Python exceptions

Defining a new exception

Use the create_exception! macro:

use pyo3::create_exception;

create_exception!(module, MyError, pyo3::exceptions::PyException);

• module is the name of the containing module.
• MyError is the name of the new exception type.

For example:

use pyo3::prelude::*;
use pyo3::create_exception;
use pyo3::types::IntoPyDict;
use pyo3::exceptions::PyException;

create_exception!(mymodule, CustomError, PyException);

fn main() -> PyResult<()> {
Python::with_gil(|py| {
    let ctx = [("CustomError", py.get_type::<CustomError>())].into_py_dict(py)?;
    pyo3::py_run!(
        py,
        *ctx,
        "assert str(CustomError) == \"<class 'mymodule.CustomError'>\""
    );
    pyo3::py_run!(py, *ctx, "assert CustomError('oops').args == ('oops',)");
  Ok(())
})
}

When using PyO3 to create an extension module, you can add the new exception to the module like this, so that it is importable from Python:

use pyo3::prelude::*;
use pyo3::exceptions::PyException;

pyo3::create_exception!(mymodule, CustomError, PyException);

#[pymodule]
fn mymodule(py: Python<'_>, m: &Bound<'_, PyModule>) -> PyResult<()> {
    // ... other elements added to module ...
    m.add("CustomError", py.get_type::<CustomError>())?;

    Ok(())
}

Raising an exception

As described in the function error handling chapter, to raise an exception from a #[pyfunction] or #[pymethods], return an Err(PyErr). PyO3 will automatically raise this exception for you when returning the result to Python.

You can also manually write and fetch errors in the Python interpreter's global state:

use pyo3::{Python, PyErr};
use pyo3::exceptions::PyTypeError;

Python::with_gil(|py| {
    PyTypeError::new_err("Error").restore(py);
    assert!(PyErr::occurred(py));
    drop(PyErr::fetch(py));
});

Checking exception types

Python has an isinstance method to check an object's type. In PyO3 every object has the PyAny::is_instance and PyAny::is_instance_of methods which do the same thing.

use pyo3::prelude::*;
use pyo3::types::{PyBool, PyList};

fn main() -> PyResult<()> {
Python::with_gil(|py| {
    assert!(PyBool::new(py, true).is_instance_of::<PyBool>());
    let list = PyList::new(py, &[1, 2, 3, 4])?;
    assert!(!list.is_instance_of::<PyBool>());
    assert!(list.is_instance_of::<PyList>());
Ok(())
})
}

To check the type of an exception, you can similarly do:

use pyo3::exceptions::PyTypeError;
use pyo3::prelude::*;
Python::with_gil(|py| {
let err = PyTypeError::new_err(());
err.is_instance_of::<PyTypeError>(py);
});

Using exceptions defined in Python code

It is possible to use an exception defined in Python code as a native Rust type. The import_exception! macro allows importing a specific exception class and defines a Rust type for that exception.

#![allow(dead_code)]
use pyo3::prelude::*;

mod io {
    pyo3::import_exception!(io, UnsupportedOperation);
}

fn tell(file: &Bound<'_, PyAny>) -> PyResult<u64> {
    match file.call_method0("tell") {
        Err(_) => Err(io::UnsupportedOperation::new_err("not supported: tell")),
        Ok(x) => x.extract::<u64>(),
    }
}

pyo3::exceptions defines exceptions for several standard library modules.