Python Function
PyO3 supports two ways to define a function in python. Both require registering the function to a module
One way is defining the function in the module definition.
# extern crate pyo3; use pyo3::prelude::*; #[pymodule] 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 #[pyfunction]
and then adding it
to the module using the add_wrapped_to_module!
macro, which takes the module
as first parameter, the function name as second and an instance of Python
as third.
# extern crate pyo3; use pyo3::prelude::*; use pyo3::wrap_pyfunction; #[pyfunction] fn double(x: usize) -> usize { x * 2 } #[pymodule] fn module_with_functions(py: Python, m: &PyModule) -> PyResult<()> { m.add_wrapped(wrap_pyfunction!(double)).unwrap(); Ok(()) } # fn main() {}
Making the function signature available to Python
In order to make the function signature available to Python to be retrieved via
inspect.signature
, simply make sure the first line of your doc-string is
formatted like in the example below. Please note that the new-line after the
--
is mandatory. The /
signifies the end of positional only arguments. This
is not a feature of this library in particular, but the general format used by
CPython for annotating signatures of built-in functions. Function signatures for
built-ins are new in Python 3 — in Python 2, it is simply considered to be part
of the doc-string.
# #![allow(unused_variables)] #fn main() { # extern crate pyo3; use pyo3::prelude::*; /// add(a, b, /) /// -- /// /// This function adds two unsigned 64-bit integers. #[pyfunction] fn add(a: u64, b: u64) -> u64 { a + b } #}
When annotated like this, signatures are also correctly displayed in IPython.
>>> pyo3_test.add?
Signature: pyo3_test.add(a, b, /)
Docstring: This function adds two unsigned 64-bit integers.
Type: builtin_function_or_method
Closures
Currently, there are no conversions between Fn
s in rust and callables in python. This would definitely be possible and very useful, so contributions are welcome. In the meantime, you can do the following:
Calling a python function in rust
You can use ObjectProtocol::is_callable
to check if you got a callable, which is true for functions (including lambdas), methods and objects with a __call__
method. You can call the object with ObjectProtocol::call
with the args as first parameter and the kwargs (or None
) as second parameter. There are also ObjectProtocol::call0
with no args and ObjectProtocol::call1
with only the args.
Calling rust Fn
s in python
If you have a static function, you can expose it with #[pyfunction]
and use wrap_pyfunction!
to get the corresponding PyObject
. For dynamic functions, e.g. lambda and functions that were passed as arguments, you must put them in some kind of owned container, e.g. a box. (Long-Term a special container similar to wasm-bindgen's Closure
should take care of that). You can than use a #[pyclass]
struct with that container as field as a way to pass the function over the ffi-barrier. You can even make that class callable with __call__
so it looks like a function in python code.