message: add namedtuple methods for backwards-compat

Thanks to @jurrian for identifying the issue in Bogdanp/django_dramatiq#143

dramatiq/message.py

@@ -18,7 +18,7 @@
 import dataclasses
 import time
 import uuid
-from typing import Any, Dict, Generic, Optional, TypeVar
+from typing import Any, Dict, Generic, Optional, Tuple, TypeVar
 
 from .broker import get_broker
 from .composition import pipeline
@@ -182,3 +182,17 @@ def __str__(self) -> str:
 
     def __lt__(self, other: "Message") -> bool:
         return dataclasses.astuple(self) < dataclasses.astuple(other)
+
+    # Backwards-compatibility with namedtuple.
+    _asdict = asdict
+
+    @property
+    def _field_defaults(self) -> Dict[str, Any]:
+        return {f.name: f.default for f in dataclasses.fields(self) if f.default is not dataclasses.MISSING}
+
+    @property
+    def _fields(self) -> Tuple[str, ...]:
+        return tuple(f.name for f in dataclasses.fields(self))
+
+    def _replace(self, **changes) -> "Message[R]":
+        return dataclasses.replace(self, **changes)

tests/test_messages.py

@@ -0,0 +1,24 @@
+import dramatiq
+
+
+def test_messages_have_namedtuple_methods(stub_broker):
+    @dramatiq.actor
+    def add(x, y):
+        return x + y
+
+    msg1 = add.message(1, 2)
+    assert msg1.asdict() == msg1._asdict()
+
+    assert msg1._field_defaults == {}
+    assert msg1._fields == (
+        "queue_name",
+        "actor_name",
+        "args",
+        "kwargs",
+        "options",
+        "message_id",
+        "message_timestamp",
+    )
+
+    msg2 = msg1._replace(queue_name="example")
+    assert msg2._asdict()["queue_name"] == "example"