PEP: 304
Title: Controlling Generation of Bytecode Files
Author: Skip Montanaro
Status: Withdrawn
Type: Standards Track
Created: 22-Jan-2003
Post-History: 27-Jan-2003, 31-Jan-2003, 17-Jun-2005

Historical Note
===============

While this original PEP was withdrawn, a variant of this feature
was eventually implemented for Python 3.8 in https://bugs.python.org/issue33499

Several of the issues and concerns originally raised in this PEP were resolved
by other changes in the intervening years:

- the introduction of isolated mode to handle potential security concerns
- the switch to ``importlib``, a fully import-hook based import system implementation
- :pep:`3147`'s change in the bytecode cache layout to use ``__pycache__``
  subdirectories, including the ``source_to_cache(path)`` and
  ``cache_to_source(path)`` APIs that allow the interpreter to automatically
  handle the redirection to a separate cache directory

Abstract
========

This PEP outlines a mechanism for controlling the generation and
location of compiled Python bytecode files.  This idea originally
arose as a patch request [1]_ and evolved into a discussion thread on
the python-dev mailing list [2]_.  The introduction of an environment
variable will allow people installing Python or Python-based
third-party packages to control whether or not bytecode files should
be generated at installation time, and if so, where they should be
written.  It will also allow users to control whether or not bytecode
files should be generated at application run-time, and if so, where
they should be written.


Proposal
========

Add a new environment variable, PYTHONBYTECODEBASE, to the mix of
environment variables which Python understands.  PYTHONBYTECODEBASE is
interpreted as follows:

- If not defined, Python bytecode is generated in exactly the same way
  as is currently done.  sys.bytecodebase is set to the root directory
  (either / on Unix and Mac OSX or the root directory of the startup
  (installation???) drive -- typically ``C:\`` -- on Windows).

- If defined and it refers to an existing directory to which the user
  has write permission, sys.bytecodebase is set to that directory and
  bytecode files are written into a directory structure rooted at that
  location.

- If defined but empty, sys.bytecodebase is set to None and generation
  of bytecode files is suppressed altogether.

- If defined and one of the following is true:

  * it does not refer to a directory,

  * it refers to a directory, but not one for which the user has write
    permission

  a warning is displayed, sys.bytecodebase is set to None and
  generation of bytecode files is suppressed altogether.

After startup initialization, all runtime references are to
sys.bytecodebase, not the PYTHONBYTECODEBASE environment variable.
sys.path is not modified.

From the above, we see sys.bytecodebase can only take on two valid
types of values: None or a string referring to a valid directory on
the system.

During import, this extension works as follows:

- The normal search for a module is conducted.  The search order is
  roughly: dynamically loaded extension module, Python source file,
  Python bytecode file.  The only time this mechanism comes into play
  is if a Python source file is found.

- Once we've found a source module, an attempt to read a byte-compiled
  file in the same directory is made.  (This is the same as before.)

- If no byte-compiled file is found, an attempt to read a
  byte-compiled file from the augmented directory is made.

- If bytecode generation is required, the generated bytecode is written
  to the augmented directory if possible.

Note that this PEP is explicitly *not* about providing
module-by-module or directory-by-directory control over the
disposition of bytecode files.


Glossary
--------

- "bytecode base" refers to the current setting of
  sys.bytecodebase.

- "augmented directory" refers to the directory formed from the
  bytecode base and the directory name of the source file.

- PYTHONBYTECODEBASE refers to the environment variable when necessary
  to distinguish it from "bytecode base".


Locating bytecode files
-----------------------

When the interpreter is searching for a module, it will use sys.path
as usual.  However, when a possible bytecode file is considered, an
extra probe for a bytecode file may be made.  First, a check is made
for the bytecode file using the directory in sys.path which holds the
source file (the current behavior).  If a valid bytecode file is not
found there (either one does not exist or exists but is out-of-date)
and the bytecode base is not None, a second probe is made using the
directory in sys.path prefixed appropriately by the bytecode base.


Writing bytecode files
----------------------

When the bytecode base is not None, a new bytecode file is written to
the appropriate augmented directory, never directly to a directory in
sys.path.


Defining augmented directories
------------------------------

Conceptually, the augmented directory for a bytecode file is the
directory in which the source file exists prefixed by the bytecode
base.  In a Unix environment this would be::

    pcb = os.path.abspath(sys.bytecodebase)
    if sourcefile[0] == os.sep: sourcefile = sourcefile[1:]
    augdir = os.path.join(pcb, os.path.dirname(sourcefile))

On Windows, which does not have a single-rooted directory tree, the
drive letter of the directory containing the source file is treated as
a directory component after removing the trailing colon.  The
augmented directory is thus derived as ::

    pcb = os.path.abspath(sys.bytecodebase)
    drive, base = os.path.splitdrive(os.path.dirname(sourcefile))
    drive = drive[:-1]
    if base[0] == "\\": base = base[1:]
    augdir = os.path.join(pcb, drive, base)


Fixing the location of the bytecode base
----------------------------------------

During program startup, the value of the PYTHONBYTECODEBASE
environment variable is made absolute, checked for validity and added
to the sys module, effectively::

    pcb = os.path.abspath(os.environ["PYTHONBYTECODEBASE"])
    probe = os.path.join(pcb, "foo")
    try:
        open(probe, "w")
    except IOError:
        sys.bytecodebase = None
    else:
        os.unlink(probe)
        sys.bytecodebase = pcb

This allows the user to specify the bytecode base as a relative path,
but not have it subject to changes to the current working directory
during program execution.  (I can't imagine you'd want it to move
around during program execution.)

There is nothing special about sys.bytecodebase.  The user may change
it at runtime if desired, but normally it will not be modified.


Rationale
=========

In many environments it is not possible for non-root users to write
into directories containing Python source files.  Most of the time,
this is not a problem as Python source is generally byte compiled
during installation.  However, there are situations where bytecode
files are either missing or need to be updated.  If the directory
containing the source file is not writable by the current user a
performance penalty is incurred each time a program importing the
module is run. [3]_ Warning messages may also be generated in certain
circumstances.  If the directory is writable, nearly simultaneous
attempts to write the bytecode file by two separate processes
may occur, resulting in file corruption. [4]_

In environments with RAM disks available, it may be desirable for
performance reasons to write bytecode files to a directory on such a
disk.  Similarly, in environments where Python source code resides on
network file systems, it may be desirable to cache bytecode files on
local disks.


Alternatives
============

The only other alternative proposed so far [1]_ seems to be to add a
-R flag to the interpreter to disable writing bytecode files
altogether.  This proposal subsumes that.  Adding a command-line
option is certainly possible, but is probably not sufficient, as the
interpreter's command line is not readily available during
installation (early during program startup???).


Issues
======

- Interpretation of a module's __file__ attribute.  I believe the
  __file__ attribute of a module should reflect the true location of
  the bytecode file.  If people want to locate a module's source code,
  they should use imp.find_module(module).

- Security - What if root has PYTHONBYTECODEBASE set?  Yes, this can
  present a security risk, but so can many other things the root user
  does.  The root user should probably not set PYTHONBYTECODEBASE
  except possibly during installation.  Still, perhaps this problem
  can be minimized.  When running as root the interpreter should check
  to see if PYTHONBYTECODEBASE refers to a directory which is writable
  by anyone other than root.  If so, it could raise an exception or
  warning and set sys.bytecodebase to None.  Or, see the next item.

- More security - What if PYTHONBYTECODEBASE refers to a general
  directory (say, /tmp)?  In this case, perhaps loading of a
  preexisting bytecode file should occur only if the file is owned by
  the current user or root.  (Does this matter on Windows?)

- The interaction of this PEP with import hooks has not been
  considered yet.  In fact, the best way to implement this idea might
  be as an import hook.  See :pep:`302`.

- In the current (pre-:pep:`304`) environment, it is safe to delete a
  source file after the corresponding bytecode file has been created,
  since they reside in the same directory.  With :pep:`304` as currently
  defined, this is not the case.  A bytecode file in the augmented
  directory is only considered when the source file is present and it
  thus never considered when looking for module files ending in
  ".pyc".  I think this behavior may have to change.


Examples
========

In the examples which follow, the urllib source code resides in
/usr/lib/python2.3/urllib.py and /usr/lib/python2.3 is in sys.path but
is not writable by the current user.

- The bytecode base is /tmp.  /usr/lib/python2.3/urllib.pyc exists and
  is valid.  When urllib is imported, the contents of
  /usr/lib/python2.3/urllib.pyc are used.  The augmented directory is
  not consulted.  No other bytecode file is generated.

- The bytecode base is /tmp.  /usr/lib/python2.3/urllib.pyc exists,
  but is out-of-date.  When urllib is imported, the generated bytecode
  file is written to urllib.pyc in the augmented directory which has
  the value /tmp/usr/lib/python2.3.  Intermediate directories will be
  created as needed.

- The bytecode base is None.  No urllib.pyc file is found.  When
  urllib is imported, no bytecode file is written.

- The bytecode base is /tmp.  No urllib.pyc file is found.  When
  urllib is imported, the generated bytecode file is written to the
  augmented directory which has the value /tmp/usr/lib/python2.3.
  Intermediate directories will be created as needed.

- At startup, PYTHONBYTECODEBASE is /tmp/foobar, which does not exist.
  A warning is emitted, sys.bytecodebase is set to None and no
  bytecode files are written during program execution unless
  sys.bytecodebase is later changed to refer to a valid,
  writable directory.

- At startup, PYTHONBYTECODEBASE is set to /, which exists, but is not
  writable by the current user.  A warning is emitted,
  sys.bytecodebase is set to None and no bytecode files are
  written during program execution unless sys.bytecodebase is
  later changed to refer to a valid, writable directory.  Note that
  even though the augmented directory constructed for a particular
  bytecode file may be writable by the current user, what counts is
  that the bytecode base directory itself is writable.

- At startup PYTHONBYTECODEBASE is set to the empty string.
  sys.bytecodebase is set to None.  No warning is generated, however.
  If no urllib.pyc file is found when urllib is imported, no bytecode
  file is written.

In the Windows examples which follow, the urllib source code resides
in ``C:\PYTHON22\urllib.py``.  ``C:\PYTHON22`` is in sys.path but is
not writable by the current user.

- The bytecode base is set to ``C:\TEMP``.  ``C:\PYTHON22\urllib.pyc``
  exists and is valid.  When urllib is imported, the contents of
  ``C:\PYTHON22\urllib.pyc`` are used.  The augmented directory is not
  consulted.

- The bytecode base is set to ``C:\TEMP``.  ``C:\PYTHON22\urllib.pyc``
  exists, but is out-of-date.  When urllib is imported, a new bytecode
  file is written to the augmented directory which has the value
  ``C:\TEMP\C\PYTHON22``.  Intermediate directories will be created as
  needed.

- At startup PYTHONBYTECODEBASE is set to ``TEMP`` and the current
  working directory at application startup is ``H:\NET``.  The
  potential bytecode base is thus ``H:\NET\TEMP``.  If this directory
  exists and is writable by the current user, sys.bytecodebase will be
  set to that value.  If not, a warning will be emitted and
  sys.bytecodebase will be set to None.

- The bytecode base is ``C:\TEMP``.  No urllib.pyc file is found.
  When urllib is imported, the generated bytecode file is written to
  the augmented directory which has the value ``C:\TEMP\C\PYTHON22``.
  Intermediate directories will be created as needed.


Implementation
==============

See the patch on Sourceforge. [6]_

References
==========

.. [1] patch 602345, Option for not writing py.[co] files, Klose
   (https://bugs.python.org/issue602345)

.. [2] python-dev thread, Disable writing .py[co], Norwitz
   (https://mail.python.org/pipermail/python-dev/2003-January/032270.html)

.. [3] Debian bug report, Mailman is writing to /usr in cron, Wegner
   (http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=96111)

.. [4] python-dev thread, Parallel pyc construction, Dubois
   (https://mail.python.org/pipermail/python-dev/2003-January/032060.html)

.. [6] patch 677103, PYTHONBYTECODEBASE patch (PEP 304), Montanaro
   (https://bugs.python.org/issue677103)


Copyright
=========

This document has been placed in the public domain.
