make the cache command write to a file by default

Compute the name of the file the loader is going to look for and write
to that if no other output is given. Use '-' to write to stdout.

Change-Id: I3f2cebb1c7a8e77cf8ed8d78c8ab85ccdd4ee325
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

reno/cache.py

@@ -10,6 +10,7 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
+from reno import loader
 from reno import scanner
 from reno import utils
 
@@ -62,10 +63,15 @@ def cache_cmd(args):
     "Generates a release notes cache"
     reporoot = args.reporoot.rstrip('/') + '/'
     notesdir = utils.get_notes_dir(args)
-    if args.output:
-        stream = open(args.output, 'w')
-    else:
+    if args.output == '-':
         stream = sys.stdout
+        close_stream = False
+    elif args.output:
+        stream = open(args.output, 'w')
+        close_stream = True
+    else:
+        stream = open(loader.get_cache_filename(reporoot, notesdir), 'w')
+        close_stream = True
     try:
         cache = build_cache_db(
             reporoot=reporoot,
@@ -83,6 +89,6 @@ def cache_cmd(args):
             encoding='utf-8',
         )
     finally:
-        if args.output:
+        if close_stream:
             stream.close()
     return

reno/main.py

@@ -127,7 +127,9 @@ def main(argv=sys.argv[1:]):
     do_cache.add_argument(
         '--output', '-o',
         default=None,
-        help='output filename, defaults to stdout',
+        help=('output filename, '
+              'defaults to the cache file within the notesdir, '
+              'use "-" for stdout'),
     )
     _build_query_arg_group(do_cache)
     do_cache.set_defaults(func=cache.cache_cmd)