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>

src/betterproto/__init__.py

@@ -428,6 +428,7 @@ class ProtoClassMetadata:
         "cls_by_field",
         "field_name_by_number",
         "meta_by_field_name",
+        "sorted_field_names",
     )
 
     def __init__(self, cls: Type["Message"]):
@@ -453,6 +454,9 @@ class ProtoClassMetadata:
         self.oneof_field_by_group = by_group
         self.field_name_by_number = by_field_number
         self.meta_by_field_name = by_field_name
+        self.sorted_field_names = tuple(
+            by_field_number[number] for number in sorted(by_field_number.keys())
+        )
 
         self.default_gen = self._get_default_gen(cls, fields)
         self.cls_by_field = self._get_cls_by_field(cls, fields)
@@ -513,23 +517,63 @@ class Message(ABC):
             if meta.group:
                 group_current.setdefault(meta.group)
 
-            if getattr(self, field_name) != PLACEHOLDER:
-                # Skip anything not set to the sentinel value
+            if self.__raw_get(field_name) != PLACEHOLDER:
+                # Found a non-sentinel value
                 all_sentinel = False
 
                 if meta.group:
                     # This was set, so make it the selected value of the one-of.
                     group_current[meta.group] = field_name
 
-                continue
-
-            setattr(self, field_name, self._get_field_default(field_name))
-
         # Now that all the defaults are set, reset it!
         self.__dict__["_serialized_on_wire"] = not all_sentinel
         self.__dict__["_unknown_fields"] = b""
         self.__dict__["_group_current"] = group_current
 
+    def __raw_get(self, name: str) -> Any:
+        return super().__getattribute__(name)
+
+    def __eq__(self, other) -> bool:
+        if type(self) is not type(other):
+            return False
+
+        for field_name in self._betterproto.meta_by_field_name:
+            self_val = self.__raw_get(field_name)
+            other_val = other.__raw_get(field_name)
+            if self_val is PLACEHOLDER:
+                if other_val is PLACEHOLDER:
+                    continue
+                self_val = self._get_field_default(field_name)
+            elif other_val is PLACEHOLDER:
+                other_val = other._get_field_default(field_name)
+
+            if self_val != other_val:
+                return False
+
+        return True
+
+    def __repr__(self) -> str:
+        parts = [
+            f"{field_name}={value!r}"
+            for field_name in self._betterproto.sorted_field_names
+            for value in (self.__raw_get(field_name),)
+            if value is not PLACEHOLDER
+        ]
+        return f"{self.__class__.__name__}({', '.join(parts)})"
+
+    def __getattribute__(self, name: str) -> Any:
+        """
+        Lazily initialize default values to avoid infinite recursion for recursive
+        message types
+        """
+        value = super().__getattribute__(name)
+        if value is not PLACEHOLDER:
+            return value
+
+        value = self._get_field_default(name)
+        super().__setattr__(name, value)
+        return value
+
     def __setattr__(self, attr: str, value: Any) -> None:
         if attr != "_serialized_on_wire":
             # Track when a field has been set.
@@ -542,9 +586,7 @@ class Message(ABC):
                     if field.name == attr:
                         self._group_current[group] = field.name
                     else:
-                        super().__setattr__(
-                            field.name, self._get_field_default(field.name)
-                        )
+                        super().__setattr__(field.name, PLACEHOLDER)
 
         super().__setattr__(attr, value)