Handle mutable default arguments cleanly

pyproject.toml

@@ -30,6 +30,7 @@ protobuf = "^3.12.2"
 pytest = "^5.4.2"
 pytest-asyncio = "^0.12.0"
 pytest-cov = "^2.9.0"
+pytest-mock = "^3.1.1"
 tox = "^3.15.1"
 
 [tool.poetry.scripts]

src/betterproto/plugin.py

@@ -369,6 +369,10 @@ def lookup_method_input_type(method, types):
             return known_type
 
 
+def is_mutable_field_type(field_type: str) -> bool:
+    return field_type.startswith("List[") or field_type.startswith("Dict[")
+
+
 def read_protobuf_service(
     service: ServiceDescriptorProto, index, proto_file, content, output_types
 ):
@@ -384,8 +388,23 @@ def read_protobuf_service(
     for j, method in enumerate(service.method):
         method_input_message = lookup_method_input_type(method, output_types)
 
+        # This section ensures that method arguments having a default
+        # value that is initialised as a List/Dict (mutable) is replaced
+        # with None and initialisation is deferred to the beginning of the
+        # method definition. This is done so to avoid any side-effects.
+        # Reference: https://docs.python-guide.org/writing/gotchas/#mutable-default-arguments
+        mutable_default_args = []
+
         if method_input_message:
             for field in method_input_message["properties"]:
+                if (
+                    not method.client_streaming
+                    and field["zero"] != "None"
+                    and is_mutable_field_type(field["type"])
+                ):
+                    mutable_default_args.append((field["py_name"], field["zero"]))
+                    field["zero"] = "None"
+
                 if field["zero"] == "None":
                     template_data["typing_imports"].add("Optional")
 
@@ -407,6 +426,7 @@ def read_protobuf_service(
                 ),
                 "client_streaming": method.client_streaming,
                 "server_streaming": method.server_streaming,
+                "mutable_default_args": mutable_default_args,
             }
         )
 

src/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 %}

tests/grpc/test_grpclib_client.py

@@ -1,11 +1,14 @@
 import asyncio
+import sys
+
 from tests.output_betterproto.service.service import (
-    DoThingResponse,
     DoThingRequest,
+    DoThingResponse,
     GetThingRequest,
     TestStub as ThingServiceClient,
 )
 import grpclib
+import grpclib.metadata
 from grpclib.testing import ChannelFor
 import pytest
 from betterproto.grpc.util.async_channel import AsyncChannel
@@ -35,6 +38,20 @@ 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

tests/inputs/service/service.proto

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