Add support for adding callables as extra fields

docs/config.rst

@@ -225,6 +225,11 @@ Options:
     to Logstash. This dictionary will be merged with any other extra
     items passed in the logging call.
 
+    Note that you can also put a callable with zero arguments. If that is
+    the case, the callable will be evaluated at the moment you log this thing
+    (ie. not in the submitter thread). If this callable returns None, extra
+    field will be skipped.
+
     *Type*: ``dict``
 
     *Default*: None

logstash_async/formatter.py

@@ -202,7 +202,11 @@ def _get_extra_fields(self, record):
         }
         # static extra fields
         if self._extra:
-            extra_fields.update(self._extra)
+            for field_name, field_value in self._extra.items():
+                if callable(field_value):
+                    field_value = field_value()
+                if field_value is not None:
+                    extra_fields[field_name] = field_value
         if getattr(record, 'taskName', None):
             extra_fields[Schema.TASK_NAME] = record.taskName
         # exceptions

tests/formatter_test.py

@@ -94,7 +94,7 @@ def test_format_timestamp_microsecond_2(self):
 
     @patch.object(LogstashFormatter, '_format_exception', lambda s, e: e)
     def test_default_schema(self):
-        formatter = LogstashFormatter(tags=['t1', 't2'])
+        formatter = LogstashFormatter(tags=['t1', 't2'], extra={'value': lambda: 5})
         result = formatter._format_to_dict(create_log_record())
         self.assertDictEqual(result, {
             '@timestamp': '2021-10-24T13:32:15.024Z',
@@ -108,6 +108,7 @@ def test_default_schema(self):
             'type': 'python-logstash',
             'tags': ['t1', 't2'],
             'extra': {
+                'value': 5,
                 'func_name': 'f',
                 'interpreter': sys.executable,
                 'interpreter_version': _interpreter_version,