python-peps/pep-0402.txt

PEP: 402
Title: Simplified Package Layout and Partitioning
Version: $Revision$
Last-Modified: $Date$
Author: P.J. Eby
Status: Rejected
Type: Standards Track
Content-Type: text/x-rst
Created: 12-Jul-2011
Python-Version: 3.3
Post-History: 20-Jul-2011
Replaces: 382

Rejection Notice
================

On the first day of sprints at US PyCon 2012 we had a long and
fruitful discussion about PEP 382 and PEP 402.  We ended up rejecting
both but a new PEP will be written to carry on in the spirit of PEP
402.  Martin von Löwis wrote up a summary: [3]_.

Abstract
========

This PEP proposes an enhancement to Python's package importing
to:

* Surprise users of other languages less,
* Make it easier to convert a module into a package, and
* Support dividing packages into separately installed components
  (ala "namespace packages", as described in PEP 382)

The proposed enhancements do not change the semantics of any
currently-importable directory layouts, but make it possible for
packages to use a simplified directory layout (that is not importable
currently).

However, the proposed changes do NOT add any performance overhead to
the importing of existing modules or packages, and performance for the
new directory layout should be about the same as that of previous
"namespace package" solutions (such as ``pkgutil.extend_path()``).


The Problem
===========

.. epigraph::

    "Most packages are like modules.  Their contents are highly
    interdependent and can't be pulled apart.  [However,] some
    packages exist to provide a separate namespace. ...  It should
    be possible to distribute sub-packages or submodules of these
    [namespace packages] independently."

    -- Jim Fulton, shortly before the release of Python 2.3 [1]_


When new users come to Python from other languages, they are often
confused by Python's package import semantics.  At Google, for example,
Guido received complaints from "a large crowd with pitchforks" [2]_
that the requirement for packages to contain an ``__init__`` module
was a "misfeature", and should be dropped.

In addition, users coming from languages like Java or Perl are
sometimes confused by a difference in Python's import path searching.

In most other languages that have a similar path mechanism to Python's
``sys.path``, a package is merely a namespace that contains modules
or classes, and can thus be spread across multiple directories in
the language's path.  In Perl, for instance, a ``Foo::Bar`` module
will be searched for in ``Foo/`` subdirectories all along the module
include path, not just in the first such subdirectory found.

Worse, this is not just a problem for new users: it prevents *anyone*
from easily splitting a package into separately-installable
components.  In Perl terms, it would be as if every possible ``Net::``
module on CPAN had to be bundled up and shipped in a single tarball!

For that reason, various workarounds for this latter limitation exist,
circulated under the term "namespace packages".  The Python standard
library has provided one such workaround since Python 2.3 (via the
``pkgutil.extend_path()`` function), and the "setuptools" package
provides another (via ``pkg_resources.declare_namespace()``).

The workarounds themselves, however, fall prey to a *third* issue with
Python's way of laying out packages in the filesystem.

Because a package *must* contain an ``__init__`` module, any attempt
to distribute modules for that package must necessarily include that
``__init__`` module, if those modules are to be importable.

However, the very fact that each distribution of modules for a package
must contain this (duplicated) ``__init__`` module, means that OS
vendors who package up these module distributions must somehow handle
the conflict caused by several module distributions installing that
``__init__`` module to the same location in the filesystem.

This led to the proposing of PEP 382 ("Namespace Packages") - a way
to signal to Python's import machinery that a directory was
importable, using unique filenames per module distribution.

However, there was more than one downside to this approach.
Performance for all import operations would be affected, and the
process of designating a package became even more complex.  New
terminology had to be invented to explain the solution, and so on.

As terminology discussions continued on the Import-SIG, it soon became
apparent that the main reason it was so difficult to explain the
concepts related to "namespace packages" was because Python's
current way of handling packages is somewhat underpowered, when
compared to other languages.

That is, in other popular languages with package systems, no special
term is needed to describe "namespace packages", because *all*
packages generally behave in the desired fashion.

Rather than being an isolated single directory with a special marker
module (as in Python), packages in other languages are typically just
the *union* of appropriately-named directories across the *entire*
import or inclusion path.

In Perl, for example, the module ``Foo`` is always found in a
``Foo.pm`` file, and a module ``Foo::Bar`` is always found in a
``Foo/Bar.pm`` file.  (In other words, there is One Obvious Way to
find the location of a particular module.)

This is because Perl considers a module to be *different* from a
package: the package is purely a *namespace* in which other modules
may reside, and is only *coincidentally* the name of a module as well.

In current versions of Python, however, the module and the package are
more tightly bound together.  ``Foo`` is always a module -- whether it
is found in ``Foo.py`` or ``Foo/__init__.py`` -- and it is tightly
linked to its submodules (if any), which *must* reside in the exact
same directory where the ``__init__.py`` was found.

On the positive side, this design choice means that a package is quite
self-contained, and can be installed, copied, etc. as a unit just by
performing an operation on the package's root directory.

On the negative side, however, it is non-intuitive for beginners, and
requires a more complex step to turn a module into a package.  If
``Foo`` begins its life as ``Foo.py``, then it must be moved and
renamed to ``Foo/__init__.py``.

Conversely, if you intend to create a ``Foo.Bar`` module from the
start, but have no particular module contents to put in ``Foo``
itself, then you have to create an empty and seemingly-irrelevant
``Foo/__init__.py`` file, just so that ``Foo.Bar`` can be imported.

(And these issues don't just confuse newcomers to the language,
either: they annoy many experienced developers as well.)

So, after some discussion on the Import-SIG, this PEP was created
as an alternative to PEP \382, in an attempt to solve *all* of the
above problems, not just the "namespace package" use cases.

And, as a delightful side effect, the solution proposed in this PEP
does not affect the import performance of ordinary modules or
self-contained (i.e. ``__init__``-based) packages.


The Solution
============

In the past, various proposals have been made to allow more intuitive
approaches to package directory layout.  However, most of them failed
because of an apparent backward-compatibility problem.

That is, if the requirement for an ``__init__`` module were simply
dropped, it would open up the possibility for a directory named, say,
``string`` on ``sys.path``, to block importing of the standard library
``string`` module.

Paradoxically, however, the failure of this approach does *not* arise
from the elimination of the ``__init__`` requirement!

Rather, the failure arises because the underlying approach takes for
granted that a package is just ONE thing, instead of two.

In truth, a package comprises two separate, but related entities: a
module (with its own, optional contents), and a *namespace* where
*other* modules or packages can be found.

In current versions of Python, however, the module part (found in
``__init__``) and the namespace for submodule imports (represented
by the ``__path__`` attribute) are both initialized at the same time,
when the package is first imported.

And, if you assume this is the *only* way to initialize these two
things, then there is no way to drop the need for an ``__init__``
module, while still being backwards-compatible with existing directory
layouts.

After all, as soon as you encounter a directory on ``sys.path``
matching the desired name, that means you've "found" the package, and
must stop searching, right?

Well, not quite.


A Thought Experiment
--------------------

Let's hop into the time machine for a moment, and pretend we're back
in the early 1990s, shortly before Python packages and ``__init__.py``
have been invented.  But, imagine that we *are* familiar with
Perl-like package imports, and we want to implement a similar system
in Python.

We'd still have Python's *module* imports to build on, so we could
certainly conceive of having ``Foo.py`` as a parent ``Foo`` module
for a ``Foo`` package.  But how would we implement submodule and
subpackage imports?

Well, if we didn't have the idea of ``__path__`` attributes yet,
we'd probably just search ``sys.path`` looking for ``Foo/Bar.py``.

But we'd *only* do it when someone actually tried to *import*
``Foo.Bar``.

NOT when they imported ``Foo``.

And *that* lets us get rid of the backwards-compatibility problem
of dropping the ``__init__`` requirement, back here in 2011.

How?

Well, when we ``import Foo``, we're not even *looking* for ``Foo/``
directories on ``sys.path``, because we don't *care* yet.  The only
point at which we care, is the point when somebody tries to actually
import a submodule or subpackage of ``Foo``.

That means that if ``Foo`` is a standard library module (for example),
and I happen to have a ``Foo`` directory on ``sys.path`` (without
an ``__init__.py``, of course), then *nothing breaks*.  The ``Foo``
module is still just a module, and it's still imported normally.


Self-Contained vs. "Virtual" Packages
-------------------------------------

Of course, in today's Python, trying to ``import Foo.Bar`` will
fail if ``Foo`` is just a ``Foo.py`` module (and thus lacks a
``__path__`` attribute).

So, this PEP proposes to *dynamically* create a ``__path__``, in the
case where one is missing.

That is, if I try to ``import Foo.Bar`` the proposed change to the
import machinery will notice that the ``Foo`` module lacks a
``__path__``, and will therefore try to *build* one before proceeding.

And it will do this by making a list of all the existing ``Foo/``
subdirectories of the directories listed in ``sys.path``.

If the list is empty, the import will fail with ``ImportError``, just
like today.  But if the list is *not* empty, then it is saved in
a new ``Foo.__path__`` attribute, making the module a "virtual
package".

That is, because it now has a valid ``__path__``, we can proceed
to import submodules or subpackages in the normal way.

Now, notice that this change does not affect "classic", self-contained
packages that have an ``__init__`` module in them.  Such packages
already *have* a ``__path__`` attribute (initialized at import time)
so the import machinery won't try to create another one later.

This means that (for example) the standard library ``email`` package
will not be affected in any way by you having a bunch of unrelated
directories named ``email`` on ``sys.path``.  (Even if they contain
``*.py`` files.)

But it *does* mean that if you want to turn your ``Foo`` module into
a ``Foo`` package, all you have to do is add a ``Foo/`` directory
somewhere on ``sys.path``, and start adding modules to it.

But what if you only want a "namespace package"?  That is, a package
that is *only* a namespace for various separately-distributed
submodules and subpackages?

For example, if you're Zope Corporation, distributing dozens of
separate tools like ``zc.buildout``, each in packages under the ``zc``
namespace, you don't want to have to make and include an empty
``zc.py`` in every tool you ship.  (And, if you're a Linux or other
OS vendor, you don't want to deal with the package installation
conflicts created by trying to install ten copies of ``zc.py`` to the
same location!)

No problem.  All we have to do is make one more minor tweak to the
import process: if the "classic" import process fails to find a
self-contained module or package (e.g., if ``import zc`` fails to find
a ``zc.py`` or ``zc/__init__.py``), then we once more try to build a
``__path__`` by searching for all the ``zc/`` directories on
``sys.path``, and putting them in a list.

If this list is empty, we raise ``ImportError``.  But if it's
non-empty, we create an empty ``zc`` module, and put the list in
``zc.__path__``.  Congratulations: ``zc`` is now a namespace-only,
"pure virtual" package!  It has no module contents, but you can still
import submodules and subpackages from it, regardless of where they're
located on ``sys.path``.

(By the way, both of these additions to the import protocol (i.e. the
dynamically-added ``__path__``, and dynamically-created modules)
apply recursively to child packages, using the parent package's
``__path__`` in place of ``sys.path`` as a basis for generating a
child ``__path__``.  This means that self-contained and virtual
packages can contain each other without limitation, with the caveat
that if you put a virtual package inside a self-contained one, it's
gonna have a really short ``__path__``!)


Backwards Compatibility and Performance
---------------------------------------

Notice that these two changes *only* affect import operations that
today would result in ``ImportError``.  As a result, the performance
of imports that do not involve virtual packages is unaffected, and
potential backward compatibility issues are very restricted.

Today, if you try to import submodules or subpackages from a module
with no ``__path__``, it's an immediate error.  And of course, if you
don't have a ``zc.py`` or ``zc/__init__.py`` somewhere on ``sys.path``
today, ``import zc`` would likewise fail.

Thus, the only potential backwards-compatibility issues are:

1. Tools that expect package directories to have an ``__init__``
   module, that expect directories without an ``__init__`` module
   to be unimportable, or that expect ``__path__`` attributes to be
   static, will not recognize virtual packages as packages.

   (In practice, this just means that tools will need updating to
   support virtual packages, e.g. by using ``pkgutil.walk_modules()``
   instead of using hardcoded filesystem searches.)

2. Code that *expects* certain imports to fail may now do something
   unexpected.  This should be fairly rare in practice, as most sane,
   non-test code does not import things that are expected not to
   exist!

The biggest likely exception to the above would be when a piece of
code tries to check whether some package is installed by importing
it.  If this is done *only* by importing a top-level module (i.e., not
checking for a ``__version__`` or some other attribute), *and* there
is a directory of the same name as the sought-for package on
``sys.path`` somewhere, *and* the package is not actually installed,
then such code could be fooled into thinking a package is installed
that really isn't.

For example, suppose someone writes a script (``datagen.py``)
containing the following code::

    try:
        import json
    except ImportError:
        import simplejson as json

And runs it in a directory laid out like this::

    datagen.py
    json/
        foo.js
        bar.js

If ``import json`` succeeded due to the mere presence of the ``json/``
subdirectory, the code would incorrectly believe that the ``json``
module was available, and proceed to fail with an error.

However, we can prevent corner cases like these from arising, simply
by making one small change to the algorithm presented so far.  Instead
of allowing you to import a "pure virtual" package (like ``zc``),
we allow only importing of the *contents* of virtual packages.

That is, a statement like ``import zc`` should raise ``ImportError``
if there is no ``zc.py`` or ``zc/__init__.py`` on ``sys.path``.  But,
doing ``import zc.buildout`` should still succeed, as long as there's
a ``zc/buildout.py`` or ``zc/buildout/__init__.py`` on ``sys.path``.

In other words, we don't allow pure virtual packages to be imported
directly, only modules and self-contained packages.  (This is an
acceptable limitation, because there is no *functional* value to
importing such a package by itself.  After all, the module object
will have no *contents* until you import at least one of its
subpackages or submodules!)

Once ``zc.buildout`` has been successfully imported, though, there
*will* be a ``zc`` module in ``sys.modules``, and trying to import it
will of course succeed.  We are only preventing an *initial* import
from succeeding, in order to prevent false-positive import successes
when clashing subdirectories are present on ``sys.path``.

So, with this slight change, the ``datagen.py`` example above will
work correctly.  When it does ``import json``, the mere presence of a
``json/`` directory will simply not affect the import process at all,
even if it contains ``.py`` files.  The ``json/`` directory will still
only be searched in the case where an import like ``import
json.converter`` is attempted.

Meanwhile, tools that expect to locate packages and modules by
walking a directory tree can be updated to use the existing
``pkgutil.walk_modules()`` API, and tools that need to inspect
packages in memory should use the other APIs described in the
`Standard Library Changes/Additions`_ section below.


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

A change is made to the existing import process, when importing
names containing at least one ``.`` -- that is, imports of modules
that have a parent package.

Specifically, if the parent package does not exist, or exists but
lacks a ``__path__`` attribute, an attempt is first made to create a
"virtual path" for the parent package (following the algorithm
described in the section on `virtual paths`_, below).

If the computed "virtual path" is empty, an ``ImportError`` results,
just as it would today.  However, if a non-empty virtual path is
obtained, the normal import of the submodule or subpackage proceeds,
using that virtual path to find the submodule or subpackage.  (Just
as it would have with the parent's ``__path__``, if the parent package
had existed and had a ``__path__``.)

When a submodule or subpackage is found (but not yet loaded),
the parent package is created and added to ``sys.modules`` (if it
didn't exist before), and its ``__path__`` is set to the computed
virtual path (if it wasn't already set).

In this way, when the actual loading of the submodule or subpackage
occurs, it will see a parent package existing, and any relative
imports will work correctly.  However, if no submodule or subpackage
exists, then the parent package will *not* be created, nor will a
standalone module be converted into a package (by the addition of a
spurious ``__path__`` attribute).

Note, by the way, that this change must be applied *recursively*: that
is, if ``foo`` and ``foo.bar`` are pure virtual packages, then
``import foo.bar.baz`` must wait until ``foo.bar.baz`` is found before
creating module objects for *both* ``foo`` and ``foo.bar``, and then
create both of them together, properly setting the ``foo`` module's
``.bar`` attribute to point to the ``foo.bar`` module.

In this way, pure virtual packages are never directly importable:
an ``import foo`` or ``import foo.bar`` by itself will fail, and the
corresponding modules will not appear in ``sys.modules`` until they
are needed to point to a *successfully* imported submodule or
self-contained subpackage.


Virtual Paths
-------------

A virtual path is created by obtaining a PEP 302 "importer" object for
each of the path entries found in ``sys.path`` (for a top-level
module) or the parent ``__path__`` (for a submodule).

(Note: because ``sys.meta_path`` importers are not associated with
``sys.path`` or ``__path__`` entry strings, such importers do *not*
participate in this process.)

Each importer is checked for a ``get_subpath()`` method, and if
present, the method is called with the full name of the module/package
the path is being constructed for.  The return value is either a
string representing a subdirectory for the requested package, or
``None`` if no such subdirectory exists.

The strings returned by the importers are added to the path list
being built, in the same order as they are found.  (``None`` values
and missing ``get_subpath()`` methods are simply skipped.)

The resulting list (whether empty or not) is then stored in a
``sys.virtual_package_paths`` dictionary, keyed by module name.

This dictionary has two purposes.  First, it serves as a cache, in
the event that more than one attempt is made to import a submodule
of a virtual package.

Second, and more importantly, the dictionary can be used by code that
extends ``sys.path`` at runtime to *update* imported packages'
``__path__`` attributes accordingly.  (See `Standard Library
Changes/Additions`_ below for more details.)

In Python code, the virtual path construction algorithm would look
something like this::

    def get_virtual_path(modulename, parent_path=None):

        if modulename in sys.virtual_package_paths:
            return sys.virtual_package_paths[modulename]

        if parent_path is None:
            parent_path = sys.path

        path = []

        for entry in parent_path:
            # Obtain a PEP 302 importer object - see pkgutil module
            importer = pkgutil.get_importer(entry)

            if hasattr(importer, 'get_subpath'):
                subpath = importer.get_subpath(modulename)
                if subpath is not None:
                    path.append(subpath)

        sys.virtual_package_paths[modulename] = path
        return path

And a function like this one should be exposed in the standard
library as e.g. ``imp.get_virtual_path()``, so that people creating
``__import__`` replacements or ``sys.meta_path`` hooks can reuse it.


Standard Library Changes/Additions
----------------------------------

The ``pkgutil`` module should be updated to handle this
specification appropriately, including any necessary changes to
``extend_path()``, ``iter_modules()``, etc.

Specifically the proposed changes and additions to ``pkgutil`` are:

* A new ``extend_virtual_paths(path_entry)`` function, to extend
  existing, already-imported virtual packages' ``__path__`` attributes
  to include any portions found in a new ``sys.path`` entry.  This
  function should be called by applications extending ``sys.path``
  at runtime, e.g. when adding a plugin directory or an egg to the
  path.

  The implementation of this function does a simple top-down traversal
  of ``sys.virtual_package_paths``, and performs any necessary
  ``get_subpath()`` calls to identify what path entries need to be
  added to the virtual path for that package, given that `path_entry`
  has been added to ``sys.path``.  (Or, in the case of sub-packages,
  adding a derived subpath entry, based on their parent package's
  virtual path.)

  (Note: this function must update both the path values in
  ``sys.virtual_package_paths`` as well as the ``__path__`` attributes
  of any corresponding modules in ``sys.modules``, even though in the
  common case they will both be the same ``list`` object.)

* A new ``iter_virtual_packages(parent='')`` function to allow
  top-down traversal of virtual packages from
  ``sys.virtual_package_paths``, by yielding the child virtual
  packages of `parent`.  For example, calling
  ``iter_virtual_packages("zope")`` might yield ``zope.app``
  and ``zope.products`` (if they are virtual packages listed in
  ``sys.virtual_package_paths``), but **not** ``zope.foo.bar``.
  (This function is needed to implement ``extend_virtual_paths()``,
  but is also potentially useful for other code that needs to inspect
  imported virtual packages.)

* ``ImpImporter.iter_modules()`` should be changed to also detect and
  yield the names of modules found in virtual packages.

In addition to the above changes, the ``zipimport`` importer should
have its ``iter_modules()`` implementation similarly changed.  (Note:
current versions of Python implement this via a shim in ``pkgutil``,
so technically this is also a change to ``pkgutil``.)

Last, but not least, the ``imp`` module (or ``importlib``, if
appropriate) should expose the algorithm described in the `virtual
paths`_ section above, as a
``get_virtual_path(modulename, parent_path=None)`` function, so that
creators of ``__import__`` replacements can use it.


Implementation Notes
--------------------

For users, developers, and distributors of virtual packages:

* While virtual packages are easy to set up and use, there is still
  a time and place for using self-contained packages.  While it's not
  strictly necessary, adding an ``__init__`` module to your
  self-contained packages lets users of the package (and Python
  itself) know that *all* of the package's code will be found in
  that single subdirectory.  In addition, it lets you define
  ``__all__``, expose a public API, provide a package-level docstring,
  and do other things that make more sense for a self-contained
  project than for a mere "namespace" package.

* ``sys.virtual_package_paths`` is allowed to contain entries for
  non-existent or not-yet-imported package names; code that uses its
  contents should not assume that every key in this dictionary is also
  present in ``sys.modules`` or that importing the name will
  necessarily succeed.

* If you are changing a currently self-contained package into a
  virtual one, it's important to note that you can no longer use its
  ``__file__`` attribute to locate data files stored in a package
  directory.  Instead, you must search ``__path__`` or use the
  ``__file__`` of a submodule adjacent to the desired files, or
  of a self-contained subpackage that contains the desired files.

  (Note: this caveat is already true for existing users of "namespace
  packages" today.  That is, it is an inherent result of being able
  to partition a package, that you must know *which* partition the
  desired data file lives in.  We mention it here simply so that
  *new* users converting from self-contained to virtual packages will
  also be aware of it.)

* XXX what is the __file__ of a "pure virtual" package?  ``None``?
  Some arbitrary string?  The path of the first directory with a
  trailing separator?  No matter what we put, *some* code is
  going to break, but the last choice might allow some code to
  accidentally work.  Is that good or bad?


For those implementing PEP 302 importer objects:

* Importers that support the ``iter_modules()`` method (used by
  ``pkgutil`` to locate importable modules and packages) and want to
  add virtual package support should modify their ``iter_modules()``
  method so that it discovers and lists virtual packages as well as
  standard modules and packages.  To do this, the importer should
  simply list all immediate subdirectory names in its jurisdiction
  that are valid Python identifiers.

  XXX This might list a lot of not-really-packages.  Should we
  require importable contents to exist?  If so, how deep do we
  search, and how do we prevent e.g. link loops, or traversing onto
  different filesystems, etc.?  Ick.  Also, if virtual packages are
  listed, they still can't be *imported*, which is a problem for the
  way that ``pkgutil.walk_modules()`` is currently implemented.

* "Meta" importers (i.e., importers placed on ``sys.meta_path``) do
  not need to implement ``get_subpath()``, because the method
  is only called on importers corresponding to ``sys.path`` entries
  and ``__path__`` entries.  If a meta importer wishes to support
  virtual packages, it must do so entirely within its own
  ``find_module()`` implementation.

  Unfortunately, it is unlikely that any such implementation will be
  able to merge its package subpaths with those of other meta
  importers or ``sys.path`` importers, so the meaning of "supporting
  virtual packages" for a meta importer is currently undefined!

  (However, since the intended use case for meta importers is to
  replace Python's normal import process entirely for some subset of
  modules, and the number of such importers currently implemented is
  quite small, this seems unlikely to be a big issue in practice.)


References
==========

.. [1] "namespace" vs "module" packages (mailing list thread)
   (http://mail.zope.org/pipermail/zope3-dev/2002-December/004251.html)

.. [2] "Dropping __init__.py requirement for subpackages"
   (http://mail.python.org/pipermail/python-dev/2006-April/064400.html)

.. [3] Namespace Packages resolution
       (http://mail.python.org/pipermail/import-sig/2012-March/000421.html)


Copyright
=========

This document has been placed in the public domain.


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