[pp:metadata] add '"mode": "print"' (#2691)

https://github.com/mikf/gallery-dl/issues/2691#issuecomment-3276158980

gallery_dl/postprocessor/metadata.py

@@ -45,6 +45,15 @@ class MetadataPP(PostProcessor):
                 cfmt = "\n".join(cfmt) + "\n"
             self._content_fmt = formatter.parse(cfmt).format_map
             ext = "txt"
+        elif mode == "print":
+            nl = "\n"
+            if isinstance(cfmt, list):
+                cfmt = f"{nl.join(cfmt)}{nl}"
+            if cfmt[-1] != nl and (cfmt[0] != "\f" or cfmt[1] == "F"):
+                cfmt = f"{cfmt}{nl}"
+            self.write = self._write_custom
+            self._content_fmt = formatter.parse(cfmt).format_map
+            filename = "-"
         elif mode == "jsonl":
             self.write = self._write_json
             self._json_encode = self._make_encoder(options).encode

test/test_postprocessor.py

@@ -555,6 +555,17 @@ class MetadataTest(BasePostprocessorTest):
         test({"mode": "custom", "format": "{foo}\n{missing}\n"})
         test({"format": "{foo}\n{missing}\n"})
 
+    def test_metadata_mode_print(self):
+        self._create(
+            {"mode": "print", "format": "{foo}\n{missing}"},
+            {"foo": "bar"},
+        )
+
+        with patch("sys.stdout", Mock()) as m:
+            self._trigger()
+
+        self.assertEqual(self._output(m), "bar\nNone\n")
+
     def test_metadata_extfmt(self):
         pp = self._create({
             "extension"       : "ignored",