[postprocessor:metadata] add 'skip' option (#3786)

docs/configuration.rst

@@ -4411,6 +4411,16 @@ Description
     i.e. fields whose name starts with an underscore.
 
 
+metadata.skip
+-------------
+Type
+    ``bool``
+Default
+    ``false``
+Description
+    Do not overwrite already existing files.
+
+
 metadata.archive
 ----------------
 Type

gallery_dl/postprocessor/metadata.py

@@ -87,6 +87,7 @@ class MetadataPP(PostProcessor):
         self.omode = options.get("open", omode)
         self.encoding = options.get("encoding", "utf-8")
         self.private = options.get("private", False)
+        self.skip = options.get("skip", False)
 
     def run(self, pathfmt):
         archive = self.archive
@@ -96,6 +97,9 @@ class MetadataPP(PostProcessor):
         directory = self._directory(pathfmt)
         path = directory + self._filename(pathfmt)
 
+        if self.skip and os.path.exists(path):
+            return
+
         try:
             with open(path, self.omode, encoding=self.encoding) as fp:
                 self.write(fp, pathfmt.kwdict)

test/test_postprocessor.py

@@ -428,11 +428,46 @@ class MetadataTest(BasePostprocessorTest):
         self.assertNotIn("baz", pdict["bar"])
         self.assertEqual(kwdict["bar"], pdict["bar"])
 
+        # no errors for deleted/undefined fields
         self._trigger()
         self.assertNotIn("foo", pdict)
         self.assertNotIn("baz", pdict["bar"])
         self.assertEqual(kwdict["bar"], pdict["bar"])
 
+    def test_metadata_option_skip(self):
+        self._create({"skip": True})
+
+        with patch("builtins.open", mock_open()) as m, \
+                patch("os.path.exists") as e:
+            e.return_value = True
+            self._trigger()
+
+        self.assertTrue(e.called)
+        self.assertTrue(not m.called)
+        self.assertTrue(not len(self._output(m)))
+
+        with patch("builtins.open", mock_open()) as m, \
+                patch("os.path.exists") as e:
+            e.return_value = False
+            self._trigger()
+
+        self.assertTrue(e.called)
+        self.assertTrue(m.called)
+        self.assertGreater(len(self._output(m)), 0)
+
+        path = self.pathfmt.realdirectory + "file.ext.json"
+        m.assert_called_once_with(path, "w", encoding="utf-8")
+
+    def test_metadata_option_skip_false(self):
+        self._create({"skip": False})
+
+        with patch("builtins.open", mock_open()) as m, \
+                patch("os.path.exists") as e:
+            self._trigger()
+
+        self.assertTrue(not e.called)
+        self.assertTrue(m.called)
+
     @staticmethod
     def _output(mock):
         return "".join(