Add '--dry-run' option.

src/zc/buildout/buildout.py

@@ -219,6 +219,7 @@ _buildout_default_options = _annotate_section({
     'log-level': 'INFO',
     'newest': 'true',
     'offline': 'false',
+    'dry-run': 'false',
     'parts-directory': 'parts',
     'prefer-final': 'false',
     'python': 'buildout',
@@ -328,6 +329,7 @@ class Buildout(UserDict.DictMixin):
         self._logger = logging.getLogger('zc.buildout')
         self.offline = (buildout_section['offline'] == 'true')
         self.newest = (buildout_section['newest'] == 'true')
+        self.dry_run = (buildout_section['dry-run'] == 'true')
         self.accept_buildout_test_releases = (
             buildout_section['accept-buildout-test-releases'] == 'true')
 
@@ -762,6 +764,8 @@ class Buildout(UserDict.DictMixin):
                 if part in installed_parts: # update
                     __doing__ = 'Updating %s.', part
                     self._logger.info(*__doing__)
+                    if self.dry_run:
+                        continue
                     old_options = installed_part_options[part]
                     old_installed_files = old_options['__buildout_installed__']
 
@@ -787,6 +791,8 @@ class Buildout(UserDict.DictMixin):
                 else: # install
                     __doing__ = 'Installing %s.', part
                     self._logger.info(*__doing__)
+                    if self.dry_run:
+                        continue
                     installed_files = self[part]._call(recipe.install)
                     if installed_files is None:
                         self._logger.warning(
@@ -806,7 +812,9 @@ class Buildout(UserDict.DictMixin):
 
         finally:
             installed_path = self['buildout']['installed']
-            if installed_parts:
+            if self.dry_run:
+                pass
+            elif installed_parts:
                 new = StringIO()
                 buildout = installed_part_options['buildout']
                 buildout['parts'] = ' '.join(installed_parts)
@@ -842,6 +850,8 @@ class Buildout(UserDict.DictMixin):
         # uninstall part
         __doing__ = 'Uninstalling %s.', part
         self._logger.info(*__doing__)
+        if self.dry_run:
+            return
 
         # run uninstall recipe
         recipe, entry = _recipe(installed_part_options[part])
@@ -1979,6 +1989,12 @@ Options:
     Squelch warnings about using an executable with a broken -S
     implementation.
 
+  --dry-run
+
+    Dry-run mode.  With this setting, buildout will display what will
+    be uninstalled and what will be installed without doing anything
+    in reality.
+
 Assignments are of the form: section:option=value and are used to
 provide configuration options that override those given in the
 configuration file.  For example, to run the buildout in offline mode,
@@ -2098,6 +2114,8 @@ def main(args=None):
                     print 'Setting socket time out to %d seconds' % timeout
                     socket.setdefaulttimeout(timeout)
 
+            elif orig_op == '--dry-run':
+                    options.append(('buildout', 'dry-run', 'true'))
             elif op:
                 if orig_op == '--help':
                     _help()