What Does PyErr_WriteUnraisable Do?

· 337 words · 2 minute read

What Does PyErr_WriteUnraisable Do? 🔗

Ever encountered a situation in Python where an error happens, but you can’t or don’t want to let it crash your program? That’s where PyErr_WriteUnraisable comes into play. Think of it as a safety net for exceptions. When an unforeseen error pops up, but you can’t or don’t want to handle it explicitly, this function is like putting your troubles in a “results pending” drawer: you note it down and move on.

How to Use PyErr_WriteUnraisable 🔗

Using PyErr_WriteUnraisable is straightforward. Here’s a sneak peek:

import sys

def some_function():
    try:
        # Some code that might raise an exception
        1 / 0
    except Exception as e:
        # Log the exception and continue running.
        sys.stderr.write(f"Exception occurred: {e}\n")
        sys.stderr.write("The operation will continue despite the exception.\n")

some_function()

In the above code, if an error occurs, rather than letting it crash the whole program, we catch it and write it out to stderr, but crucially, we keep going. That’s the basic idea behind PyErr_WriteUnraisable encapsulated in a more manual form.

Now, how about using the real deal? Here’s how:

#include <Python.h>

void raise_exception_safely() {
    PyObject *type, *value, *traceback;
    PyErr_Fetch(&type, &value, &traceback); // Fetch and clear the error indicator

    // Perform some operation
    // ...

    // Write unraisable error
    PyErr_WriteUnraisable(Py_None);

    // Clean up references
    Py_XDECREF(type);
    Py_XDECREF(value);
    Py_XDECREF(traceback);
}

In this example, PyErr_Fetch retrieves the current error and clears it. Then, PyErr_WriteUnraisable(Py_None) logs the error without interrupting the flow of the program.

How Does PyErr_WriteUnraisable Work? 🔗

Under the hood, PyErr_WriteUnraisable does a few things:

  1. Fetches the current error: It captures information about the current exception – the type, value, and traceback.

  2. Logs the error: It writes a message to stderr. The message contains the exception type and value, pointing out that the error occurred but wasn’t raised.

  3. Resets the error indicator: After logging, it clears the error indicator so that it doesn’t affect the rest of the program.

Imagine it like a vigilant assistant who keeps track of every misstep but doesn’t interrupt your meeting to point them out. Helpful, right?