pyclass: support extending Exception types

CHANGELOG.md

@@ -22,6 +22,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Add FFI definitions from `cpython/import.h`.[#1475](https://github.com/PyO3/pyo3/pull/1475)
 - Add tuple and unit struct support for `#[pyclass]` macro. [#1504](https://github.com/PyO3/pyo3/pull/1504)
 - Add FFI definition `PyDateTime_TimeZone_UTC`. [#1572](https://github.com/PyO3/pyo3/pull/1572)
+- Add support for `#[pyclass(extends=Exception)]`. [#1591](https://github.com/PyO3/pyo3/pull/1591)
 
 ### Changed
 - Change `PyTimeAcces::get_fold()` to return a `bool` instead of a `u8`. [#1397](https://github.com/PyO3/pyo3/pull/1397)

tests/test_inheritance.rs

@@ -153,7 +153,8 @@ except Exception as e:
 #[cfg(not(Py_LIMITED_API))]
 mod inheriting_native_type {
     use super::*;
-    use pyo3::types::{PyDict, PySet};
+    use pyo3::exceptions::PyException;
+    use pyo3::types::{IntoPyDict, PyDict, PySet};
 
     #[pyclass(extends=PySet)]
     #[derive(Debug)]
@@ -208,6 +209,49 @@ mod inheriting_native_type {
             r#"dict_sub[0] = 1; assert dict_sub[0] == 1; assert dict_sub._name == "Hello :)""#
         );
     }
+
+    #[pyclass(extends=PyException)]
+    struct CustomException {
+        #[pyo3(get)]
+        context: &'static str,
+    }
+
+    #[pymethods]
+    impl CustomException {
+        #[new]
+        fn new() -> Self {
+            CustomException {
+                context: "Hello :)",
+            }
+        }
+    }
+
+    #[test]
+    fn custom_exception() {
+        Python::with_gil(|py| {
+            let cls = py.get_type::<CustomException>();
+            let dict = [("cls", cls)].into_py_dict(py);
+            let res = py.run(
+            "e = cls('hello'); assert str(e) == 'hello'; assert e.context == 'Hello :)'; raise e",
+            None,
+            Some(dict)
+            );
+            let err = res.unwrap_err();
+            assert!(err.matches(py, cls), "{}", err);
+
+            // catching the exception in Python also works:
+            py_run!(
+                py,
+                cls,
+                r#"
+                    try:
+                        raise cls("foo")
+                    except cls:
+                        pass
+                "#
+            )
+        })
+    }
 }
 
 #[pyclass(subclass)]