Add support for recursive messages (#130)

Changes message initialization (`__post_init__`) so that default values
are no longer eagerly created to prevent infinite recursion when
initializing recursive messages.

As a result, `PLACEHOLDER` will be present in the message for any
uninitialized fields.  So, an implementation of `__get_attribute__` is
added that checks for `PLACEHOLDER` and lazily creates and stores
default field values.

And, because `PLACEHOLDER` values don't compare equal with zero values,
a custom implementation of `__eq__` is provided, and the code generation
template is updated so that messages generate with `@dataclass(eq=False)`.

Also add new Message __repr__ implementation that skips PLACEHOLDER 
values and orders keys by number from the proto.

Co-authored-by: Christopher Chambers <chris@peanutcode.com>
Co-authored-by: nat <n@natn.me>
Co-authored-by: James <50501825+Gobot1234@users.noreply.github.com>

tests/inputs/recursivemessage/recursivemessage.json

@@ -0,0 +1,12 @@
+{
+  "name": "Zues",
+  "child": {
+    "name": "Hercules"
+  },
+  "intermediate": {
+    "child": {
+      "name": "Douglas Adams"
+    },
+    "number": 42
+  }
+}

tests/inputs/recursivemessage/recursivemessage.proto

@@ -0,0 +1,13 @@
+syntax = "proto3";
+
+message Test {
+    string name = 1;
+    Test child = 2;
+    Intermediate intermediate = 3;
+}
+
+
+message Intermediate {
+    int32 number = 1;
+    Test child = 2;
+}

tests/test_features.py

@@ -317,3 +317,51 @@ def test_oneof_default_value_set_causes_writes_wire():
         == betterproto.which_one_of(_round_trip_serialization(foo3), "group1")
         == ("", None)
     )
+
+
+def test_recursive_message():
+    from tests.output_betterproto.recursivemessage import Test as RecursiveMessage
+
+    msg = RecursiveMessage()
+
+    assert msg.child == RecursiveMessage()
+
+    # Lazily-created zero-value children must not affect equality.
+    assert msg == RecursiveMessage()
+
+    # Lazily-created zero-value children must not affect serialization.
+    assert bytes(msg) == b""
+
+
+def test_recursive_message_defaults():
+    from tests.output_betterproto.recursivemessage import (
+        Test as RecursiveMessage,
+        Intermediate,
+    )
+
+    msg = RecursiveMessage(name="bob", intermediate=Intermediate(42))
+
+    # set values are as expected
+    assert msg == RecursiveMessage(name="bob", intermediate=Intermediate(42))
+
+    # lazy initialized works modifies the message
+    assert msg != RecursiveMessage(
+        name="bob", intermediate=Intermediate(42), child=RecursiveMessage(name="jude")
+    )
+    msg.child.child.name = "jude"
+    assert msg == RecursiveMessage(
+        name="bob",
+        intermediate=Intermediate(42),
+        child=RecursiveMessage(child=RecursiveMessage(name="jude")),
+    )
+
+    # lazily initialization recurses as needed
+    assert msg.child.child.child.child.child.child.child == RecursiveMessage()
+    assert msg.intermediate.child.intermediate == Intermediate()
+
+
+def test_message_repr():
+    from tests.output_betterproto.recursivemessage import Test
+
+    assert repr(Test(name="Loki")) == "Test(name='Loki')"
+    assert repr(Test(child=Test(), name="Loki")) == "Test(name='Loki', child=Test())"