Exceptions

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 exceptions:

C++ exception type Python exception type
std::exception RuntimeError
std::bad_alloc MemoryError
std::domain_error ValueError
std::invalid_argument ValueError
std::length_error ValueError
std::out_of_range ValueError
std::range_error ValueError
pybind11::stop_iteration StopIteration (used to implement custom iterators)
pybind11::index_error IndexError (used to indicate out of bounds accesses in __getitem__, __setitem__, etc.)
pybind11::value_error ValueError (used to indicate wrong value passed in container.remove(...)
pybind11::key_error KeyError (used to indicate out of bounds accesses in __getitem__, __setitem__ in dict-like objects, etc.)
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 error_already_set whose string payload contains a textual summary.

There is also a special exception cast_error that is thrown by handle::call() when the input arguments cannot be converted to Python objects.

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:

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:

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) {
        exc(e.what());
    } 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 section.

See also

The 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.