Add support for user defined exception translators

docs/advanced.rst

@@ -817,6 +817,8 @@ In other words, :func:`init` creates an anonymous function that invokes an
 in-place constructor. Memory allocation etc. is already take care of beforehand
 within pybind11.
 
+.. _catching_and_throwing_exceptions:
+
 Catching and throwing exceptions
 ================================
 
@@ -869,6 +871,65 @@ There is also a special exception :class:`cast_error` that is thrown by
 :func:`handle::call` when the input arguments cannot be converted to Python
 objects.
 
+Registering custom exception translators
+========================================
+
+If the default exception conversion policy described
+:ref:`above <catching_and_throwing_exceptions>`
+is insufficient, pybind11 also provides support for registering custom
+exception translators.
+
+The function ``register_exception_translator(translator)`` takes a stateless
+callable (e.g. a function pointer or a lambda function without captured
+variables) with the following call signature: ``void(std::exception_ptr)``.
+
+When a C++ exception is thrown, registered exception translators are tried
+in reverse order of registration (i.e. the last registered translator gets
+a first shot at handling the exception).
+
+Inside the translator, ``std::rethrow_exception`` should be used within
+a try block to re-throw the exception. A catch clause can then use
+``PyErr_SetString`` to set a Python exception as demonstrated
+in :file:`example19.cpp``.
+
+This example also demonstrates how to create custom exception types
+with ``py::exception``.
+
+The following example demonstrates this for a hypothetical exception class
+``MyCustomException``:
+
+.. code-block:: cpp
+
+    py::register_exception_translator([](std::exception_ptr p) {
+        try {
+            if (p) std::rethrow_exception(p);
+        } catch (const MyCustomException &e) {
+            PyErr_SetString(PyExc_RuntimeError, e.what());
+        }
+    });
+
+Multiple exceptions can be handled by a single translator. If the exception is
+not caught by the current translator, the previously registered one gets a
+chance.
+
+If none of the registered exception translators is able to handle the
+exception, it is handled by the default converter as described in the previous
+section.
+
+.. note::
+
+    You must either call ``PyErr_SetString`` for every exception caught in a
+    custom exception translator. Failure to do so will cause Python to crash
+    with ``SystemError: error return without exception set``.
+
+    Exceptions that you do not plan to handle should simply not be caught.
+
+    You may also choose to explicity (re-)throw the exception to delegate it to
+    the other existing exception translators.
+
+    The ``py::exception`` wrapper for creating custom exceptions cannot (yet)
+    be used as a ``py::base``.
+
 .. _opaque:
 
 Treating STL data structures as opaque objects

example/CMakeLists.txt

@@ -25,6 +25,7 @@ set(PYBIND11_EXAMPLES
   example16.cpp
   example17.cpp
   example18.cpp
+  example19.cpp
   issues.cpp
 )
 

example/example.cpp

@@ -27,6 +27,7 @@ void init_ex15(py::module &);
 void init_ex16(py::module &);
 void init_ex17(py::module &);
 void init_ex18(py::module &);
+void init_ex19(py::module &);
 void init_issues(py::module &);
 
 #if defined(PYBIND11_TEST_EIGEN)
@@ -54,6 +55,7 @@ PYBIND11_PLUGIN(example) {
     init_ex16(m);
     init_ex17(m);
     init_ex18(m);
+    init_ex19(m);
     init_issues(m);
 
     #if defined(PYBIND11_TEST_EIGEN)

example/example19.cpp

+/*
+    example/example19.cpp -- exception translation
+
+    Copyright (c) 2016 Pim Schellart <P.Schellart@princeton.edu>
+
+    All rights reserved. Use of this source code is governed by a
+    BSD-style license that can be found in the LICENSE file.
+*/
+
+#include "example.h"
+
+// A type that should be raised as an exeption in Python
+class MyException : public std::exception {
+public:
+    explicit MyException(const char * m) : message{m} {}
+    virtual const char * what() const noexcept override {return message.c_str();}
+private:
+    std::string message = "";
+};
+
+// A type that should be translated to a standard Python exception
+class MyException2 : public std::exception {
+public:
+    explicit MyException2(const char * m) : message{m} {}
+    virtual const char * what() const noexcept override {return message.c_str();}
+private:
+    std::string message = "";
+};
+
+// A type that is not derived from std::exception (and is thus unknown)
+class MyException3 {
+public:
+    explicit MyException3(const char * m) : message{m} {}
+    virtual const char * what() const noexcept {return message.c_str();}
+private:
+    std::string message = "";
+};
+
+// A type that should be translated to MyException
+// and delegated to its exception translator
+class MyException4 : public std::exception {
+public:
+    explicit MyException4(const char * m) : message{m} {}
+    virtual const char * what() const noexcept override {return message.c_str();}
+private:
+    std::string message = "";
+};
+
+void throws1() {
+    throw MyException("this error should go to a custom type");
+}
+
+void throws2() {
+    throw MyException2("this error should go to a standard Python exception");
+}
+
+void throws3() {
+    throw MyException3("this error cannot be translated");
+}
+
+void throws4() {
+    throw MyException4("this error is rethrown");
+}
+
+void throws_logic_error() {
+    throw std::logic_error("this error should fall through to the standard handler");
+}
+
+void init_ex19(py::module &m) {
+    // make a new custom exception and use it as a translation target
+    static py::exception<MyException> ex(m, "MyException");
+    py::register_exception_translator([](std::exception_ptr p) {
+        try {
+            if (p) std::rethrow_exception(p);
+        } catch (const MyException &e) {
+            PyErr_SetString(ex.ptr(), e.what());
+        }
+    });
+
+    // register new translator for MyException2
+    // no need to store anything here because this type will
+    // never by visible from Python
+    py::register_exception_translator([](std::exception_ptr p) {
+        try {
+            if (p) std::rethrow_exception(p);
+        } catch (const MyException2 &e) {
+            PyErr_SetString(PyExc_RuntimeError, e.what());
+        }
+    });
+
+    // register new translator for MyException4
+    // which will catch it and delegate to the previously registered
+    // translator for MyException by throwing a new exception
+    py::register_exception_translator([](std::exception_ptr p) {
+        try {
+            if (p) std::rethrow_exception(p);
+        } catch (const MyException4 &e) {
+            throw MyException(e.what());
+        }
+    });
+
+    m.def("throws1", &throws1);
+    m.def("throws2", &throws2);
+    m.def("throws3", &throws3);
+    m.def("throws4", &throws4);
+    m.def("throws_logic_error", &throws_logic_error);
+}
+

example/example19.py

+#!/usr/bin/env python
+from __future__ import print_function
+import sys
+sys.path.append('.')
+
+import example
+
+print("Can we catch a MyException?")
+try:
+    example.throws1()
+except example.MyException as e:
+    print(e.__class__.__name__, ":", e)
+print("")
+
+print("Can we translate to standard Python exceptions?")
+try:
+    example.throws2()
+except Exception as e:
+    print(e.__class__.__name__, ":", e)
+print("")
+
+print("Can we handle unknown exceptions?")
+try:
+    example.throws3()
+except Exception as e:
+    print(e.__class__.__name__, ":", e)
+print("")
+
+print("Can we delegate to another handler by rethrowing?")
+try:
+    example.throws4()
+except example.MyException as e:
+    print(e.__class__.__name__, ":", e)
+print("")
+
+print("Can we fall-through to the default handler?")
+try:
+    example.throws_logic_error()
+except Exception as e:
+    print(e.__class__.__name__, ":", e)
+print("")
+

example/example19.ref

+Can we catch a MyException?
+MyException : this error should go to a custom type
+
+Can we translate to standard Python exceptions?
+RuntimeError : this error should go to a standard Python exception
+
+Can we handle unknown exceptions?
+RuntimeError : Caught an unknown exception!
+
+Can we delegate to another handler by rethrowing?
+MyException : this error is rethrown
+
+Can we fall-through to the default handler?
+RuntimeError : this error should fall through to the standard handler
+

include/pybind11/cast.h

@@ -49,6 +49,27 @@ PYBIND11_NOINLINE inline internals &get_internals() {
             internals_ptr->istate = tstate->interp;
         #endif
         builtins[id] = capsule(internals_ptr);
+        internals_ptr->registered_exception_translators.push_front(
+            [](std::exception_ptr p) -> void {
+                try {
+                    if (p) std::rethrow_exception(p);
+                } catch (const error_already_set &)      {                                                 return;
+                } catch (const index_error &e)           { PyErr_SetString(PyExc_IndexError,    e.what()); return;
+                } catch (const value_error &e)           { PyErr_SetString(PyExc_ValueError,    e.what()); return;
+                } catch (const stop_iteration &e)        { PyErr_SetString(PyExc_StopIteration, e.what()); return;
+                } catch (const std::bad_alloc &e)        { PyErr_SetString(PyExc_MemoryError,   e.what()); return;
+                } catch (const std::domain_error &e)     { PyErr_SetString(PyExc_ValueError,    e.what()); return;
+                } catch (const std::invalid_argument &e) { PyErr_SetString(PyExc_ValueError,    e.what()); return;
+                } catch (const std::length_error &e)     { PyErr_SetString(PyExc_ValueError,    e.what()); return;
+                } catch (const std::out_of_range &e)     { PyErr_SetString(PyExc_IndexError,    e.what()); return;
+                } catch (const std::range_error &e)      { PyErr_SetString(PyExc_ValueError,    e.what()); return;
+                } catch (const std::exception &e)        { PyErr_SetString(PyExc_RuntimeError,  e.what()); return;
+                } catch (...) {
+                    PyErr_SetString(PyExc_RuntimeError, "Caught an unknown exception!");
+                    return;
+                }
+            }
+        );
     }
     return *internals_ptr;
 }

include/pybind11/common.h

@@ -66,6 +66,7 @@
 #  pragma warning(pop)
 #endif
 
+#include <forward_list>
 #include <vector>
 #include <string>
 #include <stdexcept>
@@ -264,6 +265,7 @@ struct internals {
     std::unordered_map<const void *, void*> registered_types_py;     // PyTypeObject* -> type_info
     std::unordered_map<const void *, void*> registered_instances;    // void * -> PyObject*
     std::unordered_set<std::pair<const PyObject *, const char *>, overload_hash> inactive_overload_cache;
+    std::forward_list<void (*) (std::exception_ptr)> registered_exception_translators;
 #if defined(WITH_THREAD)
     decltype(PyThread_create_key()) tstate = 0; // Usually an int but a long on Cygwin64 with Python 3.x
     PyInterpreterState *istate = nullptr;

include/pybind11/pybind11.h

@@ -401,19 +401,32 @@ protected:
                 if (result.ptr() != PYBIND11_TRY_NEXT_OVERLOAD)
                     break;
             }
-        } catch (const error_already_set &)      {                                                 return nullptr;
-        } catch (const index_error &e)           { PyErr_SetString(PyExc_IndexError,    e.what()); return nullptr;
-        } catch (const value_error &e)           { PyErr_SetString(PyExc_ValueError,    e.what()); return nullptr;
-        } catch (const stop_iteration &e)        { PyErr_SetString(PyExc_StopIteration, e.what()); return nullptr;
-        } catch (const std::bad_alloc &e)        { PyErr_SetString(PyExc_MemoryError,   e.what()); return nullptr;
-        } catch (const std::domain_error &e)     { PyErr_SetString(PyExc_ValueError,    e.what()); return nullptr;
-        } catch (const std::invalid_argument &e) { PyErr_SetString(PyExc_ValueError,    e.what()); return nullptr;
-        } catch (const std::length_error &e)     { PyErr_SetString(PyExc_ValueError,    e.what()); return nullptr;
-        } catch (const std::out_of_range &e)     { PyErr_SetString(PyExc_IndexError,    e.what()); return nullptr;
-        } catch (const std::range_error &e)      { PyErr_SetString(PyExc_ValueError,    e.what()); return nullptr;
-        } catch (const std::exception &e)        { PyErr_SetString(PyExc_RuntimeError,  e.what()); return nullptr;
+        } catch (const error_already_set &) {
+            return nullptr;
         } catch (...) {
-            PyErr_SetString(PyExc_RuntimeError, "Caught an unknown exception!");
+            /* When an exception is caught, give each registered exception
+               translator a chance to translate it to a Python exception
+               in reverse order of registration.
+              
+               A translator may choose to do one of the following:
+              
+                - catch the exception and call PyErr_SetString or PyErr_SetObject
+                  to set a standard (or custom) Python exception, or
+                - do nothing and let the exception fall through to the next translator, or
+                - delegate translation to the next translator by throwing a new type of exception. */
+
+            auto last_exception = std::current_exception(); 
+            auto &registered_exception_translators = pybind11::detail::get_internals().registered_exception_translators;
+            for (auto& translator : registered_exception_translators) {
+                try {
+                    translator(last_exception);
+                } catch (...) {
+                    last_exception = std::current_exception();
+                    continue;
+                }
+                return nullptr;
+            }
+            PyErr_SetString(PyExc_SystemError, "Exception escaped from default exception translator!");
             return nullptr;
         }
 
@@ -1089,6 +1102,31 @@ template <typename InputType, typename OutputType> void implicitly_convertible()
     ((detail::type_info *) it->second)->implicit_conversions.push_back(implicit_caster);
 }
 
+template <typename ExceptionTranslator>
+void register_exception_translator(ExceptionTranslator&& translator) {
+    detail::get_internals().registered_exception_translators.push_front(
+        std::forward<ExceptionTranslator>(translator));
+}
+
+/* Wrapper to generate a new Python exception type.
+ *
+ * This should only be used with PyErr_SetString for now.
+ * It is not (yet) possible to use as a py::base.
+ * Template type argument is reserved for future use.
+ */
+template <typename type>
+class exception : public object {
+public:
+    exception(module &m, const std::string name, PyObject* base=PyExc_Exception) {
+        std::string full_name = std::string(PyModule_GetName(m.ptr()))
+                + std::string(".") + name;
+        char* exception_name = const_cast<char*>(full_name.c_str());
+        m_ptr = PyErr_NewException(exception_name, base, NULL);
+        inc_ref(); // PyModule_AddObject() steals a reference
+        PyModule_AddObject(m.ptr(), name.c_str(), m_ptr);
+    }
+};
+
 #if defined(WITH_THREAD)
 
 /* The functions below essentially reproduce the PyGILState_* API using a RAII