From 69dfe9cafc26248029502df865d2da63a977562c Mon Sep 17 00:00:00 2001
From: James <50501825+Gobot1234@users.noreply.github.com>
Date: Tue, 24 Nov 2020 18:35:09 +0000
Subject: [PATCH] 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                |  2 +-
 src/betterproto/__init__.py | 12 ++++++++++++
 tests/test_features.py      | 30 ++++++++++++++++++++++++++++++
 3 files changed, 43 insertions(+), 1 deletion(-)

diff --git a/docs/api.rst b/docs/api.rst
index ceae2d2..9f99b51 100644
--- a/docs/api.rst
+++ b/docs/api.rst
@@ -12,7 +12,7 @@ Message
 
 .. autoclass:: betterproto.Message
     :members:
-    :special-members: __bytes__
+    :special-members: __bytes__, __bool__
 
 
 .. autofunction:: betterproto.serialized_on_wire
diff --git a/src/betterproto/__init__.py b/src/betterproto/__init__.py
index e6daeb3..b26ec4b 100644
--- a/src/betterproto/__init__.py
+++ b/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:
         """
diff --git a/tests/test_features.py b/tests/test_features.py
index f548264..ef33d87 100644
--- a/tests/test_features.py
+++ b/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