Implement Message.__bool__ (#142)

* Implement Message.__bool__ with similar semantics to a collection, such that any value being set on the message (i.e. having a non-default value) make the Message value truthy .

Co-authored-by: nat <n@natn.me>

docs/api.rst

@@ -12,7 +12,7 @@ Message
 
 .. autoclass:: betterproto.Message
     :members:
-    :special-members: __bytes__
+    :special-members: __bytes__, __bool__
 
 
 .. autofunction:: betterproto.serialized_on_wire

src/betterproto/__init__.py

@@ -516,6 +516,10 @@ class Message(ABC):
         .. describe:: bytes(x)
 
             Calls :meth:`__bytes__`.
+
+        .. describe:: bool(x)
+
+            Calls :meth:`__bool__`.
     """
 
     _serialized_on_wire: bool
@@ -606,6 +610,14 @@ class Message(ABC):
 
         super().__setattr__(attr, value)
 
+    def __bool__(self) -> bool:
+        """True if the Message has any fields with non-default values."""
+        return any(
+            self.__raw_get(field_name)
+            not in (PLACEHOLDER, self._get_field_default(field_name))
+            for field_name in self._betterproto.meta_by_field_name
+        )
+
     @property
     def _betterproto(self) -> ProtoClassMetadata:
         """

tests/test_features.py

@@ -365,3 +365,33 @@ def test_message_repr():
 
     assert repr(Test(name="Loki")) == "Test(name='Loki')"
     assert repr(Test(child=Test(), name="Loki")) == "Test(name='Loki', child=Test())"
+
+
+def test_bool():
+    """Messages should evaluate similarly to a collection
+    >>> test = []
+    >>> bool(test)
+    ... False
+    >>> test.append(1)
+    >>> bool(test)
+    ... True
+    >>> del test[0]
+    >>> bool(test)
+    ... False
+    """
+
+    @dataclass
+    class Falsy(betterproto.Message):
+        pass
+
+    @dataclass
+    class Truthy(betterproto.Message):
+        bar: int = betterproto.int32_field(1)
+
+    assert not Falsy()
+    t = Truthy()
+    assert not t
+    t.bar = 1
+    assert t
+    t.bar = 0
+    assert not t