[postprocessor:metadata] implement 'extension-format' option

closes #477

docs/configuration.rst

@@ -1474,7 +1474,20 @@ metadata.extension
 =========== =====
 Type        ``string``
 Default     ``"json"`` or ``"txt"``
-Description Filename extension for metadata files.
+Description Filename extension for metadata files that will be appended to the
+            original file names.
+=========== =====
+
+metadata.extension-format
+-------------------------
+=========== =====
+Type        ``string``
+Example     * ``"{extension}.json"``
+            * ``"json"``
+Description Custom format string to build filename extensions for metadata
+            files with, which will replace the original filename extensions.
+
+            Note: `metadata.extension`_ is ignored if this option is set.
 =========== =====
 
 metadata.format
@@ -1482,7 +1495,7 @@ metadata.format
 =========== =====
 Type        ``string``
 Example     ``"tags:\n\n{tags:J\n}\n"``
-Description Custom format string to build content of metadata files.
+Description Custom format string to build the content of metadata files with.
 
             Note: Only applies for ``"mode": "custom"``.
 =========== =====

gallery_dl/postprocessor/metadata.py

@@ -18,29 +18,45 @@ class MetadataPP(PostProcessor):
         PostProcessor.__init__(self)
 
         mode = options.get("mode", "json")
-        ext = "txt"
-
         if mode == "custom":
             self.write = self._write_custom
-            self.formatter = util.Formatter(options.get("format"))
+            self.contentfmt = util.Formatter(options.get("format")).format_map
+            ext = "txt"
         elif mode == "tags":
             self.write = self._write_tags
+            ext = "txt"
         else:
             self.write = self._write_json
             self.indent = options.get("indent", 4)
             self.ascii = options.get("ascii", False)
             ext = "json"
 
+        extfmt = options.get("extension-format")
+        if extfmt:
+            self.path = self._path_format
+            self.extfmt = util.Formatter(extfmt).format_map
+        else:
+            self.path = self._path_append
             self.extension = options.get("extension", ext)
 
     def run(self, pathfmt):
-        path = "{}.{}".format(pathfmt.realpath, self.extension)
-        with open(path, "w", encoding="utf-8") as file:
+        with open(self.path(pathfmt), "w", encoding="utf-8") as file:
             self.write(file, pathfmt.kwdict)
 
+    def _path_append(self, pathfmt):
+        return "{}.{}".format(pathfmt.realpath, self.extension)
+
+    def _path_format(self, pathfmt):
+        kwdict = pathfmt.kwdict
+        ext = kwdict["extension"]
+        kwdict["extension"] = pathfmt.extension
+        kwdict["extension"] = pathfmt.prefix + self.extfmt(kwdict)
+        path = pathfmt.realdirectory + pathfmt.build_filename()
+        kwdict["extension"] = ext
+        return path
+
     def _write_custom(self, file, kwdict):
-        output = self.formatter.format_map(kwdict)
-        file.write(output)
+        file.write(self.contentfmt(kwdict))
 
     def _write_tags(self, file, kwdict):
         tags = kwdict.get("tags") or kwdict.get("tag_string")

test/test_postprocessor.py

@@ -156,6 +156,7 @@ class MetadataTest(BasePostprocessorTest):
             "_private" : "world",
         })
 
+        self.assertEqual(pp.path     , pp._path_append)
         self.assertEqual(pp.write    , pp._write_json)
         self.assertEqual(pp.ascii    , True)
         self.assertEqual(pp.indent   , 2)
@@ -228,13 +229,41 @@ class MetadataTest(BasePostprocessorTest):
         )
         self.assertEqual(pp.write, pp._write_custom)
         self.assertEqual(pp.extension, "txt")
-        self.assertTrue(pp.formatter)
+        self.assertTrue(pp.contentfmt)
 
         with patch("builtins.open", mock_open()) as m:
             pp.prepare(self.pathfmt)
             pp.run(self.pathfmt)
         self.assertEqual(self._output(m), "bar\nNone\n")
 
+    def test_metadata_extfmt(self):
+        pp = self._create({
+            "extension"       : "ignored",
+            "extension-format": "json",
+        })
+
+        self.assertEqual(pp.path, pp._path_format)
+
+        with patch("builtins.open", mock_open()) as m:
+            pp.prepare(self.pathfmt)
+            pp.run(self.pathfmt)
+
+        path = self.pathfmt.realdirectory + "file.json"
+        m.assert_called_once_with(path, "w", encoding="utf-8")
+
+    def test_metadata_extfmt_2(self):
+        pp = self._create({
+            "extension-format": "{extension!u}-data:{category:Res/ES/}",
+        })
+
+        self.pathfmt.prefix = "2."
+        with patch("builtins.open", mock_open()) as m:
+            pp.prepare(self.pathfmt)
+            pp.run(self.pathfmt)
+
+        path = self.pathfmt.realdirectory + "file.2.EXT-data:tESt"
+        m.assert_called_once_with(path, "w", encoding="utf-8")
+
     @staticmethod
     def _output(mock):
         return "".join(