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Revision 116013 - (download) (annotate)
Sun Aug 29 20:58:59 2010 UTC (4 years, 2 months ago) by gary
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mention updated recipes so far.
System Python and zc.buildout 1.5
*********************************

The 1.5 line of zc.buildout introduced a number of changes.

Problems
========

As usual, please send questions and comments to the `distutils SIG
mailing list <mailto://distutils-sig@python.org>`_. Report bugs using
the `zc.buildout Launchpad Bug Tracker
<https://launchpad.net/zc.buildout/+bugs>`_.

If problems are keeping you from your work, here's an easy way to
revert to the old code temporarily: switch to a custom "emergency"
bootstrap script, available from
http://svn.zope.org/repos/main/zc.buildout/branches/1.4/bootstrap/bootstrap.py .

This customized script will select zc.buildout 1.4.4 by default.
zc.buildout 1.4.4 will not upgrade itself unless you explicitly specify
a new version.  It will also prefer older versions of zc.recipe.egg and
some other common recipes.  If you have trouble with other recipes,
consider using a standard buildout "versions" section to specify older
versions of these, as described in the Buildout documentation
(http://pypi.python.org/pypi/zc.buildout#repeatable-buildouts-controlling-eggs-used).

Working with a System Python
============================

While there are a number of new features available in zc.buildout 1.5,
the biggest is that Buildout itself supports usage with a system Python.
This can work if you follow a couple of simple rules.

1. Use the new bootstrap.py (available from
   svn://svn.zope.org/repos/main/zc.buildout/trunk/bootstrap/bootstrap.py).

2. Use buildout recipes that have been upgraded to work with zc.buildout 1.5
   and higher.  Specifically, they should use
   ``zc.buildout.easy_install.sitepackage_safe_scripts`` to generate
   their scripts, if any, rather than ``zc.buildout.easy_install.scripts``.
   See the `Recipes That Support a System Python`_ section below for more
   details on recipes that are available as of this writing, and
   `Updating Recipes to Support a System Python`_ for instructions on
   how to update a recipe.  Note that you should generally only need to
   update recipes that generate scripts.

It's important to note that recipes not upgraded for zc.buildout 1.5.0
should continue to work--just not with a system Python.

Using a system Python is inherently fragile.  Using a clean,
freshly-installed Python without customization in site-packages is more
robust and repeatable.  See some of the regression tests added to the
1.5.0 line for the kinds of issues that you can encounter with a system
Python, and see
http://pypi.python.org/pypi/z3c.recipe.scripts#including-site-packages-and-sitecustomize
for more discussion.

However, using a system Python can be very convenient, and the
zc.buildout code for this feature has been tested by many users already.
Moreover, it has automated tests to exercise the problems that have been
encountered and fixed.

Recipes That Support a System Python
====================================

zc.recipe.egg continues to generate old-style scripts that are not safe
for use with a system Python.  This was done for backwards
compatibility, because it is integral to so many buildouts and used as a
dependency of so many other recipes.

If you want to generate new-style scripts that do support system Python
usage, use z3c.recipe.scripts instead
(http://pypi.python.org/pypi/z3c.recipe.scripts). z3c.recipe.scripts has
the same script and interpreter generation options as zc.recipe.egg,
plus a few more for the new features mentioned above.  In the simplest
case, you should be able to simply change ``recipe = zc.recipe.egg`` to
``recipe = z3c.recipe.scripts`` in the pertinent sections of your
buildout configuration and your generated scripts will work with a system
Python.

Other updated recipes include zc.recipe.testrunner 1.4.0 and
z3c.recipe.tag 0.4.0.  Others should be updated soon: see their change
documents for details, or see `Updating Recipes to Support a System
Python`_ for instructions on how to update recipes yourself.

Templates for creating Python scripts with the z3c.recipe.filetemplate
recipe can be easily changed to support a system Python.

- If you don't care about supporting relative paths, simply using a
  generated interpreter with the eggs you want should be sufficient, as
  it was before. For instance, if the interpreter is named "py", use
  ``#!${buildout:bin-directory/py}`` or ``#!/usr/bin/env
  ${buildout:bin-directory/py}``).

- If you do care about relative paths,  (``relative-paths = true`` in
  your buildout configuration), then z3c.recipe.scripts does require a
  bit more changes, as is usual for the relative path support in that
  package.  First, use z3c.recipe.scripts to generate a script or
  interpreter with the dependencies you want.  This will create a
  directory in ``parts`` that has a site.py and sitecustomize.py.  Then,
  begin your script as in the snippet below.  The example assumes that
  the z3c.recipe.scripts generated were from a Buildout configuration
  section labeled "scripts": adjust accordingly.

  ::

    #!${buildout:executable} -S
    ${python-relative-path-setup}
    import sys
    sys.path.insert(0, ${scripts:parts-directory|path-repr})
    import site

Updating Recipes to Support a System Python
===========================================

You should generally only need to update recipes that generate scripts.
These recipes need to change from using ``zc.buildout.easy_install.scripts``
to be using ``zc.buildout.easy_install.sitepackage_safe_scripts``.
The signatures of the two functions are different.  Please compare::

    def scripts(
        reqs, working_set, executable, dest,
        scripts=None,
        extra_paths=(),
        arguments='',
        interpreter=None,
        initialization='',
        relative_paths=False,
        ):

    def sitepackage_safe_scripts(
        dest, working_set, executable, site_py_dest,
        reqs=(),
        scripts=None,
        interpreter=None,
        extra_paths=(),
        initialization='',
        include_site_packages=False,
        exec_sitecustomize=False,
        relative_paths=False,
        script_arguments='',
        script_initialization='',
        ):

In most cases, the arguments are merely reordered.  The ``reqs``
argument is no longer required in order to make it easier to generate an
interpreter alone.  The ``arguments`` argument was renamed to
``script_arguments`` to clarify that it did not affect interpreter
generation.

The only new required argument is ``site_py_dest``.  It must be the path
to a directory in which the customized site.py and sitecustomize.py
files will be written.  A typical generation in a recipe will look like
this.

(In the recipe's __init__ method...)

::

        self.options = options
        b_options = buildout['buildout']
        options['parts-directory'] = os.path.join(
            b_options['parts-directory'], self.name)

(In the recipe's install method...)

::

        options = self.options
        generated = []
        if not os.path.exists(options['parts-directory']):
            os.mkdir(options['parts-directory'])
            generated.append(options['parts-directory'])

Then ``options['parts-directory']`` can be used for the ``site_py_dest``
value.

If you want to support the other arguments (``include_site_packages``,
``exec_sitecustomize``, ``script_initialization``, as well as the
``allowed-eggs-from-site-packages`` option),  you might want to look at
some of the code in
svn://svn.zope.org/repos/main/zc.buildout/trunk/z3c.recipe.scripts\_/src/z3c/recipe/scripts/scripts.py .
You might even be able to adopt some of it by subclassing or delegating.
The Scripts class in that file is the closest to what you might be used
to from zc.recipe.egg.

Important note for recipe authors: the code in recipes is *always run
without access to the site-packages*.  This is irrespective of the
``include-site-packages`` option discussed elsewhere, which controls the
software being built, but not the environment in which Buildout itself runs.

virtualenv
==========

Using virtualenv (http://pypi.python.org/pypi/virtualenv) with the
--no-site-packages option already provided a simple way of using a
system Python.  This is intended to continue to work, and some automated
tests exist to demonstrate this.

However, it is only supported to the degree that people have found it to
work in the past.  The existing Buildout tests for virtualenv are only
for problems encountered previously.  They are very far from
comprehensive.

Using Buildout with a system python has at least three advantages over
using Buildout in conjunction with virtualenv.  They may or may not be
pertinent to your desired usage.

- Unlike ``virtualenv --no-site-packages``, Buildout's support allows you
  to choose to let packages from your system Python be available to your
  software (see ``include-site-packages`` in
  http://pypi.python.org/pypi/z3c.recipe.scripts).

  You can even specify which eggs installed in your system Python can be
  allowed to fulfill some of your packages' dependencies (see
  ``allowed-eggs-from-site-packages`` in
  http://pypi.python.org/pypi/z3c.recipe.scripts).

  At the expense of some repeatability and platform dependency, this
  flexibility means that, for instance, you can rely on
  difficult-to-build eggs like lxml coming from your system Python.

- Buildout's implementation has a full set of automated tests.

- An integral Buildout implementation means fewer steps and fewer dependencies
  to work with a system Python.

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