Set serialized_on_wire when message contains only lists

betterproto/__init__.py

@@ -741,6 +741,9 @@ def parse(self: T, data: bytes) -> T:
         Parse the binary encoded Protobuf into this message instance. This
         returns the instance itself and is therefore assignable and chainable.
         """
+        # Got some data over the wire
+        self._serialized_on_wire = True
+
         for parsed in parse_fields(data):
             field_name = self._betterproto.field_name_by_number.get(parsed.number)
             if not field_name:

betterproto/tests/test_features.py

@@ -1,6 +1,6 @@
 import betterproto
 from dataclasses import dataclass
-from typing import Optional
+from typing import Optional, List, Dict
 
 
 def test_has_field():
@@ -32,6 +32,25 @@ class Foo(betterproto.Message):
     foo.bar = Bar()
     assert betterproto.serialized_on_wire(foo.bar) == False
 
+    @dataclass
+    class WithCollections(betterproto.Message):
+        test_list: List[str] = betterproto.string_field(1)
+        test_map: Dict[str, str] = betterproto.map_field(
+            2, betterproto.TYPE_STRING, betterproto.TYPE_STRING
+        )
+
+    # Is always set from parse, even if all collections are empty
+    with_collections_empty = WithCollections().parse(bytes(WithCollections()))
+    assert betterproto.serialized_on_wire(with_collections_empty) == True
+    with_collections_list = WithCollections().parse(
+        bytes(WithCollections(test_list=["a", "b", "c"]))
+    )
+    assert betterproto.serialized_on_wire(with_collections_list) == True
+    with_collections_map = WithCollections().parse(
+        bytes(WithCollections(test_map={"a": "b", "c": "d"}))
+    )
+    assert betterproto.serialized_on_wire(with_collections_map) == True
+
 
 def test_class_init():
     @dataclass