Clone of @virtuald's PR #2112 with minor enhancements. (#3215)

* Add py::raise_from to enable chaining exceptions on Python 3.3+

* Use 'raise from' in initialization

* Documenting the exact base version of _PyErr_FormatVFromCause, adding back `assert`s.

Co-authored-by: Dustin Spicuzza <dustin@virtualroadside.com>

docs/advanced/exceptions.rst

@@ -323,6 +323,34 @@ Alternately, to ignore the error, call `PyErr_Clear
 Any Python error must be thrown or cleared, or Python/pybind11 will be left in
 an invalid state.
 
+Chaining exceptions ('raise from')
+==================================
+
+In Python 3.3 a mechanism for indicating that exceptions were caused by other
+exceptions was introduced:
+
+.. code-block:: py
+
+    try:
+        print(1 / 0)
+    except Exception as exc:
+        raise RuntimeError("could not divide by zero") from exc
+
+To do a similar thing in pybind11, you can use the ``py::raise_from`` function. It
+sets the current python error indicator, so to continue propagating the exception
+you should ``throw py::error_already_set()`` (Python 3 only).
+
+.. code-block:: cpp
+
+    try {
+        py::eval("print(1 / 0"));
+    } catch (py::error_already_set &e) {
+        py::raise_from(e, PyExc_RuntimeError, "could not divide by zero");
+        throw py::error_already_set();
+    }
+
+.. versionadded:: 2.8
+
 .. _unraisable_exceptions:
 
 Handling unraisable exceptions

include/pybind11/detail/common.h

@@ -315,6 +315,19 @@ extern "C" {
         }                                                                      \
     }
 
+#if PY_VERSION_HEX >= 0x03030000
+
+#define PYBIND11_CATCH_INIT_EXCEPTIONS \
+        catch (pybind11::error_already_set &e) {                                 \
+            pybind11::raise_from(e, PyExc_ImportError, "initialization failed"); \
+            return nullptr;                                                      \
+        } catch (const std::exception &e) {                                      \
+            PyErr_SetString(PyExc_ImportError, e.what());                        \
+            return nullptr;                                                      \
+        }                                                                        \
+
+#else
+
 #define PYBIND11_CATCH_INIT_EXCEPTIONS \
         catch (pybind11::error_already_set &e) {                               \
             PyErr_SetString(PyExc_ImportError, e.what());                      \
@@ -324,6 +337,8 @@ extern "C" {
             return nullptr;                                                    \
         }                                                                      \
 
+#endif
+
 /** \rst
     ***Deprecated in favor of PYBIND11_MODULE***
 

include/pybind11/pytypes.h

@@ -382,6 +382,47 @@ private:
 #  pragma warning(pop)
 #endif
 
+#if PY_VERSION_HEX >= 0x03030000
+
+/// Replaces the current Python error indicator with the chosen error, performing a
+/// 'raise from' to indicate that the chosen error was caused by the original error.
+inline void raise_from(PyObject *type, const char *message) {
+    // Based on _PyErr_FormatVFromCause:
+    // https://github.com/python/cpython/blob/467ab194fc6189d9f7310c89937c51abeac56839/Python/errors.c#L405
+    // See https://github.com/pybind/pybind11/pull/2112 for details.
+    PyObject *exc = nullptr, *val = nullptr, *val2 = nullptr, *tb = nullptr;
+
+    assert(PyErr_Occurred());
+    PyErr_Fetch(&exc, &val, &tb);
+    PyErr_NormalizeException(&exc, &val, &tb);
+    if (tb != nullptr) {
+        PyException_SetTraceback(val, tb);
+        Py_DECREF(tb);
+    }
+    Py_DECREF(exc);
+    assert(!PyErr_Occurred());
+
+    PyErr_SetString(type, message);
+
+    PyErr_Fetch(&exc, &val2, &tb);
+    PyErr_NormalizeException(&exc, &val2, &tb);
+    Py_INCREF(val);
+    PyException_SetCause(val2, val);
+    PyException_SetContext(val2, val);
+    PyErr_Restore(exc, val2, tb);
+}
+
+/// Sets the current Python error indicator with the chosen error, performing a 'raise from'
+/// from the error contained in error_already_set to indicate that the chosen error was
+/// caused by the original error. After this function is called error_already_set will
+/// no longer contain an error.
+inline void raise_from(error_already_set& err, PyObject *type, const char *message) {
+    err.restore();
+    raise_from(type, message);
+}
+
+#endif
+
 /** \defgroup python_builtins _
     Unless stated otherwise, the following C++ functions behave the same
     as their Python counterparts.

tests/test_embed/test_interpreter.cpp

@@ -74,8 +74,24 @@ TEST_CASE("Import error handling") {
     REQUIRE_NOTHROW(py::module_::import("widget_module"));
     REQUIRE_THROWS_WITH(py::module_::import("throw_exception"),
                         "ImportError: C++ Error");
+#if PY_VERSION_HEX >= 0x03030000
+    REQUIRE_THROWS_WITH(py::module_::import("throw_error_already_set"),
+                        Catch::Contains("ImportError: initialization failed"));
+
+    auto locals = py::dict("is_keyerror"_a=false, "message"_a="not set");
+    py::exec(R"(
+        try:
+            import throw_error_already_set
+        except ImportError as e:
+            is_keyerror = type(e.__cause__) == KeyError
+            message = str(e.__cause__)
+    )", py::globals(), locals);
+    REQUIRE(locals["is_keyerror"].cast<bool>() == true);
+    REQUIRE(locals["message"].cast<std::string>() == "'missing'");
+#else
     REQUIRE_THROWS_WITH(py::module_::import("throw_error_already_set"),
                         Catch::Contains("ImportError: KeyError"));
+#endif
 }
 
 TEST_CASE("There can be only one interpreter") {

tests/test_exceptions.cpp

@@ -262,4 +262,24 @@ TEST_SUBMODULE(exceptions, m) {
     m.def("simple_bool_passthrough", [](bool x) {return x;});
 
     m.def("throw_should_be_translated_to_key_error", []() { throw shared_exception(); });
+
+#if PY_VERSION_HEX >= 0x03030000
+
+    m.def("raise_from", []() {
+        PyErr_SetString(PyExc_ValueError, "inner");
+        py::raise_from(PyExc_ValueError, "outer");
+        throw py::error_already_set();
+    });
+
+    m.def("raise_from_already_set", []() {
+        try {
+            PyErr_SetString(PyExc_ValueError, "inner");
+            throw py::error_already_set();
+        } catch (py::error_already_set& e) {
+            py::raise_from(e, PyExc_ValueError, "outer");
+            throw py::error_already_set();
+        }
+    });
+
+#endif
 }

tests/test_exceptions.py

@@ -24,6 +24,22 @@ def test_error_already_set(msg):
     assert msg(excinfo.value) == "foo"
 
 
+@pytest.mark.skipif("env.PY2")
+def test_raise_from(msg):
+    with pytest.raises(ValueError) as excinfo:
+        m.raise_from()
+    assert msg(excinfo.value) == "outer"
+    assert msg(excinfo.value.__cause__) == "inner"
+
+
+@pytest.mark.skipif("env.PY2")
+def test_raise_from_already_set(msg):
+    with pytest.raises(ValueError) as excinfo:
+        m.raise_from_already_set()
+    assert msg(excinfo.value) == "outer"
+    assert msg(excinfo.value.__cause__) == "inner"
+
+
 def test_cross_module_exceptions(msg):
     with pytest.raises(RuntimeError) as excinfo:
         cm.raise_runtime_error()