Add conversion to bool for Maybe (#2178)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: sobolevn <[email protected]>

Author: 3 people

returns/maybe.py

@@ -303,6 +303,9 @@ def from_optional(
             return _Nothing(inner_value)
         return Some(inner_value)
 
+    def __bool__(self) -> bool:
+        """Convert (or treat) an instance of ``Maybe`` as a boolean."""
+
 
 @final
 class _Nothing(Maybe[Any]):
@@ -378,6 +381,10 @@ def failure(self) -> None:
         """Returns failed value."""
         return self._inner_value
 
+    def __bool__(self):
+        """Returns ``False``."""
+        return False
+
 
 @final
 class Some(Maybe[_ValueType_co]):
@@ -435,6 +442,15 @@ def failure(self):
         """Raises exception for successful container."""
         raise UnwrapFailedError(self)
 
+    def __bool__(self):
+        """
+        Returns ``True```.
+
+        Any instance of ``Something`` is treated
+        as ``True``, even ``Something(None)``.
+        """
+        return True
+
 
 #: Public unit value of protected :class:`~_Nothing` type.
 Nothing: Maybe[Never] = _Nothing()

tests/test_maybe/test_maybe_conversions.py

@@ -0,0 +1,23 @@
+from returns.maybe import Nothing, Some
+
+
+def test_some_is_true() -> None:
+    """Ensures that ``Something(...)`` is ``True`` when treated as a boolean."""
+    assert bool(Some(123))
+    assert bool(Some('abc'))
+
+
+def test_nothing_is_false() -> None:
+    """Ensures that ``Nothing`` is ``False`` when treated as a boolean."""
+    assert not bool(Nothing)
+
+
+def test_some_none_is_true() -> None:
+    """
+    Ensures that ``Something(None)`` is ``True`` when treated as a boolean.
+
+    See <https://github.com/dry-python/returns/issues/2177> for the discussion
+    of this design choice.
+    """
+    assert bool(Some(None))
+    assert bool(Some(Nothing))