Emulating callable objects

Classes can be callable if they have a #[pymethod] named __call__. This allows instances of a class to behave similar to functions.

This method's signature must look like __call__(<self>, ...) -> object - here, any argument list can be defined as for normal pymethods

Example: Implementing a call counter

The following pyclass is a basic decorator - its constructor takes a Python object as argument and calls that object when called. An equivalent Python implementation is linked at the end.

An example crate containing this pyclass can be found here

#![allow(unused)]
fn main() {
use pyo3::prelude::*;
use pyo3::types::{PyDict, PyTuple};

/// A function decorator that keeps track how often it is called.
///
/// It otherwise doesn't do anything special.
#[pyclass(name = "Counter")]
pub struct PyCounter {
    // We use `#[pyo3(get)]` so that python can read the count but not mutate it.
    #[pyo3(get)]
    count: u64,

    // This is the actual function being wrapped.
    wraps: Py<PyAny>,
}

#[pymethods]
impl PyCounter {
    // Note that we don't validate whether `wraps` is actually callable.
    //
    // While we could use `PyAny::is_callable` for that, it has some flaws:
    //    1. It doesn't guarantee the object can actually be called successfully
    //    2. We still need to handle any exceptions that the function might raise
    #[new]
    fn __new__(wraps: Py<PyAny>) -> Self {
        PyCounter { count: 0, wraps }
    }

    #[args(args = "*", kwargs = "**")]
    fn __call__(
        &mut self,
        py: Python,
        args: &PyTuple,
        kwargs: Option<&PyDict>,
    ) -> PyResult<Py<PyAny>> {
        self.count += 1;
        let name = self.wraps.getattr(py, "__name__")?;

        println!("{} has been called {} time(s).", name, self.count);

        // After doing something, we finally forward the call to the wrapped function
        let ret = self.wraps.call(py, args, kwargs)?;

        // We could do something with the return value of
        // the function before returning it
        Ok(ret)
    }
}

#[pymodule]
pub fn decorator(_py: Python, module: &PyModule) -> PyResult<()> {
    module.add_class::<PyCounter>()?;
    Ok(())
}
}

Python code:

@Counter
def say_hello():
    print("hello")


say_hello()
say_hello()
say_hello()
say_hello()

assert say_hello.count == 4

Output:

say_hello has been called 1 time(s).
hello
say_hello has been called 2 time(s).
hello
say_hello has been called 3 time(s).
hello
say_hello has been called 4 time(s).
hello

Pure Python implementation

A Python implementation of this looks similar to the Rust version:

class Counter:
    def __init__(self, wraps):
        self.count = 0
        self.wraps = wraps

    def __call__(self, *args, **kwargs):
        self.count += 1
        print(f"{self.wraps.__name__} has been called {self.count} time(s)")
        self.wraps(*args, **kwargs)

Note that it can also be implemented as a higher order function:

def Counter(wraps):
    count = 0
    def call(*args, **kwargs):
        nonlocal count
        count += 1
        print(f"{wraps.__name__} has been called {count} time(s)")
        return wraps(*args, **kwargs)
    return call