export: Add --format=jsonlines option

This adds support for the JSON Lines format as documented at
https://jsonlines.org/.

In this mode the data is output incrementally, whereas the other
modes load every item into memory and don't produce output until
the end.

beetsplug/export.py

@@ -54,6 +54,14 @@ class ExportPlugin(BeetsPlugin):
                     'sort_keys': True
                 }
             },
+            'jsonlines': {
+                # JSON Lines formatting options.
+                'formatting': {
+                    'ensure_ascii': False,
+                    'separators': (',', ': '),
+                    'sort_keys': True
+                }
+            },
             'csv': {
                 # CSV module formatting options.
                 'formatting': {
@@ -95,7 +103,7 @@ class ExportPlugin(BeetsPlugin):
         )
         cmd.parser.add_option(
             u'-f', u'--format', default='json',
-            help=u"the output format: json (default), csv, or xml"
+            help=u"the output format: json (default), jsonlines, csv, or xml"
         )
         return [cmd]
 
@@ -103,6 +111,7 @@ class ExportPlugin(BeetsPlugin):
         file_path = opts.output
         file_mode = 'a' if opts.append else 'w'
         file_format = opts.format or self.config['default_format'].get(str)
+        file_format_is_line_based = (file_format == 'jsonlines')
         format_options = self.config[file_format]['formatting'].get(dict)
 
         export_format = ExportFormat.factory(
@@ -130,9 +139,14 @@ class ExportPlugin(BeetsPlugin):
                 continue
 
             data = key_filter(data)
-            items += [data]
 
-        export_format.export(items, **format_options)
+            if file_format_is_line_based:
+                export_format.export(data, **format_options)
+            else:
+                items += [data]
+
+        if not file_format_is_line_based:
+            export_format.export(items, **format_options)
 
 
 class ExportFormat(object):
@@ -147,7 +161,7 @@ class ExportFormat(object):
 
     @classmethod
     def factory(cls, file_type, **kwargs):
-        if file_type == "json":
+        if file_type in ["json", "jsonlines"]:
             return JsonFormat(**kwargs)
         elif file_type == "csv":
             return CSVFormat(**kwargs)
@@ -167,6 +181,7 @@ class JsonFormat(ExportFormat):
 
     def export(self, data, **kwargs):
         json.dump(data, self.out_stream, cls=ExportEncoder, **kwargs)
+        self.out_stream.write('\n')
 
 
 class CSVFormat(ExportFormat):

docs/changelog.rst

@@ -25,7 +25,7 @@ New features:
   `discogs_artistid`
   :bug: `3413`
 * :doc:`/plugins/export`: Added new ``-f`` (``--format``) flag; 
-  which allows for the ability to export in json, csv and xml.
+  which allows for the ability to export in json, jsonlines, csv and xml.
   Thanks to :user:`austinmm`.
   :bug:`3402`
 * :doc:`/plugins/unimported`: lets you find untracked files in your library directory.

docs/plugins/export.rst

@@ -39,14 +39,15 @@ The ``export`` command has these command-line options:
 
 * ``--append``: Appends the data to the file instead of writing.
 
-* ``--format`` or ``-f``: Specifies the format the data will be exported as. If not informed, JSON will be used by default. The format options include csv, json and xml.
+* ``--format`` or ``-f``: Specifies the format the data will be exported as. If not informed, JSON will be used by default. The format options include csv, json, `jsonlines <https://jsonlines.org/>`_ and xml.
 
 Configuration
 -------------
 
 To configure the plugin, make a ``export:`` section in your configuration
 file.
-For JSON export, these options are available under the ``json`` key:
+For JSON export, these options are available under the ``json`` and
+``jsonlines`` keys:
 
 - **ensure_ascii**: Escape non-ASCII characters with ``\uXXXX`` entities.
 - **indent**: The number of spaces for indentation.

test/test_export.py

@@ -66,6 +66,17 @@ class ExportPluginTest(unittest.TestCase, TestHelper):
             self.assertTrue(key in json_data)
             self.assertEqual(val, json_data[key])
 
+    def test_jsonlines_output(self):
+        item1 = self.create_item()
+        out = self.execute_command(
+            format_type='jsonlines',
+            artist=item1.artist
+        )
+        json_data = json.loads(out)
+        for key, val in self.test_values.items():
+            self.assertTrue(key in json_data)
+            self.assertEqual(val, json_data[key])
+
     def test_csv_output(self):
         item1 = self.create_item()
         out = self.execute_command(