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Visions and ideas for further development of the py lib
=======================================================

.. contents::
.. sectnum::

This document tries to describe directions and guiding ideas
for the near-future development of the py lib.  *Note that all
statements within this document - even if they sound factual -
mostly just express thoughts and ideas. They not always refer to 
real code so read with some caution.  This is not a reference guide
(tm). Moreover, the order in which appear here in the file does 
not reflect the order in which they may be implemented.* 

.. _`general-path`: 
.. _`a more general view on path objects`:

A more general view on ``py.path`` objects 
==========================================

Seen from a more general persective, the current ``py.path.extpy`` path 
offers a way to go from a file to the structured content of 
a file, namely a python object.  The ``extpy`` path retains some
common ``path`` operations and semantics but offers additional
methods, e.g. ``resolve()`` gets you a true python object.   

But apart from python files there are many other examples 
of structured content like xml documents or INI-style 
config files.  While some tasks will only be convenient 
to perform in a domain specific manner (e.g. applying xslt 
etc.pp) ``py.path`` offers a common behaviour for 
structured content paths. So far only ``py.path.extpy``
is implemented and used by py.test to address tests 
and traverse into test files. 

*You are in a maze of twisty passages, all alike*
-------------------------------------------------

Now, for the sake of finding out a good direction, 
let's consider some code that wants to find all 
*sections* which have a certain *option* value
within some given ``startpath``:: 

    def find_option(startpath, optionname): 
        for section in startpath.listdir(dir=1): 
            opt = section.join(optionname) 
            if opt.check(): # does the option exist here? 
                print section.basename, "found:", opt.read() 

Now the point is that ``find_option()`` would obviously work
when ``startpath`` is a filesystem-like path like a local
filesystem path or a subversion URL path. It would then see
directories as sections and files as option-names and the
content of the file as values. 

But it also works (today) for ``extpy`` paths if you put the following
python code in a file:: 

    class Section1:
        someoption = "i am an option value" 

    class Section2:
        someoption = "i am another option value" 

An ``extpy()`` path maps classes and modules to directories and 
name-value bindings to file/read() operations. 

And it could also work for 'xml' paths if you put
the following xml string in a file:: 

    <xml ...>
    <root>
        <section1>      
            <someoption>value</name></section1>
        <section2>
            <someoption>value</name></section2></root>

where tags containing non-text tags map to directories 
and tags with just text-children map to files (which
upon read() return the joined content of the text 
tags possibly as unicode. 

Now, to complete the picture, we could make Config-Parser 
*ini-style* config files also available::

    [section1]
    name = value 
    
    [section2]
    othername = value

where sections map to directories and name=value mappings
to file/contents. 

So it seems that our above ``find_option()`` function would
work nicely on all these *mappings*. 

Of course, the somewhat open question is how to make the
transition from a filesystem path to structured content
useful and unified, as much as possible without overdoing it. 

Again, there are tasks that will need fully domain specific
solutions (DOM/XSLT/...) but i think the above view warrants
some experiments and refactoring.  The degree of uniformity 
still needs to be determined and thought about. 

path objects should be stackable
--------------------------------
 
Oh, and btw, a ``py.path.extpy`` file could live on top of a 
'py.path.xml' path as well, i.e. take::

    <xml ...>
    <code>
        <py>      
            <magic>
                <assertion>
                    import py 
                    ... </assertion>
                <exprinfo> 
                    def getmsg(x): pass </exprino></magic></py></code>

and use it to have a ``extpy`` path living on it::

    p = py.path.local(xmlfilename)
    xmlp = py.path.extxml(p, 'py/magic/exprinfo')
    p = py.path.extpy(xmlp, 'getmsg')
  
    assert p.check(func=1, basename='getmsg') 
    getmsg = p.resolve() 
    # we now have a *live* getmsg() function taken and compiled from 
    # the above xml fragment

There could be generic converters which convert between 
different content formats ... allowing configuration files to e.g. 
be in XML/Ini/python or filesystem-format with some common way 
to find and iterate values. 

*After all the unix filesystem and the python namespaces are 
two honking great ideas, why not do more of them? :-)*


.. _importexport: 

Revising and improving the import/export system 
===============================================

    or let's wrap the world all around 

the export/import interface 
---------------------------

The py lib already incorporates a mechanism to select which
namespaces and names get exposed to a user of the library.
Apart from reducing the outside visible namespaces complexity 
this allows to quickly rename and refactor stuff in the
implementation without affecting the caller side.  This export
control can be used by other python packages as well. 

However, all is not fine as the import/export has a 
few major deficiencies and shortcomings:

- it doesn't allow to specify doc-strings 
- it is a bit hackish (see py/initpkg.py)
- it doesn't present a complete and consistent view of the API. 
- ``help(constructed_namespace)`` doesn't work for the root 
  package namespace
- when the py lib implementation accesses parts of itself 
  it uses the native python import mechanism which is 
  limiting in some respects.  Especially for distributed
  programs as encouraged by `py.execnet`_ it is not clear
  how the mechanism can nicely integrate to support remote
  lazy importing. 

Discussions have been going on for a while but it is
still not clear how to best tackle the problem.  Personally, 
i believe the main missing thing for the first major release 
is the docstring one.   The current specification 
of exported names is dictionary based.  It would be 
better to declare it in terms of Objects. 


Example sketch for a new export specification 
---------------------------------------------

Here is a sketch of how the py libs ``__init__.py`` file 
might or should look like:: 

    """
        the py lib version 0.8
        http://codespeak.net/py/0.8
    """

    from py import pkg
    pkg.export(__name__,
        pkg.Module('path',
            '''provides path objects for local filesystem, 
               subversion url and working copy, and extension paths.
            ''',
            pkg.Class('local', '''
               the local filesystem path offering a single
               point of interaction for many purposes.
               ''', extpy='./path/local.LocalPath'),

            pkg.Class('svnurl', '''
               the subversion url path.
            ''', extpy='./path/local/svn/urlcommand.SvnUrlPath'),
        ),
    # it goes on ... 
    )

The current ``initpkg.py`` code can be cleaned up to support
this new more explicit style of stating things. Note that
in principle there is nothing that stops us from retrieving
implementations over the network, e.g. a subversion repository. 


Let there be alternatives 
-------------------------

We could also specify alternative implementations easily::

    pkg.Class('svnwc', '''
       the subversion working copy.
    ''', extpy=('./path/local/svn/urlbinding.SvnUrlPath', 
                './path/local/svn/urlcommand.SvnUrlPath',)
    )

This would prefer the python binding based implementation over
the one working through he 'svn' command line utility.  And
of course, it could uniformly signal if no implementation is 
available at all. 


Problems problems  
-----------------

Now there are reasons there isn't a clear conclusion so far. 
For example, the above approach has some implications, the
main one being that implementation classes like
``py/path/local.LocalPath`` are visible to the caller side but
this presents an inconsistency because the user started out with
``py.path.local`` and expects that the two classes are really much
the same.  We have the same problem today, of course. 

The naive solution strategy of wrapping the "implementation
level" objects into their exported representations may remind
of the `wrapping techniques PyPy uses`_.  But it
*may* result in a slightly heavyweight mechanism that affects
runtime speed.  However, I guess that this standard strategy
is probably the cleanest. 


Every problem can be solved with another level ... 
--------------------------------------------------

The wrapping of implementation level classes in their export
representations objects adds another level of indirection.
But this indirection would have interesting advantages: 

- we could easily present a consistent view of the library 
- it could take care of exceptions as well 
- it provides natural interception points for logging 
- it enables remote lazy loading of implementations 
  or certain versions of interfaces 

And quite likely the extra indirection wouldn't hurt so much
as it is not much more than a function call and we cared
we could even generate some c-code (with PyPy :-) to speed
it up.   

But it can lead to new problems ...
-----------------------------------

However, it is critical to avoid to burden the implementation
code of being aware of its wrapping.  This is what we have 
to do in PyPy but the import/export mechanism works at 
a higher level of the language, i think.  

Oh, and we didn't talk about bootstrapping :-) 

.. _`py.execnet`: ../execnet.html 
.. _`wrapping techniques PyPy uses`: http://codespeak.net/pypy/index.cgi?doc/wrapping.html
.. _`lightweight xml generation`: 

Extension of py.path.local.sysexec()
====================================

The `sysexec mechanism`_ allows to directly execute 
binaries on your system.  Especially after we'll have this
nicely integrated into Win32 we may also want to run python 
scripts both locally and from the net::

    vadm = py.path.svnurl('http://codespeak.net/svn/vadm/dist/vadm/cmdline.py') 
    stdoutput = vadm.execute('diff')

To be able to execute this code fragement, we need either or all of 

- an improved import system that allows remote imports 

- a way to specify what the "neccessary" python import
  directories are. for example, the above scriptlet will
  require a certain root included in the python search for module 
  in order to execute something like "import vadm". 

- a way to specify dependencies ... which opens up another
  interesting can of worms, suitable for another chapter
  in the neverending `future book`_. 

.. _`sysexec mechanism`: ../misc.html#sysexec
.. _`compile-on-the-fly`: 

we need a persistent storage for the py lib 
-------------------------------------------

A somewhat open question is where to store the underlying
generated pyc-files and other files generated on the fly 
with `CPython's distutils`_.  We want to have a 
*persistent location* in order to avoid runtime-penalties
when switching python versions and platforms (think NFS). 

A *persistent location* for the py lib would be a good idea
maybe also for other reasons. We could cache some of the
expensive test setups, like the multi-revision subversion
repository that is created for each run of the tests. 

.. _`CPython's distutils`: http://www.python.org/dev/doc/devel/lib/module-distutils.html

.. _`getting started`: ../getting-started.html 
.. _`restructured text`: http://docutils.sourceforge.net/docs/user/rst/quickref.html
.. _`python standard library`: http://www.python.org/doc/2.3.4/lib/lib.html
.. _`xpython EuroPython 2004 talk`: http://codespeak.net/svn/user/hpk/talks/xpython-talk.txt
.. _`under the xpy tree`: http://codespeak.net/svn/user/hpk/xpy/xml.py
.. _`future book`: future.html 
.. _`PEP-324 subprocess module`: http://www.python.org/peps/pep-0324.html
.. _`subprocess implementation`: http://www.lysator.liu.se/~astrand/popen5/
.. _`py.test`: ../test.html

Refactor path implementations to use a Filesystem Abstraction 
============================================================= 

It seems like a good idea to refactor all python implementations to
use an internal Filesystem abstraction.  The current code base
would be transformed to have Filesystem implementations for e.g. 
local, subversion and subversion "working copy" filesystems. Today 
the according code is scattered through path-handling code. 

On a related note, Armin Rigo has hacked `pylufs`_ which allows to 
implement kernel-level linux filesystems with pure python. Now 
the idea is that the mentioned filesystem implementations would 
be directly usable for such linux-filesystem glue code. 

In other words, implementing a `memoryfs`_ or a `dictfs`_ would 
give you two things for free: a filesystem mountable at kernel level
as well as a uniform "path" object allowing you to access your
filesystem in convenient ways.  (At some point it might
even become interesting to think about interfacing to  
`reiserfs v4 features`_ at the Filesystem level but that
is a can of subsequent worms).  

.. _`memoryfs`: http://codespeak.net/svn/user/arigo/hack/pyfuse/memoryfs.py
.. _`dictfs`: http://codespeak.net/pipermail/py-dev/2005-January/000191.html 
.. _`pylufs`: http://codespeak.net/svn/user/arigo/hack/pylufs/
.. _`reiserfs v4 features`: http://www.namesys.com/v4/v4.html


Improve and unify Path API 
==========================

visit() grows depth control 
--------------------------- 

Add a ``maxdepth`` argument to the path.visit() method, 
which will limit traversal to subdirectories. Example:: 

    x = py.path.local.get_tmproot()
    for x in p.visit('bin', stop=N): 
        ... 

This will yield all file or directory paths whose basename
is 'bin', depending on the values of ``stop``:: 

    p                       # stop == 0 or higher (and p.basename == 'bin')
    p / bin                 # stop == 1 or higher
    p / ... / bin           # stop == 2 or higher
    p / ... / ... / bin     # stop == 3 or higher

The default for stop would be `255`. 

But what if `stop < 0`?  We could let that mean to go upwards:: 

    for x in x.visit('py/bin', stop=-255): 
        # will yield all parent direcotires which have a 
        # py/bin subpath 

visit() returning a lazy list? 
------------------------------ 

There is a very nice "no-API" `lazy list`_ implementation from 
Armin Rigo which presents a complete list interface, given some 
iterable.  The iterable is consumed only on demand and retains 
memory efficiency as much as possible.  The lazy list 
provides a number of advantages in addition to the fact that
a list interface is nicer to deal with than an iterator. 
For example it lets you do:: 

    for x in p1.visit('*.cfg') + p2.visit('*.cfg'): 
        # will iterate through all results 

Here the for-iter expression will retain all lazyness (with
the result of adding lazy lists being another another lazy
list) by internally concatenating the underlying
lazylists/iterators.  Moreover, the lazylist implementation
will know that there are no references left to the lazy list
and throw away iterated elements.  This makes the iteration
over the sum of the two visit()s as efficient as if we had 
used iterables to begin with! 

For this, we would like to move the lazy list into the 
py lib's namespace, most probably at `py.builtin.lazylist`. 

.. _`lazy list`: http://codespeak.net/svn/user/arigo/hack/misc/collect.py