Try harder by looking for a __bool__ magic method when extracing bool values from Python objects.

I decided to not implement the full protocol for truth value testing [1] as it
seems confusing in the context of function arguments if basically any instance
of custom class or non-empty collection turns into `true`.

[1] https://docs.python.org/3/library/stdtypes.html#truth

Author: adamreichold

newsfragments/3638.changed.md

@@ -0,0 +1 @@
+Values of type `bool` can now be extracted from all Python values defining a `__bool__` magic method.

src/types/any.rs

@@ -144,7 +144,6 @@ impl PyAny {
     ///
     /// To avoid repeated temporary allocations of Python strings, the [`intern!`] macro can be used
     /// to intern `attr_name`.
-    #[allow(dead_code)] // Currently only used with num-complex+abi3, so dead without that.
     pub(crate) fn lookup_special<N>(&self, attr_name: N) -> PyResult<Option<&PyAny>>
     where
         N: IntoPy<Py<PyString>>,

src/types/boolobject.rs

@@ -1,6 +1,9 @@
 #[cfg(feature = "experimental-inspect")]
 use crate::inspect::types::TypeInfo;
-use crate::{ffi, FromPyObject, IntoPy, PyAny, PyObject, PyResult, Python, ToPyObject};
+use crate::{
+    exceptions::PyTypeError, ffi, intern, FromPyObject, IntoPy, PyAny, PyObject, PyResult, Python,
+    ToPyObject,
+};
 
 /// Represents a Python `bool`.
 #[repr(transparent)]
@@ -56,7 +59,16 @@ impl IntoPy<PyObject> for bool {
 /// Fails with `TypeError` if the input is not a Python `bool`.
 impl<'source> FromPyObject<'source> for bool {
     fn extract(obj: &'source PyAny) -> PyResult<Self> {
-        Ok(obj.downcast::<PyBool>()?.is_true())
+        if let Ok(obj) = obj.downcast::<PyBool>() {
+            return Ok(obj.is_true());
+        }
+
+        let meth = obj
+            .lookup_special(intern!(obj.py(), "__bool__"))?
+            .ok_or_else(|| PyTypeError::new_err("object has no __bool__ magic method"))?;
+
+        let obj = meth.call0()?.downcast::<PyBool>()?;
+        Ok(obj.is_true())
     }
 
     #[cfg(feature = "experimental-inspect")]
@@ -67,7 +79,7 @@ impl<'source> FromPyObject<'source> for bool {
 
 #[cfg(test)]
 mod tests {
-    use crate::types::{PyAny, PyBool};
+    use crate::types::{PyAny, PyBool, PyModule};
     use crate::Python;
     use crate::ToPyObject;
 
@@ -90,4 +102,47 @@ mod tests {
             assert!(false.to_object(py).is(PyBool::new(py, false)));
         });
     }
+
+    #[test]
+    fn test_magic_method() {
+        Python::with_gil(|py| {
+            let module = PyModule::from_code(
+                py,
+                r#"
+class A:
+    def __bool__(self): return True
+class B:
+    def __bool__(self): return "not a bool"
+class C:
+    def __len__(self): return 23
+class D:
+    pass
+                "#,
+                "test.py",
+                "test",
+            )
+            .unwrap();
+
+            let a = module.getattr("A").unwrap().call0().unwrap();
+            assert!(a.extract::<bool>().unwrap());
+
+            let b = module.getattr("B").unwrap().call0().unwrap();
+            assert_eq!(
+                b.extract::<bool>().unwrap_err().to_string(),
+                "TypeError: 'str' object cannot be converted to 'PyBool'",
+            );
+
+            let c = module.getattr("C").unwrap().call0().unwrap();
+            assert_eq!(
+                c.extract::<bool>().unwrap_err().to_string(),
+                "TypeError: object has no __bool__ magic method",
+            );
+
+            let d = module.getattr("D").unwrap().call0().unwrap();
+            assert_eq!(
+                d.extract::<bool>().unwrap_err().to_string(),
+                "TypeError: object has no __bool__ magic method",
+            );
+        });
+    }
 }