Got printing of picked versions to file working.

An 'update-versions-file' option points at a file you want updated.

src/zc/buildout/buildout.py

@@ -29,6 +29,7 @@ except ImportError:
 
 import zc.buildout.configparser
 import copy
+import datetime
 import distutils.errors
 import glob
 import itertools
@@ -138,6 +139,7 @@ _buildout_default_options = _annotate_section({
     'python': 'buildout',
     'show-picked-versions': 'false',
     'socket-timeout': '',
+    'update-versions-file': '',
     'use-dependency-links': 'true',
     }, 'DEFAULT_VALUE')
 
@@ -336,6 +338,7 @@ class Buildout(DictMixin):
                 bool_option(options, 'allow-picked-versions'))
         self.show_picked_versions = bool_option(options,
                                                 'show-picked-versions')
+        self.update_versions_file = options['update-versions-file']
         zc.buildout.easy_install.show_picked_versions(
             self.show_picked_versions)
 
@@ -1022,18 +1025,19 @@ class Buildout(DictMixin):
         print_("Versions had to be automatically picked.")
         print_("The following part definition lists the versions picked:")
         print_('\n'.join(output))
-        # REINOUT Print to file
-        # if file_name:
-        #     if os.path.exists(file_name):
-        #         output[:1] = [
-        #             '',
-        #             '# Added by Buildout Versions at %s' % datetime.now(),
-        #             ]
-        #     output.append('')
-        #     f = open(file_name,'a')
-        #     f.write('\n'.join(output))
-        #     f.close()
-        #     print("This information has been written to %r" % file_name)
+        if self.update_versions_file:
+            # Also 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('')
+            f = open(self.update_versions_file, 'a')
+            f.write('\n'.join(output))
+            f.close()
+            print_("This information has been written to " +
+                   self.update_versions_file)
 
     def setup(self, args):
         if not args:

src/zc/buildout/repeatable.txt

@@ -304,6 +304,72 @@ and case differences won't impact the pinning:
     Updating foo.
     recipe v2
 
+Sometimes it is handy to have a separate file with versions. This is a regular
+buildout file with a single ``[versions]`` section. You include it by
+extending from that versions file:
+
+    >>> write('my_versions.cfg',
+    ... '''
+    ... [versions]
+    ... distribute = 0.6.34
+    ... spam = 2
+    ... ''')
+    >>> write('buildout.cfg',
+    ... '''
+    ... [buildout]
+    ... parts = foo
+    ... extends = my_versions.cfg
+    ... find-links = %s
+    ... show-picked-versions = true
+    ...
+    ... [foo]
+    ... recipe = spam
+    ... ''' % join('recipe', 'dist'))
+    >>> print_(system(buildout), end='') # doctest: +ELLIPSIS
+    Updating foo.
+    recipe v2
+
+If not everything is pinned and buildout has to pick versions, you can tell
+buildout to append the versions to your versions file. It simply appends them
+at the end.
+
+    >>> write('my_versions.cfg',
+    ... '''
+    ... [versions]
+    ... distribute = 0.6.34
+    ... ''')
+    >>> write('buildout.cfg',
+    ... '''
+    ... [buildout]
+    ... parts = foo
+    ... extends = my_versions.cfg
+    ... update-versions-file = my_versions.cfg
+    ... find-links = %s
+    ... show-picked-versions = true
+    ...
+    ... [foo]
+    ... recipe = spam
+    ... ''' % join('recipe', 'dist'))
+    >>> print_(system(buildout), end='') # doctest: +ELLIPSIS
+    Updating foo.
+    recipe v2
+    Versions had to be automatically picked.
+    The following part definition lists the versions picked:
+    [versions]
+    spam = 2
+    This information has been written to my_versions.cfg
+
+The versions file now contains the extra pin:
+
+    >>> 'spam = 2' in open('my_versions.cfg').read()
+    True
+
+And re-running buildout doesn't report any picked versions anymore:
+
+    >>> print_(system(buildout), end='') # doctest: +ELLIPSIS
+    Updating foo.
+    recipe v2
+
 
 Controlling the python version
 -------------------------------