# Python Function
Pyo3 supports two ways to define a function in python. Both require registering
the function to a [module](./module.md)
One way is defining the function in the module definition.
```rust
#![feature(proc_macro)]
extern crate pyo3;
use pyo3::prelude::*;
#[pymodinit]
fn rust2py(py: Python, m: &PyModule) -> PyResult<()> {
// Note that the `#[pyfn()]` annotation automatically converts the arguments from
// Python objects to Rust values; and the Rust return value back into a Python object.
#[pyfn(m, "sum_as_string")]
fn sum_as_string_py(_py: Python, a:i64, b:i64) -> PyResult<String> {
Ok(format!("{}", a + b).to_string())
}
Ok(())
}
# fn main() {}
```
The other is annotating a function with `#[py::function]` and then adding it
to the module using the `add_function_to_module!` macro, which takes the module
as first parameter, the function name as second and an instance of `Python`
as third.
```rust
#![feature(specialization, use_extern_macros)]
#[macro_use]
extern crate pyo3;
use pyo3::prelude::*;
#[pyfunction]
fn double(x: usize) -> usize {
x * 2
}
#[pymodinit]
fn module_with_functions(py: Python, m: &PyModule) -> PyResult<()> {
m.add_function(wrap_function!(double)).unwrap();
Ok(())
}
# fn main() {}
```