Fix serialization of 'datetime' objects in '--write-metadata'

Simplified universal serialization support in json.dump() can be achieved
by passing 'default=str', which was already the case in DataJob.run()
for -j/--dump-json, but not for the 'metadata' post-processor.

This commit introduces util.dump_json() that (more or less) unifies the
JSON output procedure of both --write-metadata and --dump-json.

(#251, #252)

gallery_dl/job.py

@@ -8,7 +8,6 @@
 
 import sys
 import time
-import json
 import logging
 from . import extractor, downloader, postprocessor
 from . import config, text, util, output, exception
@@ -469,16 +468,13 @@ def run(self):
         except BaseException:
             pass
 
+        # convert numbers to string
         if config.get(("output", "num-to-str"), False):
             for msg in self.data:
                 util.transform_dict(msg[-1], util.number_to_string)
 
         # dump to 'file'
-        json.dump(
-            self.data, self.file,
-            sort_keys=True, indent=2, ensure_ascii=self.ascii, default=str,
-        )
-        self.file.write("\n")
+        util.dump_json(self.data, self.file, self.ascii, 2)
 
     def handle_url(self, url, kwdict):
         self.data.append((Message.Url, url, self._filter(kwdict)))

gallery_dl/postprocessor/metadata.py

@@ -10,7 +10,6 @@
 
 from .common import PostProcessor
 from .. import util
-import json
 
 
 class MetadataPP(PostProcessor):
@@ -61,13 +60,7 @@ def _write_tags(self, file, pathfmt):
         file.write("\n")
 
     def _write_json(self, file, pathfmt):
-        json.dump(
-            pathfmt.keywords,
-            file,
-            sort_keys=True,
-            indent=self.indent,
-            ensure_ascii=self.ascii,
-        )
+        util.dump_json(pathfmt.keywords, file, self.ascii, self.indent)
 
 
 __postprocessor__ = MetadataPP

gallery_dl/util.py

@@ -11,6 +11,7 @@
 import re
 import os
 import sys
+import json
 import shutil
 import string
 import _string
@@ -92,6 +93,18 @@ def to_string(value):
     return str(value)
 
 
+def dump_json(obj, fp=sys.stdout, ensure_ascii=True, indent=4):
+    """Serialize 'obj' as JSON and write it to 'fp'"""
+    json.dump(
+        obj, fp,
+        ensure_ascii=ensure_ascii,
+        indent=indent,
+        default=str,
+        sort_keys=True,
+    )
+    fp.write("\n")
+
+
 def expand_path(path):
     """Expand environment variables and tildes (~)"""
     if not path: