Handle mutable default arguments cleanly

When generating code, ensure that default list/dict arguments are
initialised in local scope if unspecified or `None`.

Author: abn

betterproto/plugin.py

@@ -314,6 +314,24 @@ def generate_code(request, response):
                                     output["typing_imports"].add("Optional")
                             break
 
+                    mutable_default_args = []
+                    if (
+                        not method.client_streaming
+                        and input_message
+                        and input_message.get("properties")
+                    ):
+                        properties = []
+                        for f in input_message.get("properties"):
+                            if f["zero"] != "None" and (
+                                f["type"].startswith("List[")
+                                or f["type"].startswith("Dict[")
+                            ):
+                                output["typing_imports"].add("Optional")
+                                mutable_default_args.append((f["py_name"], f["zero"]))
+                                f["zero"] = "None"
+                            properties.append(f)
+                        input_message["properties"] = properties
+
                     data["methods"].append(
                         {
                             "name": method.name,
@@ -332,6 +350,7 @@ def generate_code(request, response):
                             ),
                             "client_streaming": method.client_streaming,
                             "server_streaming": method.server_streaming,
+                            "mutable_default_args": mutable_default_args,
                         }
                     )
 

betterproto/templates/template.py.j2

@@ -80,6 +80,10 @@ class {{ service.py_name }}Stub(betterproto.ServiceStub):
 {{ method.comment }}
 
         {% endif %}
+        {%- for py_name, zero in method.mutable_default_args %}
+        {{ py_name }} = {{ py_name }} or {{ zero }}
+        {% endfor %}
+
         {% if not method.client_streaming %}
         request = {{ method.input }}()
         {% for field in method.input_message.properties %}

betterproto/tests/grpc/test_grpclib_client.py

@@ -1,14 +1,18 @@
 import asyncio
+import sys
+
+import grpclib
+import grpclib.metadata
+import pytest
+from grpclib.testing import ChannelFor
+
+from betterproto.grpc.util.async_channel import AsyncChannel
 from betterproto.tests.output_betterproto.service.service import (
-    DoThingResponse,
     DoThingRequest,
+    DoThingResponse,
     GetThingRequest,
     TestStub as ThingServiceClient,
 )
-import grpclib
-from grpclib.testing import ChannelFor
-import pytest
-from betterproto.grpc.util.async_channel import AsyncChannel
 from .thing_service import ThingService
 
 
@@ -35,6 +39,20 @@ async def test_simple_service_call():
         await _test_client(ThingServiceClient(channel))
 
 
+@pytest.mark.asyncio
+@pytest.mark.skipif(
+    sys.version_info < (3, 8), reason="async mock spy does works for python3.8+"
+)
+async def test_service_call_mutable_defaults(mocker):
+    async with ChannelFor([ThingService()]) as channel:
+        client = ThingServiceClient(channel)
+        spy = mocker.spy(client, "_unary_unary")
+        await _test_client(client)
+        comments = spy.call_args_list[-1].args[1].comments
+        await _test_client(client)
+        assert spy.call_args_list[-1].args[1].comments is not comments
+
+
 @pytest.mark.asyncio
 async def test_service_call_with_upfront_request_params():
     # Setting deadline

betterproto/tests/inputs/service/service.proto

@@ -4,6 +4,7 @@ package service;
 
 message DoThingRequest {
   string name = 1;
+  repeated string comments = 2;
 }
 
 message DoThingResponse {