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PEP 382: Namespace Packages

 
 
Martin v. Lwis
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Posts: n/a
 
      04-02-2009
I propose the following PEP for inclusion to Python 3.1.
Please comment.

Regards,
Martin

Abstract
========

Namespace packages are a mechanism for splitting a single Python
package across multiple directories on disk. In current Python
versions, an algorithm to compute the packages __path__ must be
formulated. With the enhancement proposed here, the import machinery
itself will construct the list of directories that make up the
package.

Terminology
===========

Within this PEP, the term package refers to Python packages as defined
by Python's import statement. The term distribution refers to
separately installable sets of Python modules as stored in the Python
package index, and installed by distutils or setuptools. The term
vendor package refers to groups of files installed by an operating
system's packaging mechanism (e.g. Debian or Redhat packages install
on Linux systems).

The term portion refers to a set of files in a single directory (possibly
stored in a zip file) that contribute to a namespace package.

Namespace packages today
========================

Python currently provides the pkgutil.extend_path to denote a package as
a namespace package. The recommended way of using it is to put::

from pkgutil import extend_path
__path__ = extend_path(__path__, __name__)

int the package's ``__init__.py``. Every distribution needs to provide
the same contents in its ``__init__.py``, so that extend_path is
invoked independent of which portion of the package gets imported
first. As a consequence, the package's ``__init__.py`` cannot
practically define any names as it depends on the order of the package
fragments on sys.path which portion is imported first. As a special
feature, extend_path reads files named ``*.pkg`` which allow to
declare additional portions.

setuptools provides a similar function pkg_resources.declare_namespace
that is used in the form::

import pkg_resources
pkg_resources.declare_namespace(__name__)

In the portion's __init__.py, no assignment to __path__ is necessary,
as declare_namespace modifies the package __path__ through sys.modules.
As a special feature, declare_namespace also supports zip files, and
registers the package name internally so that future additions to sys.path
by setuptools can properly add additional portions to each package.

setuptools allows declaring namespace packages in a distribution's
setup.py, so that distribution developers don't need to put the
magic __path__ modification into __init__.py themselves.

Rationale
=========

The current imperative approach to namespace packages has lead to
multiple slightly-incompatible mechanisms for providing namespace
packages. For example, pkgutil supports ``*.pkg`` files; setuptools
doesn't. Likewise, setuptools supports inspecting zip files, and
supports adding portions to its _namespace_packages variable, whereas
pkgutil doesn't.

In addition, the current approach causes problems for system vendors.
Vendor packages typically must not provide overlapping files, and an
attempt to install a vendor package that has a file already on disk
will fail or cause unpredictable behavior. As vendors might chose to
package distributions such that they will end up all in a single
directory for the namespace package, all portions would contribute
conflicting __init__.py files.

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

Rather than using an imperative mechanism for importing packages, a
declarative approach is proposed here, as an extension to the existing
``*.pkg`` mechanism.

The import statement is extended so that it directly considers ``*.pkg``
files during import; a directory is considered a package if it either
contains a file named __init__.py, or a file whose name ends with
".pkg".

In addition, the format of the ``*.pkg`` file is extended: a line with
the single character ``*`` indicates that the entire sys.path will
be searched for portions of the namespace package at the time the
namespace packages is imported.

Importing a package will immediately compute the package's __path__;
the ``*.pkg`` files are not considered anymore after the initial import.
If a ``*.pkg`` package contains an asterisk, this asterisk is prepended
to the package's __path__ to indicate that the package is a namespace
package (and that thus further extensions to sys.path might also
want to extend __path__). At most one such asterisk gets prepended
to the path.

extend_path will be extended to recognize namespace packages according
to this PEP, and avoid adding directories twice to __path__.

No other change to the importing mechanism is made; searching
modules (including __init__.py) will continue to stop at the first
module encountered.

Discussion
==========

With the addition of ``*.pkg`` files to the import mechanism, namespace
packages can stop filling out the namespace package's __init__.py.
As a consequence, extend_path and declare_namespace become obsolete.

It is recommended that distributions put a file <distribution>.pkg
into their namespace packages, with a single asterisk. This allows
vendor packages to install multiple portions of namespace package
into a single directory, with no risk of overlapping files.

Namespace packages can start providing non-trivial __init__.py
implementations; to do so, it is recommended that a single distribution
provides a portion with just the namespace package's __init__.py
(and potentially other modules that belong to the namespace package
proper).

The mechanism is mostly compatible with the existing namespace
mechanisms. extend_path will be adjusted to this specification;
any other mechanism might cause portions to get added twice to
__path__.

Copyright
=========

This document has been placed in the public domain.
 
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Carl Banks
Guest
Posts: n/a
 
      04-02-2009
On Apr 2, 8:32*am, "Martin v. Lwis" <(E-Mail Removed)> wrote:
> I propose the following PEP for inclusion to Python 3.1.
> Please comment.
>
> Regards,
> Martin
>
> Abstract
> ========
>
> Namespace packages are a mechanism for splitting a single Python
> package across multiple directories on disk. In current Python
> versions, an algorithm to compute the packages __path__ must be
> formulated. With the enhancement proposed here, the import machinery
> itself will construct the list of directories that make up the
> package.


-0

My main concern is that we'll start seeing all kinds of packages with
names like:

com.dusinc.sarray.ptookkit.v_1_34_beta.btree.BTree

The current lack of global package namespace effectively prevents
bureaucratic package naming, which in my mind makes it worth the
cost. However, I'd be willing to believe this can be kept under
control some other way.


Carl Banks
 
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Kay Schluehr
Guest
Posts: n/a
 
      04-02-2009
On 2 Apr., 17:32, "Martin v. Lwis" <(E-Mail Removed)> wrote:
> I propose the following PEP for inclusion to Python 3.1.
> Please comment.
>
> Regards,
> Martin
>
> Abstract
> ========
>
> Namespace packages are a mechanism for splitting a single Python
> package across multiple directories on disk. In current Python
> versions, an algorithm to compute the packages __path__ must be
> formulated. With the enhancement proposed here, the import machinery
> itself will construct the list of directories that make up the
> package.
>
> Terminology
> ===========
>
> Within this PEP, the term package refers to Python packages as defined
> by Python's import statement. The term distribution refers to
> separately installable sets of Python modules as stored in the Python
> package index, and installed by distutils or setuptools. The term
> vendor package refers to groups of files installed by an operating
> system's packaging mechanism (e.g. Debian or Redhat packages install
> on Linux systems).
>
> The term portion refers to a set of files in a single directory (possibly
> stored in a zip file) that contribute to a namespace package.
>
> Namespace packages today
> ========================
>
> Python currently provides the pkgutil.extend_path to denote a package as
> a namespace package. The recommended way of using it is to put::
>
> from pkgutil import extend_path
> __path__ = extend_path(__path__, __name__)
>
> int the package's ``__init__.py``. Every distribution needs to provide
> the same contents in its ``__init__.py``, so that extend_path is
> invoked independent of which portion of the package gets imported
> first. As a consequence, the package's ``__init__.py`` cannot
> practically define any names as it depends on the order of the package
> fragments on sys.path which portion is imported first. As a special
> feature, extend_path reads files named ``*.pkg`` which allow to
> declare additional portions.
>
> setuptools provides a similar function pkg_resources.declare_namespace
> that is used in the form::
>
> import pkg_resources
> pkg_resources.declare_namespace(__name__)
>
> In the portion's __init__.py, no assignment to __path__ is necessary,
> as declare_namespace modifies the package __path__ through sys.modules.
> As a special feature, declare_namespace also supports zip files, and
> registers the package name internally so that future additions to sys.path
> by setuptools can properly add additional portions to each package.
>
> setuptools allows declaring namespace packages in a distribution's
> setup.py, so that distribution developers don't need to put the
> magic __path__ modification into __init__.py themselves.
>
> Rationale
> =========
>
> The current imperative approach to namespace packages has lead to
> multiple slightly-incompatible mechanisms for providing namespace
> packages. For example, pkgutil supports ``*.pkg`` files; setuptools
> doesn't. Likewise, setuptools supports inspecting zip files, and
> supports adding portions to its _namespace_packages variable, whereas
> pkgutil doesn't.
>
> In addition, the current approach causes problems for system vendors.
> Vendor packages typically must not provide overlapping files, and an
> attempt to install a vendor package that has a file already on disk
> will fail or cause unpredictable behavior. As vendors might chose to
> package distributions such that they will end up all in a single
> directory for the namespace package, all portions would contribute
> conflicting __init__.py files.
>
> Specification
> =============
>
> Rather than using an imperative mechanism for importing packages, a
> declarative approach is proposed here, as an extension to the existing
> ``*.pkg`` mechanism.
>
> The import statement is extended so that it directly considers ``*.pkg``
> files during import; a directory is considered a package if it either
> contains a file named __init__.py, or a file whose name ends with
> ".pkg".
>
> In addition, the format of the ``*.pkg`` file is extended: a line with
> the single character ``*`` indicates that the entire sys.path will
> be searched for portions of the namespace package at the time the
> namespace packages is imported.
>
> Importing a package will immediately compute the package's __path__;
> the ``*.pkg`` files are not considered anymore after the initial import.
> If a ``*.pkg`` package contains an asterisk, this asterisk is prepended
> to the package's __path__ to indicate that the package is a namespace
> package (and that thus further extensions to sys.path might also
> want to extend __path__). At most one such asterisk gets prepended
> to the path.
>
> extend_path will be extended to recognize namespace packages according
> to this PEP, and avoid adding directories twice to __path__.
>
> No other change to the importing mechanism is made; searching
> modules (including __init__.py) will continue to stop at the first
> module encountered.
>
> Discussion
> ==========
>
> With the addition of ``*.pkg`` files to the import mechanism, namespace
> packages can stop filling out the namespace package's __init__.py.
> As a consequence, extend_path and declare_namespace become obsolete.
>
> It is recommended that distributions put a file <distribution>.pkg
> into their namespace packages, with a single asterisk. This allows
> vendor packages to install multiple portions of namespace package
> into a single directory, with no risk of overlapping files.
>
> Namespace packages can start providing non-trivial __init__.py
> implementations; to do so, it is recommended that a single distribution
> provides a portion with just the namespace package's __init__.py
> (and potentially other modules that belong to the namespace package
> proper).
>
> The mechanism is mostly compatible with the existing namespace
> mechanisms. extend_path will be adjusted to this specification;
> any other mechanism might cause portions to get added twice to
> __path__.
>
> Copyright
> =========
>
> This document has been placed in the public domain.


Wow. You python-dev guys are really jumping the shark. Isn't your Rube
Goldberg "import machinery" already complex enough for you?
 
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Chris Rebert
Guest
Posts: n/a
 
      04-02-2009
On Thu, Apr 2, 2009 at 11:38 AM, Carl Banks <(E-Mail Removed)> wrote:
> On Apr 2, 8:32*am, "Martin v. Löwis" <(E-Mail Removed)> wrote:
>> I propose the following PEP for inclusion to Python 3.1.
>> Please comment.
>>
>> Regards,
>> Martin
>>
>> Abstract
>> ========
>>
>> Namespace packages are a mechanism for splitting a single Python
>> package across multiple directories on disk. In current Python
>> versions, an algorithm to compute the packages __path__ must be
>> formulated. With the enhancement proposed here, the import machinery
>> itself will construct the list of directories that make up the
>> package.

>
> -0
>
> My main concern is that we'll start seeing all kinds of packages with
> names like:
>
> com.dusinc.sarray.ptookkit.v_1_34_beta.btree.BTree
>
> The current lack of global package namespace effectively prevents
> bureaucratic package naming, which in my mind makes it worth the
> cost. *However, I'd be willing to believe this can be kept under
> control some other way.


Agreed, although I'd be slightly less optimistic on its usage being
kept under control. It seems this goes a bit against the "Flat is
better than nested" principle.
Then again, we also have the "Namespaces are honkingly great"
principle to contend with as well, so it's definitely a balancing act.

Cheers,
Chris

--
I have a blog:
http://blog.rebertia.com
 
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drobinow@gmail.com
Guest
Posts: n/a
 
      04-02-2009
On Apr 2, 5:59*pm, Ben Finney <(E-Mail Removed)> wrote:
> Kay Schluehr <(E-Mail Removed)> writes:
> > Wow. You python-dev guys are really jumping the shark. Isn't your
> > Rube Goldberg "import machinery" already complex enough for you?

>
> Thanks for your constructive criticism, and your considerate quote
> trimming.

Ben, you should use google groups. No trimming necessary.
 
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Martin v. Lwis
Guest
Posts: n/a
 
      04-04-2009
> -0
>
> My main concern is that we'll start seeing all kinds of packages with
> names like:
>
> com.dusinc.sarray.ptookkit.v_1_34_beta.btree.BTree
>
> The current lack of global package namespace effectively prevents
> bureaucratic package naming, which in my mind makes it worth the
> cost. However, I'd be willing to believe this can be kept under
> control some other way.


In principle, people can do this today already. That they are not
doing it is a good sign.

I think this bureaucratic naming in Java originates more from an
explicitly stated policy that people should use such naming than
from the ability to actually do so easily.

Regards,
Martin
 
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