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);
Python::with_gil(|py| {
let ctx = [("CustomError", py.get_type_bound::<CustomError>())].into_py_dict_bound(py);
pyo3::py_run!(
py,
*ctx,
"assert str(CustomError) == \"<class 'mymodule.CustomError'>\""
);
pyo3::py_run!(py, *ctx, "assert CustomError('oops').args == ('oops',)");
});
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_bound::<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};
Python::with_gil(|py| {
assert!(PyBool::new_bound(py, true).is_instance_of::<PyBool>());
let list = PyList::new_bound(py, &[1, 2, 3, 4]);
assert!(!list.is_instance_of::<PyBool>());
assert!(list.is_instance_of::<PyList>());
});
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.