Question

I have some trouble handling custom C++ exceptions when calling from Cython.
My situation is the following: I have a library that uses CustomLibraryException for all exceptions. What I want is basically get the error message and raise a Python error with it.

The user guide has some hints but it is a bit unspecific.
The first possibility is to do:

cdef int bar() except +ValueError

This converts the CustomLibraryException to a ValueError, but loses the error message.

The other possibility is to explicitly convert the error using

cdef int raise_py_error()
cdef int something_dangerous() except +raise_py_error

I don’t really understant this solution. I understood that raise_py_error has to be a C++ function that somehow handles the error. I am not sure how to handle it though. The function doesn’t get an argument and is called inside the catch block in C++.

If any one has an working example of handling a C++ exception in Cython, that would be of great help.

Answer 1

If CustomLibraryException derives from std::runtime_error (as a well-behaved C++ exception should), then the behavior you’re seeing is a bug in Cython.

If it doesn’t, then the easiest thing to do is to wrap the C++ function you’re calling in a C++ helper function that translates the exception:

double foo(char const *, Bla const &);  // this is the one we're wrapping

double foo_that_throws_runtime_error(char const *str, Bla const &blaref)
{
    try {
        return foo(str, blaref);
    } catch (CustomLibraryException const &e) {
        throw std::runtime_error(e.get_the_message());
    }
}

This will cause a RuntimeError to be raised on the Python side. Alternatively, throw an std::invalid_argument to raise a ValueError, etc. (see the table in the page you linked to).