Refactored picked versions logic to make it easier to use for plugins

This includes renaming `store_picked_versions` to `store_required_by`
to better signify its intent and adding `get_picked_versions`
to `zc.buidlout.easy_install`.

src/zc/buildout/buildout.py

@@ -120,6 +120,24 @@ def _unannotate(data):
         data[key] = _unannotate_section(data[key])
     return data
 
+
+def _format_picked_versions(picked_versions, required_by):
+    output = ['[versions]']
+    required_output = []
+    for dist_, version in picked_versions:
+        if dist_ in required_by:
+            required_output.append('')
+            required_output.append('# Required by:')
+            for req_ in sorted(required_by[dist_]):
+                required_output.append('# '+req_)
+            target = required_output
+        else:
+            target = output
+        target.append("%s = %s" % (dist_, version))
+    output.extend(required_output)
+    return '\n'.join(output)
+
+
 _buildout_default_options = _annotate_section({
     'allow-hosts': '*',
     'allow-picked-versions': 'true',
@@ -323,8 +341,8 @@ class Buildout(DictMixin):
         self.show_picked_versions = bool_option(options,
                                                 'show-picked-versions')
         self.update_versions_file = options['update-versions-file']
-        zc.buildout.easy_install.store_picked_versions(
-            self.show_picked_versions or self.update_versions_file)
+        zc.buildout.easy_install.store_required_by(self.show_picked_versions or
+                                                   self.update_versions_file)
 
         download_cache = options.get('download-cache')
         if download_cache:
@@ -637,6 +655,7 @@ class Buildout(DictMixin):
         elif (not installed_parts) and installed_exists:
             os.remove(self['buildout']['installed'])
 
+        if self.show_picked_versions or self.update_versions_file:
             self._print_picked_versions()
         self._unload_extensions()
 
@@ -987,40 +1006,27 @@ class Buildout(DictMixin):
                 ep.load()(self)
 
     def _print_picked_versions(self):
-        Installer = zc.buildout.easy_install.Installer
-        if not Installer._picked_versions:
+        picked_versions, required_by = (zc.buildout.easy_install
+                                        .get_picked_versions())
+        if not picked_versions:
             # Don't print empty output.
             return
-        output = ['[versions]']
-        required_output = []
-        for dist_, version in sorted(Installer._picked_versions.items()):
-            if dist_ in Installer._required_by:
-                required_output.append('')
-                required_output.append('# Required by:')
-                for req_ in sorted(Installer._required_by[dist_]):
-                    required_output.append('# '+req_)
-                target = required_output
-            else:
-                target = output
-            target.append("%s = %s" % (dist_, version))
 
-        output.extend(required_output)
+        output = _format_picked_versions(picked_versions, required_by)
 
         if self.show_picked_versions:
             print_("Versions had to be automatically picked.")
             print_("The following part definition lists the versions picked:")
-            print_('\n'.join(output))
+            print_(output)
 
         if self.update_versions_file:
             # Write to the versions file.
             if os.path.exists(self.update_versions_file):
-                output[:1] = [
-                    '',
-                    '# Added by buildout at %s' % datetime.datetime.now(),
-                    ]
-            output.append('')
+                output += ('\n# Added by buildout at %s' %
+                           datetime.datetime.now())
+            output += '\n'
             f = open(self.update_versions_file, 'a')
-            f.write('\n'.join(output))
+            f.write(output)
             f.close()
             print_("Picked versions have been written to " +
                    self.update_versions_file)

src/zc/buildout/easy_install.py

@@ -150,7 +150,7 @@ class Installer:
     _prefer_final = True
     _use_dependency_links = True
     _allow_picked_versions = True
-    _store_picked_versions = False
+    _store_required_by = False
 
     def __init__(self,
                  dest=None,
@@ -755,12 +755,17 @@ def allow_picked_versions(setting=None):
         Installer._allow_picked_versions = bool(setting)
     return old
 
-def store_picked_versions(setting=None):
-    old = Installer._store_picked_versions
+def store_required_by(setting=None):
+    old = Installer._store_required_by
     if setting is not None:
-        Installer._store_picked_versions = bool(setting)
+        Installer._store_required_by = bool(setting)
     return old
 
+def get_picked_versions():
+    picked_versions = sorted(Installer._picked_versions.items())
+    required_by = Installer._required_by
+    return (picked_versions, required_by)
+
 
 def install(specs, dest,
             links=(), index=None,
@@ -1283,7 +1288,7 @@ class MissingDistribution(zc.buildout.UserError):
 
 def _log_requirement(ws, req):
     if (not logger.isEnabledFor(logging.DEBUG) and
-        not Installer._store_picked_versions):
+        not Installer._store_required_by):
         # Sorting the working set and iterating over it's requirements
         # is expensive, so short circuit the work if it won't even be
         # logged.  When profiling a simple buildout with 10 parts with