What does error_already_set in Boost.python do and how to handle exceptions similarly in Python C API

Question

I have been working on a project where I want to remove the boost dependencies and replace it with the Python C API.

I spent some time understanding the Python C API and I saw this catch (error_already_set const &)

I read the boost docs but it explains where it is used. But I want to know why it is needed and how can I achieve the same functionality using the native Python C api.

Answer 1

Boost throws error_already_set when a Python error has occurred. So if you see code like this:

try {
    bp::exec(bp::str("garbage code is garbage"));
} catch (const bp::error_already_set&) {
    // your code here to examine Python traceback etc.
}

you'll replace it with:

your_ptr<PyObject> res = PyRun_String("garbage code is garbage");
if (!res) {
    // your code here to examine Python traceback etc.
}

In other words, wherever you see catch(error_already_set), there you will likely want to do some error handling using whatever PyObject* or other value is involved to recognize when an error has occurred (and therefore you can examine the traceback, or convert it into a C++ exception).