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299 lines
13 KiB
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Exceptions
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##########
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Built-in C++ to Python exception translation
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============================================
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When Python calls C++ code through pybind11, pybind11 provides a C++ exception handler
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that will trap C++ exceptions, translate them to the corresponding Python exception,
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and raise them so that Python code can handle them.
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pybind11 defines translations for ``std::exception`` and its standard
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subclasses, and several special exception classes that translate to specific
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Python exceptions. Note that these are not actually Python exceptions, so they
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cannot be examined using the Python C API. Instead, they are pure C++ objects
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that pybind11 will translate the corresponding Python exception when they arrive
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at its exception handler.
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.. tabularcolumns:: |p{0.5\textwidth}|p{0.45\textwidth}|
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+--------------------------------------+--------------------------------------+
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| Exception thrown by C++ | Translated to Python exception type |
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+======================================+======================================+
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| :class:`std::exception` | ``RuntimeError`` |
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+--------------------------------------+--------------------------------------+
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| :class:`std::bad_alloc` | ``MemoryError`` |
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+--------------------------------------+--------------------------------------+
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| :class:`std::domain_error` | ``ValueError`` |
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+--------------------------------------+--------------------------------------+
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| :class:`std::invalid_argument` | ``ValueError`` |
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+--------------------------------------+--------------------------------------+
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| :class:`std::length_error` | ``ValueError`` |
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+--------------------------------------+--------------------------------------+
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| :class:`std::out_of_range` | ``IndexError`` |
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+--------------------------------------+--------------------------------------+
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| :class:`std::range_error` | ``ValueError`` |
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+--------------------------------------+--------------------------------------+
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| :class:`std::overflow_error` | ``OverflowError`` |
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+--------------------------------------+--------------------------------------+
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| :class:`pybind11::stop_iteration` | ``StopIteration`` (used to implement |
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| | custom iterators) |
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+--------------------------------------+--------------------------------------+
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| :class:`pybind11::index_error` | ``IndexError`` (used to indicate out |
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| | of bounds access in ``__getitem__``, |
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| | ``__setitem__``, etc.) |
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+--------------------------------------+--------------------------------------+
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| :class:`pybind11::value_error` | ``ValueError`` (used to indicate |
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| | wrong value passed in |
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| | ``container.remove(...)``) |
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+--------------------------------------+--------------------------------------+
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| :class:`pybind11::key_error` | ``KeyError`` (used to indicate out |
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| | of bounds access in ``__getitem__``, |
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| | ``__setitem__`` in dict-like |
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| | objects, etc.) |
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+--------------------------------------+--------------------------------------+
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Exception translation is not bidirectional. That is, *catching* the C++
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exceptions defined above above will not trap exceptions that originate from
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Python. For that, catch :class:`pybind11::error_already_set`. See :ref:`below
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<handling_python_exceptions_cpp>` for further details.
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There is also a special exception :class:`cast_error` that is thrown by
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:func:`handle::call` when the input arguments cannot be converted to Python
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objects.
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Registering custom translators
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==============================
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If the default exception conversion policy described above is insufficient,
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pybind11 also provides support for registering custom exception translators.
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To register a simple exception conversion that translates a C++ exception into
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a new Python exception using the C++ exception's ``what()`` method, a helper
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function is available:
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.. code-block:: cpp
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py::register_exception<CppExp>(module, "PyExp");
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This call creates a Python exception class with the name ``PyExp`` in the given
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module and automatically converts any encountered exceptions of type ``CppExp``
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into Python exceptions of type ``PyExp``.
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It is possible to specify base class for the exception using the third
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parameter, a `handle`:
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.. code-block:: cpp
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py::register_exception<CppExp>(module, "PyExp", PyExc_RuntimeError);
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Then `PyExp` can be caught both as `PyExp` and `RuntimeError`.
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The class objects of the built-in Python exceptions are listed in the Python
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documentation on `Standard Exceptions <https://docs.python.org/3/c-api/exceptions.html#standard-exceptions>`_.
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The default base class is `PyExc_Exception`.
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When more advanced exception translation is needed, the function
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``py::register_exception_translator(translator)`` can be used to register
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functions that can translate arbitrary exception types (and which may include
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additional logic to do so). The function takes a stateless callable (e.g. a
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function pointer or a lambda function without captured variables) with the call
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signature ``void(std::exception_ptr)``.
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When a C++ exception is thrown, the registered exception translators are tried
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in reverse order of registration (i.e. the last registered translator gets the
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first shot at handling the exception).
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Inside the translator, ``std::rethrow_exception`` should be used within
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a try block to re-throw the exception. One or more catch clauses to catch
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the appropriate exceptions should then be used with each clause using
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``PyErr_SetString`` to set a Python exception or ``ex(string)`` to set
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the python exception to a custom exception type (see below).
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To declare a custom Python exception type, declare a ``py::exception`` variable
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and use this in the associated exception translator (note: it is often useful
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to make this a static declaration when using it inside a lambda expression
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without requiring capturing).
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The following example demonstrates this for a hypothetical exception classes
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``MyCustomException`` and ``OtherException``: the first is translated to a
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custom python exception ``MyCustomError``, while the second is translated to a
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standard python RuntimeError:
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.. code-block:: cpp
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static py::exception<MyCustomException> exc(m, "MyCustomError");
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py::register_exception_translator([](std::exception_ptr p) {
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try {
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if (p) std::rethrow_exception(p);
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} catch (const MyCustomException &e) {
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exc(e.what());
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} catch (const OtherException &e) {
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PyErr_SetString(PyExc_RuntimeError, e.what());
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}
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});
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Multiple exceptions can be handled by a single translator, as shown in the
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example above. If the exception is not caught by the current translator, the
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previously registered one gets a chance.
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If none of the registered exception translators is able to handle the
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exception, it is handled by the default converter as described in the previous
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section.
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.. seealso::
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The file :file:`tests/test_exceptions.cpp` contains examples
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of various custom exception translators and custom exception types.
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.. note::
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Call either ``PyErr_SetString`` or a custom exception's call
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operator (``exc(string)``) for every exception caught in a custom exception
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translator. Failure to do so will cause Python to crash with ``SystemError:
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error return without exception set``.
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Exceptions that you do not plan to handle should simply not be caught, or
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may be explicitly (re-)thrown to delegate it to the other,
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previously-declared existing exception translators.
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.. _handling_python_exceptions_cpp:
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Handling exceptions from Python in C++
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======================================
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When C++ calls Python functions, such as in a callback function or when
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manipulating Python objects, and Python raises an ``Exception``, pybind11
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converts the Python exception into a C++ exception of type
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:class:`pybind11::error_already_set` whose payload contains a C++ string textual
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summary and the actual Python exception. ``error_already_set`` is used to
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propagate Python exception back to Python (or possibly, handle them in C++).
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.. tabularcolumns:: |p{0.5\textwidth}|p{0.45\textwidth}|
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+--------------------------------------+--------------------------------------+
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| Exception raised in Python | Thrown as C++ exception type |
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+======================================+======================================+
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| Any Python ``Exception`` | :class:`pybind11::error_already_set` |
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+--------------------------------------+--------------------------------------+
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For example:
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.. code-block:: cpp
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try {
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// open("missing.txt", "r")
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auto file = py::module_::import("io").attr("open")("missing.txt", "r");
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auto text = file.attr("read")();
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file.attr("close")();
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} catch (py::error_already_set &e) {
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if (e.matches(PyExc_FileNotFoundError)) {
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py::print("missing.txt not found");
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} else if (e.match(PyExc_PermissionError)) {
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py::print("missing.txt found but not accessible");
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} else {
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throw;
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}
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}
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Note that C++ to Python exception translation does not apply here, since that is
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a method for translating C++ exceptions to Python, not vice versa. The error raised
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from Python is always ``error_already_set``.
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This example illustrates this behavior:
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.. code-block:: cpp
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try {
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py::eval("raise ValueError('The Ring')");
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} catch (py::value_error &boromir) {
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// Boromir never gets the ring
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assert(false);
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} catch (py::error_already_set &frodo) {
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// Frodo gets the ring
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py::print("I will take the ring");
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}
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try {
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// py::value_error is a request for pybind11 to raise a Python exception
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throw py::value_error("The ball");
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} catch (py::error_already_set &cat) {
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// cat won't catch the ball since
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// py::value_error is not a Python exception
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assert(false);
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} catch (py::value_error &dog) {
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// dog will catch the ball
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py::print("Run Spot run");
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throw; // Throw it again (pybind11 will raise ValueError)
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}
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Handling errors from the Python C API
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=====================================
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Where possible, use :ref:`pybind11 wrappers <wrappers>` instead of calling
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the Python C API directly. When calling the Python C API directly, in
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addition to manually managing reference counts, one must follow the pybind11
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error protocol, which is outlined here.
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After calling the Python C API, if Python returns an error,
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``throw py::error_already_set();``, which allows pybind11 to deal with the
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exception and pass it back to the Python interpreter. This includes calls to
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the error setting functions such as ``PyErr_SetString``.
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.. code-block:: cpp
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PyErr_SetString(PyExc_TypeError, "C API type error demo");
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throw py::error_already_set();
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// But it would be easier to simply...
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throw py::type_error("pybind11 wrapper type error");
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Alternately, to ignore the error, call `PyErr_Clear
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<https://docs.python.org/3/c-api/exceptions.html#c.PyErr_Clear>`_.
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Any Python error must be thrown or cleared, or Python/pybind11 will be left in
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an invalid state.
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.. _unraisable_exceptions:
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Handling unraisable exceptions
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==============================
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If a Python function invoked from a C++ destructor or any function marked
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``noexcept(true)`` (collectively, "noexcept functions") throws an exception, there
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is no way to propagate the exception, as such functions may not throw.
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Should they throw or fail to catch any exceptions in their call graph,
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the C++ runtime calls ``std::terminate()`` to abort immediately.
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Similarly, Python exceptions raised in a class's ``__del__`` method do not
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propagate, but are logged by Python as an unraisable error. In Python 3.8+, a
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`system hook is triggered
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<https://docs.python.org/3/library/sys.html#sys.unraisablehook>`_
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and an auditing event is logged.
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Any noexcept function should have a try-catch block that traps
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class:`error_already_set` (or any other exception that can occur). Note that
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pybind11 wrappers around Python exceptions such as
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:class:`pybind11::value_error` are *not* Python exceptions; they are C++
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exceptions that pybind11 catches and converts to Python exceptions. Noexcept
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functions cannot propagate these exceptions either. A useful approach is to
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convert them to Python exceptions and then ``discard_as_unraisable`` as shown
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below.
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.. code-block:: cpp
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void nonthrowing_func() noexcept(true) {
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try {
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// ...
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} catch (py::error_already_set &eas) {
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// Discard the Python error using Python APIs, using the C++ magic
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// variable __func__. Python already knows the type and value and of the
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// exception object.
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eas.discard_as_unraisable(__func__);
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} catch (const std::exception &e) {
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// Log and discard C++ exceptions.
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third_party::log(e);
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}
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}
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.. versionadded:: 2.6
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