====================================
Miscellaneous features of the py lib 
====================================

.. contents::
.. sectnum::

Mapping the standard python library into py 
===========================================

    Warning: This feature is very young and thus experimental.
    Be prepared to adapt your code later if you use it. 

After you have worked with the py lib a bit, you might enjoy
the lazy importing, i.e. you only have to do ``import py`` and
work your way to your desired object.  Using the full path 
also ensures that there remains a focus on getting short paths 
to objects. 

The :api:`py.std` hook
----------------------

Of course, no matter what, everybody will continue to use the
python standard library because it is a very usable code base.
However, to properly support lazyness the py lib offers a way
to get to many standard modules without requiring "import"
statements.  For example, to get to the print-exception
functionality of the standard library you can write:: 

    py.std.traceback.print_exc()

without having to do anything else than the usual ``import py`` 
at the beginning.  Note that not having imports for the 
`python standard library` obviously gets rid of the *unused 
import* problem. Modules only get imported when you actually
need them. 

Moreover, this approach resolves some of the issues stated in
`the relative/absolute import PEP-328`_, as with the above
approach you never have ambiguity problems.  The above
traceback-usage is an absolute path that will not be
accidentally get confused with local names.  (Well, never put
a file ``py.py`` in an importable path, btw, mind you :-) 

Automagically accessing sub packages doesn't work (yet?) 
--------------------------------------------------------

If you use the :api:`py.std` hook you currently cannot magically
import nested packages which otherwise need explicit imports of
their sub-packages.  For example, the suversion bindings
require you to do something like:: 

    import svn.client 

If you just do the naive thing with the py lib, i.e. write
``py.std.svn.client`` it will not work unless you previously
imported it already.  The py lib currently doesn't try to
magically make this work.  The :api:`py.std` hook really is
intended for Python standard modules which very seldomly (if
at all) provide such nested packages. 

**Note that you may never rely** on module identity, i.e. 
that ``X is py.std.X`` for any ``X``. This is to allow
us later to lazyly import nested packages. Yes, lazyness
is hard to resist :-) 

Note: you get an AttributeError, not an ImportError
---------------------------------------------------

If you say ``py.std.XYZ`` and importing ``XYZ`` produces an
``ImportError`` , it will actually show up as an
``AttributeError``. It is deemed more important to adhere to
the standard ``__getattr__`` protocol than to let the
``ImportError`` pass through.  For example, you might want to
do::

    getattr(py.std.cStringIO, 'StringIO', py.std.StringIO.StringIO) 

and you would expect that it works. It does work although it will 
take away some lazyness because ``py.std.StringIO.StringIO`` will
be imported in any case. 

.. _`the relative/absolute import PEP-328`: http://www.python.org/peps/pep-0328.html

Support for interaction with system utilities/binaries
======================================================

sources:

  * :source:`py/process/`
  * :source:`py/path/local/`

Currently, the py lib offers two ways to interact with
system executables. :api:`py.process.cmdexec()` invokes
the shell in order to execute a string.  The other
one, :api:`py.path.local`'s 'sysexec()' method lets you 
directly execute a binary. 

Both approaches will raise an exception in case of a return-
code other than 0 and otherwise return the stdout-output 
of the child process.

The shell based approach 
------------------------

You can execute a command via your system shell 
by doing something like:: 

    out = py.process.cmdexec('ls -v')

However, the ``cmdexec`` approach has a few shortcomings: 

- it relies on the underlying system shell
- it neccessitates shell-escaping for expressing arguments
- it does not easily allow to "fix" the binary you want to run.  
- it only allows to execute executables from the local 
  filesystem 

.. _sysexec: 

local paths have ``sysexec``
---------------------------- 

The py lib currently offers a stripped down functionality of what
the new `PEP-324 subprocess module`_ offers.  The main functionality 
of synchronously executing a system executable has a straightforward API:: 

    binsvn.sysexec('ls', 'http://codespeak.net/svn') 

where ``binsvn`` is a path that points to the ``svn`` commandline
binary. Note that this function would not offer any shell-escaping
so you really have to pass in separated arguments.  This idea
fits nicely into `a more general view on path objects`_. 

For a first go, we are just reusing the existing `subprocess
implementation`_ but don't expose any of its API apart
from the above ``sysexec()`` method. 

Note, however, that currently the support for the ``sysexec`` interface on
win32 is not thoroughly tested. If you run into problems with it, we are
interested to hear about them. If you are running a Python older than 2.4 you
will have to install the `pywin32 package`_.


.. _`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/
.. _`a more general view on path objects`: future.html#general-path
.. _`pywin32 package`: http://pywin32.sourceforge.net/

finding an executable local path
--------------------------------

Finding an executable is quite different on multiple platforms. 
Currently, the ``PATH`` environment variable based search on
unix platforms is supported:: 

    py.path.local.sysfind('svn')

which returns the first path whose ``basename`` matches ``svn``. 
In principle, `sysfind` deploys platform specific algorithms
to perform the search.  On Windows, for example, it may look
at the registry (XXX). 

To make the story complete, we allow to pass in a second ``checker`` 
argument that is called for each found executable.  For example, if 
you have multiple binaries available you may want to select the
right version:: 

    def mysvn(p):
        """ check that the given svn binary has version 1.1. """
        line = p.execute('--version'').readlines()[0]
        if line.find('version 1.1'): 
            return p 
    binsvn = py.path.local.sysfind('svn', checker=mysvn) 



Cross-Python Version compatibility helpers
=============================================

sources: 

  * :source:`py/compat/`
  * :source:`py/builtin/`

The py-lib contains some helpers that make writing scripts that work on various
Python versions easier.

:api:`py.compat`
----------------

:api:`py.compat` provides fixed versions (currently from Python 2.4.4) of
various newer modules to be able to use them in various Python versions.
Currently these are:

 * doctest
 * optparse
 * subprocess
 * textwrap

They are used by replacing the normal ``import ...`` byr
``from py.compat import ...``.

:api:`py.builtin`
-----------------

:api:`py.builtin` provides various builtins that were added in later Python
versions. If the used Python version used does not provide these builtins, they
are pure-Python reimplementations. These currently are:

 * enumerate
 * reversed
 * sorted
 * BaseException
 * set and frozenset (using either the builtin, if available, or the sets
   module)

:api:`py.builtin.BaseException` is just ``Exception`` before Python 2.5.