support limiting packages from site-packages

.bzrignore

+.installed.cfg
+bin
+build
+develop-eggs
+eggs
+parts
+src/zc.buildout.egg-info
+z3c.recipe.scripts_/src/z3c.recipe.scripts.egg-info
+zc.recipe.egg_/src/zc.recipe.egg.egg-info

src/zc/buildout/easy_install.txt

@@ -89,6 +89,14 @@ use_dependency_links
    for using dependency_links in preference to other
    locations. Defaults to true.
 
+include_site_packages
+    A flag indicating whether Python's non-standard-library packages should
+    be available for finding dependencies.  Defaults to true.
+
+    Paths outside of Python's standard library--or more precisely, those that
+    are not included when Python is started with the -S argument--are loosely
+    referred to as "site-packages" here.
+
 relative_paths
    Adjust egg paths so they are relative to the script path.  This
    allows scripts to work when scripts and eggs are moved, as long as
@@ -399,6 +407,68 @@ dictionary:
     >>> [d.version for d in ws]
     ['0.3', '1.1']
 
+Dependencies in Site Packages
+-----------------------------
+
+Paths outside of Python's standard library--or more precisely, those that are
+not included when Python is started with the -S argument--are loosely referred
+to as "site-packages" here.  These site-packages are searched by default for
+distributions.  This can be disabled, so that, for instance, a system Python
+can be used with buildout, cleaned of any packages installed by a user or
+system package manager.
+
+The default behavior can be controlled and introspected using
+zc.buildout.easy_install.include_site_packages.
+
+    >>> zc.buildout.easy_install.include_site_packages()
+    True
+
+Here's an example of using a Python executable that includes our dependencies.
+
+Our "py_path" will have the "demoneeded," and "demo" packages available.
+ We'll simply be asking for "demoneeded" here, but without any external
+ index or links.
+
+    >>> from zc.buildout.tests import create_sample_sys_install
+    >>> py_path, site_packages_path = make_py()
+    >>> create_sample_sys_install(site_packages_path)
+
+    >>> example_dest = tmpdir('site-packages-example-install')
+    >>> workingset = zc.buildout.easy_install.install(
+    ...     ['demoneeded'], example_dest, links=[], executable=py_path,
+    ...     index=None)
+    >>> [dist.project_name for dist in workingset]
+    ['demoneeded']
+
+That worked fine.  Let's try again with site packages not allowed.  We'll
+change the policy by changing the default.  Notice that the function for
+changing the default value returns the previous value.
+
+    >>> zc.buildout.easy_install.include_site_packages(False)
+    True
+
+    >>> zc.buildout.easy_install.include_site_packages()
+    False
+
+    >>> zc.buildout.easy_install.clear_index_cache()
+    >>> rmdir(example_dest)
+    >>> example_dest = tmpdir('site-packages-example-install')
+    >>> workingset = zc.buildout.easy_install.install(
+    ...     ['demoneeded'], example_dest, links=[], executable=py_path,
+    ...     index=None)
+    Traceback (most recent call last):
+        ...
+    MissingDistribution: Couldn't find a distribution for 'demoneeded'.
+    >>> zc.buildout.easy_install.clear_index_cache()
+
+Now we'll reset the default.
+
+    >>> zc.buildout.easy_install.include_site_packages(True)
+    False
+
+    >>> zc.buildout.easy_install.include_site_packages()
+    True
+
 Dependency links
 ----------------
 
@@ -1259,6 +1329,7 @@ call to another text fixture to create.
     >>> namespace_eggs = tmpdir('namespace_eggs')
     >>> create_sample_namespace_eggs(namespace_eggs)
 
+    >>> reset_interpreter()
     >>> ws = zc.buildout.easy_install.install(
     ...     ['demo', 'tellmy.fortune'], join(interpreter_dir, 'eggs'),
     ...     links=[link_server, namespace_eggs], index=link_server+'index/')
@@ -1319,6 +1390,7 @@ The most complex that this function gets is if you use namespace packages,
 include site-packages, and use relative paths.  For completeness, we'll look
 at that result.
 
+    >>> reset_interpreter()
     >>> generated = zc.buildout.easy_install.generate_scripts(
     ...     interpreter_bin_dir, ws, sys.executable, interpreter_parts_dir,
     ...     interpreter='py', add_site_packages=True,

src/zc/buildout/testing.py

@@ -222,11 +222,37 @@ def wait_until(label, func, *args, **kw):
         time.sleep(0.01)
     raise ValueError('Timed out waiting for: '+label)
 
+def get_installer_values():
+    """Get the current values for the easy_install module.
+
+    This is necessary because instantiating a Buildout will force the
+    Buildout's values on the installer.
+
+    Returns a dict of names-values suitable for set_installer_values."""
+    names = ('default_versions', 'download_cache', 'install_from_cache',
+             'prefer_final', 'include_site_packages',
+             'allowed_eggs_from_site_packages', 'use_dependency_links',
+             'allow_picked_versions', 'always_unzip'
+            )
+    values = {}
+    for name in names:
+        values[name] = getattr(zc.buildout.easy_install, name)()
+    return values
+
+def set_installer_values(values):
+    """Set the given values on the installer."""
+    for name, value in values.items():
+        getattr(zc.buildout.easy_install, name)(value)
+
 def make_buildout():
-    # Create a basic buildout.cfg to avoid a warning from buildout:
+    """Make a buildout that uses this version of zc.buildout."""
+    # Create a basic buildout.cfg to avoid a warning from buildout.
     open('buildout.cfg', 'w').write(
         "[buildout]\nparts =\n"
         )
+    # Get state of installer defaults so we can reinstate them (instantiating
+    # a Buildout will force the Buildout's defaults on the installer).
+    installer_values = get_installer_values()
     # Use the buildout bootstrap command to create a buildout
     zc.buildout.buildout.Buildout(
         'buildout.cfg',
@@ -234,20 +260,23 @@ def make_buildout():
          # trick bootstrap into putting the buildout develop egg
          # in the eggs dir.
          ('buildout', 'develop-eggs-directory', 'eggs'),
-         ]
+         ],
+        user_defaults=False,
         ).bootstrap([])
     # Create the develop-eggs dir, which didn't get created the usual
     # way due to the trick above:
     os.mkdir('develop-eggs')
+    # Reinstate the default values of the installer.
+    set_installer_values(installer_values)
 
 def buildoutSetUp(test):
 
     test.globs['__tear_downs'] = __tear_downs = []
     test.globs['register_teardown'] = register_teardown = __tear_downs.append
 
-    prefer_final = zc.buildout.easy_install.prefer_final()
+    installer_values = get_installer_values()
     register_teardown(
-        lambda: zc.buildout.easy_install.prefer_final(prefer_final)
+        lambda: set_installer_values(installer_values)
         )
 
     here = os.getcwd()
@@ -367,8 +396,6 @@ def buildoutSetUp(test):
         make_py = make_py
         ))
 
-    zc.buildout.easy_install.prefer_final(prefer_final)
-
 def buildoutTearDown(test):
     for f in test.globs['__tear_downs']:
         f()