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Built-in exception translation
When C++ code invoked from Python throws an ``std::exception``, it is
automatically converted into a Python ``Exception``. pybind11 defines multiple
special exception classes that will map to different types of Python
.. tabularcolumns:: |p{0.5\textwidth}|p{0.45\textwidth}|
| C++ exception type | Python exception type |
| :class:`std::exception` | ``RuntimeError`` |
| :class:`std::bad_alloc` | ``MemoryError`` |
| :class:`std::domain_error` | ``ValueError`` |
| :class:`std::invalid_argument` | ``ValueError`` |
| :class:`std::length_error` | ``ValueError`` |
| :class:`std::out_of_range` | ``ValueError`` |
| :class:`std::range_error` | ``ValueError`` |
| :class:`pybind11::stop_iteration` | ``StopIteration`` (used to |
| | implement custom iterators) |
| :class:`pybind11::index_error` | ``IndexError`` (used to |
| | indicate out of bounds |
| | accesses in ``__getitem__``, |
| | ``__setitem__``, etc.) |
| :class:`pybind11::value_error` | ``ValueError`` (used to |
| | indicate wrong value passed |
| | in ``container.remove(...)`` |
| :class:`pybind11::key_error` | ``KeyError`` (used to |
| | indicate out of bounds |
| | accesses in ``__getitem__``, |
| | ``__setitem__`` in dict-like |
| | objects, etc.) |
| :class:`pybind11::error_already_set` | Indicates that the Python |
| | exception flag has already |
| | been initialized |
When a Python function invoked from C++ throws an exception, it is converted
into a C++ exception of type :class:`error_already_set` whose string payload
contains a textual summary.
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
Registering custom translators
If the default exception conversion policy described above is insufficient,
pybind11 also provides support for registering custom exception translators.
To register a simple exception conversion that translates a C++ exception into
a new Python exception using the C++ exception's ``what()`` method, a helper
function is available:
.. code-block:: cpp
py::register_exception<CppExp>(module, "PyExp");
This call creates a Python exception class with the name ``PyExp`` in the given
module and automatically converts any encountered exceptions of type ``CppExp``
into Python exceptions of type ``PyExp``.
When more advanced exception translation is needed, the function
``py::register_exception_translator(translator)`` can be used to register
functions that can translate arbitrary exception types (and which may include
additional logic to do so). The function takes a stateless callable (e.g. a
function pointer or a lambda function without captured variables) with the call
signature ``void(std::exception_ptr)``.
When a C++ exception is thrown, the registered exception translators are tried
in reverse order of registration (i.e. the last registered translator gets the
first shot at handling the exception).
Inside the translator, ``std::rethrow_exception`` should be used within
a try block to re-throw the exception. One or more catch clauses to catch
the appropriate exceptions should then be used with each clause using
``PyErr_SetString`` to set a Python exception or ``ex(string)`` to set
the python exception to a custom exception type (see below).
To declare a custom Python exception type, declare a ``py::exception`` variable
and use this in the associated exception translator (note: it is often useful
to make this a static declaration when using it inside a lambda expression
without requiring capturing).
The following example demonstrates this for a hypothetical exception classes
``MyCustomException`` and ``OtherException``: the first is translated to a
custom python exception ``MyCustomError``, while the second is translated to a
standard python RuntimeError:
.. code-block:: cpp
static py::exception<MyCustomException> exc(m, "MyCustomError");
py::register_exception_translator([](std::exception_ptr p) {
try {
if (p) std::rethrow_exception(p);
} catch (const MyCustomException &e) {
} catch (const OtherException &e) {
PyErr_SetString(PyExc_RuntimeError, e.what());
Multiple exceptions can be handled by a single translator, as shown in the
example above. 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
.. seealso::
The file :file:`tests/test_exceptions.cpp` contains examples
of various custom exception translators and custom exception types.
.. note::
You must call either ``PyErr_SetString`` or a custom exception's call
operator (``exc(string)``) 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, or
may be explicity (re-)thrown to delegate it to the other,
previously-declared existing exception translators.