# Python exceptions
## Defining a new exception
You can use the [`create_exception!`] macro to define a new exception type:
```rust
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:
```rust
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::<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',)");
});
```
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:
```rust
use pyo3::prelude::*;
use pyo3::exceptions::PyException;
pyo3::create_exception!(mymodule, CustomError, PyException);
#[pymodule]
fn mymodule(py: Python<'_>, m: &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](./function/error_handling.md) 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:
```rust
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`](https://docs.python.org/3/library/functions.html#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.
```rust
use pyo3::Python;
use pyo3::types::{PyBool, PyList};
Python::with_gil(|py| {
assert!(PyBool::new(py, true).is_instance_of::<PyBool>().unwrap());
let list = PyList::new(py, &[1, 2, 3, 4]);
assert!(!list.is_instance_of::<PyBool>().unwrap());
assert!(list.is_instance_of::<PyList>().unwrap());
});
```
To check the type of an exception, you can similarly do:
```rust
# 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.
```rust
#![allow(dead_code)]
use pyo3::prelude::*;
mod io {
pyo3::import_exception!(io, UnsupportedOperation);
}
fn tell(file: &PyAny) -> PyResult<u64> {
match file.call_method0("tell") {
Err(_) => Err(io::UnsupportedOperation::new_err("not supported: tell")),
Ok(x) => x.extract::<u64>(),
}
}
```
[`pyo3::exceptions`]({{#PYO3_DOCS_URL}}/pyo3/exceptions/index.html)
defines exceptions for several standard library modules.
[`create_exception!`]: {{#PYO3_DOCS_URL}}/pyo3/macro.create_exception.html
[`import_exception!`]: {{#PYO3_DOCS_URL}}/pyo3/macro.import_exception.html
[`PyErr`]: {{#PYO3_DOCS_URL}}/pyo3/struct.PyErr.html
[`PyResult`]: {{#PYO3_DOCS_URL}}/pyo3/type.PyResult.html
[`PyErr::from_value`]: {{#PYO3_DOCS_URL}}/pyo3/struct.PyErr.html#method.from_value
[`PyAny::is_instance`]: {{#PYO3_DOCS_URL}}/pyo3/struct.PyAny.html#method.is_instance
[`PyAny::is_instance_of`]: {{#PYO3_DOCS_URL}}/pyo3/struct.PyAny.html#method.is_instance_of