Add default/fallback behavior for json.dumps

CHANGELOG.md

@@ -1,5 +1,9 @@
 # Changelog
 
+## Unreleased
+
+* Add default/fallback handling for json.dumps ([#47](https://github.com/elastic/ecs-logging-python/pull/47))
+
 ## 1.0.0 (2021-02-08)
 
 * Remove "beta" designation

ecs_logging/_utils.py

@@ -151,17 +151,46 @@ def json_dumps(value):
         # case the given values aren't strings (even though
         # they should be according to the spec)
         ordered_json = ",".join(
-            '"%s":%s' % (k, json.dumps(v, sort_keys=True, separators=(",", ":")))
+            '"%s":%s'
+            % (
+                k,
+                json.dumps(
+                    v,
+                    sort_keys=True,
+                    separators=(",", ":"),
+                    default=_json_dumps_fallback,
+                ),
+            )
             for k, v in ordered_fields
         )
         if value:
             return "{%s,%s" % (
                 ordered_json,
-                json.dumps(value, sort_keys=True, separators=(",", ":"))[1:],
+                json.dumps(
+                    value,
+                    sort_keys=True,
+                    separators=(",", ":"),
+                    default=_json_dumps_fallback,
+                )[1:],
             )
         else:
             return "{%s}" % ordered_json
     # If there are no fields with ordering requirements we
     # pass everything into json.dumps()
     else:
-        return json.dumps(value, sort_keys=True, separators=(",", ":"))
+        return json.dumps(
+            value, sort_keys=True, separators=(",", ":"), default=_json_dumps_fallback
+        )
+
+
+def _json_dumps_fallback(value):
+    # type: (Any) -> Any
+    """
+    Fallback handler for json.dumps to handle objects json doesn't know how to
+    serialize.
+    """
+    try:
+        # This is what structlog's json fallback does
+        return value.__structlog__()
+    except AttributeError:
+        return repr(value)

tests/test_structlog_formatter.py

@@ -21,8 +21,17 @@
 from .compat import StringIO
 
 
+class NotSerializable:
+    def __repr__(self):
+        return "<NotSerializable>"
+
+
 def make_event_dict():
-    return {"event": "test message", "log.logger": "logger-name"}
+    return {
+        "event": "test message",
+        "log.logger": "logger-name",
+        "baz": NotSerializable(),
+    }
 
 
 @mock.patch("time.time")
@@ -32,7 +41,9 @@ def test_event_dict_formatted(time, spec_validator):
     formatter = ecs_logging.StructlogFormatter()
     assert spec_validator(formatter(None, "debug", make_event_dict())) == (
         '{"@timestamp":"2020-03-20T16:16:37.187Z","log.level":"debug",'
-        '"message":"test message","ecs":{"version":"1.6.0"},'
+        '"message":"test message",'
+        '"baz":"<NotSerializable>",'
+        '"ecs":{"version":"1.6.0"},'
         '"log":{"logger":"logger-name"}}'
     )