the mapping between tags and magic
+numbers. By convention, the tag will contain the Python
+implementation name and version nickname, where the nickname is
+generally the major version number and minor version number. Magic
+numbers should not change between Python micro releases, but some
+other convention can be used for changes in magic number between
+pre-release development versions.
+
+For example, CPython 3.2 would have a magic identifier tag of
+`cpython-32` and write pyc files like this: `foo.cpython-32.pyc`.
+When the `-O` flag is used, it would write `foo.cpython-32.pyo`. For
+backports of this feature to Python 2, when the `-U` flag is used, a
+file such as `foo.cpython-27u.pyc` can be written.
+
+
+Alternative Python implementations
+==================================
+
+Alternative Python implementations such as Jython [11]_, IronPython
+[12]_, PyPy [13]_, Pynie [14]_, and Unladen Swallow can also use the
+`__pycache__` directory to store whatever compilation artifacts make
+sense for their platforms. For example, Jython could store the class
+file for the module in `__pycache__/foo.jython-32.class`.
Implementation strategy
@@ -166,13 +318,97 @@
they see fit.
+Effects on existing code
+========================
+
+Adoption of this PEP will affect existing code and idioms, both inside
+Python and outside. This section enumerates some of these effects.
+
+
+__file__
+---------
+
+in Python 3, when you import a module, its `__file__` attribute points
+to its source `py` file (in Python 2, it points to the `pyc` file). A
+package's `__file__` points to the `py` file for its `__init__.py`.
+E.g.::
+
+ >>> import foo
+ >>> foo.__file__
+ 'foo.py'
+ # baz is a package
+ >>> import baz
+ >>> baz.__file__
+ 'baz/__init__.py'
+
+The implementation of this PEP would have to ensure that the same
+directory level is returned from `__file__` as it currently does so
+that the common idiom above continues to work.
+
+As part of this PEP, we will add an `__cached__` attribute to modules,
+which will always point to the actual `pyc` file that was read or
+written. When the environment variable `$PYTHONDONTWRITEBYTECODE` is
+set, or the `-B` option is given, or if the source lives on a
+read-only filesystem, then the `__cached__` attribute will point to
+the location that the `pyc` file *would* have been written to if it
+didn't exist. This location of course includes the `__pycache__`
+subdirectory in its path.
+
+For alternative Python implementations which do not support `pyc`
+files, the `__cached__` attribute may point to whatever
+version-specific binary file was read for the module code. E.g. on
+Jython, this might be the `.class` file for the module:
+`__pycache__/foo.jython-32.class`. Alternative implementations for
+which this scheme does not make sense should set the `__cached__`
+attribute to `None`.
+
+
+File extension checks
+---------------------
+
+There exists some code which checks for files ending in `.pyc` and
+simply chops off the last character to find the matching `.py` file.
+This code will obviously fail once this PEP is implemented.
+
+To support this use case, we'll add two new methods to the `imp`
+package [15]_:
+
+ * `imp.source_from_cache(py_path)` -> `pyc_path`
+ * `imp.cache_from_source(pyc_path)` -> `py_path`
+
+Alternative implementations are free to override these functions to
+return reasonable values based on their own support for this PEP.
+
+
+PEP 302 loaders
+---------------
+
+PEP 302 [16]_ defined loaders have a `.get_filename()` method which
+points to the `__file__` for a module. As part of this PEP, we will
+extend this API, to include a new method `.get_paths()` which will
+return a 2-tuple containing the path to the source file and the path
+to where the matching `pyc` file is (or would be).
+
+
+Backports
+---------
+
+For versions of Python earlier than 3.2 (and possibly 2.7), it is
+possible to backport this PEP. However, in Python 3.2 (and possibly
+2.7), this behavior will be turned on by default, and in fact, it will
+replace the old behavior. Backports will need to support the old
+layout by default. We suggest supporting PEP 3147 through the use of
+an environment variable called `$PYTHONCACHEDIR` or the command line
+switch `-Xcachedir` to enable the feature.
+
+
Alternatives
============
PEP 304
-------
-There is some overlap between the goals of this PEP and PEP 304 [12]_,
+There is some overlap between the goals of this PEP and PEP 304 [17]_,
which has been withdrawn. However PEP 304 would allow a user to
create a shadow file system hierarchy in which to store `pyc` files.
This concept of a shadow hierarchy for `pyc` files could be used to
@@ -197,37 +433,6 @@
possible to find the `my.dat` file relative to the source directory
from there.
-On the other hand, this PEP keeps all byte code artifacts co-located
-with the source file. Some adjustment will have to be made for the
-fact that the `pyc` file lives in a subdirectory. For example, in
-current Python, when you import a module, its `__file__` attribute
-points to its `pyc` file. A package's `__file__` points to the `pyc`
-file for its `__init__.py`. E.g.::
-
- >>> import foo
- >>> foo.__file__
- 'foo.pyc'
- # baz is a package
- >>> import baz
- >>> baz.__file__
- 'baz/__init__.pyc'
-
-The implementation of this PEP would have to ensure that the same
-directory level is returned from `__file__` as it does without the
-`pyr` directory, so that the common idiom above continues to work::
-
- >>> import foo
- >>> foo.__file__
- 'foo.pyr'
- # baz is a package
- >>> import baz
- >>> baz.__file__
- 'baz/__init__.pyr'
-
-Note that some existing Python code only checks for `.py` and `.pyc`
-file extensions (and possibly `.pyo`). These would have to be
-extended to also check for `.pyr` extensions.
-
Fat byte compilation files
--------------------------
@@ -240,8 +445,10 @@
but with extension lookup tables available to scale `pyf` byte code
objects as large as necessary.
-The fat byte compilation files were fairly complex, so the current
-simplification of using directories was suggested.
+The fat byte compilation files were fairly complex, and inherently
+introduced difficult race conditions, so the current simplification of
+using directories was suggested. The same problem applies to using
+zip files as the fat pyc file format.
Multiple file extensions
@@ -256,49 +463,16 @@
tools that are dependent on the file extension.
-Open questions
-==============
-
-* Are there any concurrency issues added by this PEP, above those that
- already exist? For example, what if two Python processes attempt to
- write the same `<magic>.pyc` file? Is that any different than two
- Python processes trying to write to the same `foo.pyc` file?
- Current thinking is that there isn't since the exclusive open
- mechanism currently used, will still be used to open `pyc` files
- inside a `pyr` directory.
-
-* How do the imp [13]_ and importlib [14]_ modules need to be updated
- to conform to the `pyr` directories?
-
-* What about `py` source files that are compatible with most but not
- all installed Python versions. We might need a way to say "this py
- file should be hidden from Python versions X.Y or earlier". There
- are three options:
-
- - Use file system tricks to only share py files that are actually
- sharable in all installed Python versions (e.g. different search
- directories for Python X.Y and Python X.Z).
- - Introduce Python syntax that is legal before __future__ imports
- and is evaluated to determine if the py file is compatible,
- raising an `ImportError('no module named foo')` if not.
- - Add an optional metadata file co-located with the py file that
- declares which Python versions it is compatible with.
-
- How does this requirement interact with PEP 382 namespace packages [15]_?
-
-* Are there any opportunities for also sharing extension modules
- (.so/.dll files) in a `pyr` directory?
-
-* Would a moratorium on byte code changes, similar to the language
- moratorium described in PEP 3003 [16]_ be a better approach to
- pursue, and would that solve the problem for vendors? At the time
- of this writing, PEP 3003 is silent on the issue.
-
-
Reference implementation
========================
-TBD
+A pure-Python reference implementation will be written using
+importlib [18]_, which may need some modifications to its API and
+abstract base classes. Once the semantics are agreed upon and the
+implementation details are settled, we'll port this to the C
+implementation in `import.c`. We will have extensive tests that
+guarantee that the pure-Python implementation and the built-in
+implementation remain in sync.
References
@@ -306,41 +480,45 @@
.. [1] PEP 3146
-.. [2] Ubuntu: <http://www.ubuntu.com>
+.. [2] The marshal module:
+ http://www.python.org/doc/current/library/marshal.html
+
+.. [3] import.c:
+ http://svn.python.org/view/python/branches/py3k/Python/import.c?view=markup
-.. [3] Debian: <http://www.debian.org>
+.. [4] Ubuntu: <http://www.ubuntu.com>
-.. [4] Debian Python Policy:
- http://www.debian.org/doc/packaging-manuals/python-policy/
+.. [5] Debian: <http://www.debian.org>
-.. [5] import.c:
- http://svn.python.org/view/python/branches/py3k/Python/import.c?view=markup
+.. [6] Debian Python Policy:
+ http://www.debian.org/doc/packaging-manuals/python-policy/
-.. [6] PEP 384
+.. [7] PEP 384
-.. [7] python-support:
+.. [8] python-support:
http://wiki.debian.org/DebianPythonFAQ#Whatispython-support.3F
-.. [8] python-central:
+.. [9] python-central:
http://wiki.debian.org/DebianPythonFAQ#Whatispython-central.3F
-.. [9] http://www.filesuffix.com/?m=search&e=pyr&submit=Search
-
.. [10] binascii.hexlify():
http://www.python.org/doc/current/library/binascii.html#binascii.hexlify
-.. [11] The marshal module:
- http://www.python.org/doc/current/library/marshal.html
+.. [11] Jython: http://www.jython.org/
+
+.. [12] IronPython: http://ironpython.net/
+
+.. [13] PyPy: http://codespeak.net/pypy/dist/pypy/doc/
-.. [12] PEP 304:
+.. [14] Pynie: http://code.google.com/p/pynie/
-.. [13] imp: http://www.python.org/doc/current/library/imp.html
+.. [15] imp: http://www.python.org/doc/current/library/imp.html
-.. [14] importlib: http://docs.python.org/3.1/library/importlib.html
+.. [16] PEP 302
-.. [15] PEP 382
+.. [17] PEP 304
-.. [16] PEP 3003
+.. [18] importlib: http://docs.python.org/3.1/library/importlib.html
ACKNOWLEDGMENTS
@@ -350,7 +528,7 @@
Martin von Loewis reviewed an early draft of the PEP and suggested the
simplification to store traditional `pyc` and `pyo` files in a
directory. Many other people reviewed early versions of this PEP and
-provided useful feedback including:
+provided useful feedback including but not limited to:
* David Malcolm
* Josselin Mouette
@@ -368,26 +546,6 @@
This document has been placed in the public domain.
-Notes from python-dev
-=====================
-
-The python-dev discussion has been very fruitful. Here are some
-in-progress notes from that thread which still needs to be reconciled
-into the body of the PEP.
-
-* Rarity of the use of this feature. Important for distros but
- probably much less so for individual users (who may never even see
- these things).
-* Sibling vs folder-per-folder. Do performance measurements. Do stat
- calls outweigh everything else? We need to do an analysis of the
- current implementation as a baseline.
-* Magic numbers in file names are magical; no one really knows the
- mappings. Maybe we should use magic strings (with a lookup table?),
- e.g. 'foo.cython-27.py'
-* Modules should unambiguously name their __source__ and __cache__
- file names. __file__ is ambiguous.
-
-
..
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