# Calling Python in Rust code

This chapter of the guide documents some ways to interact with Python code from Rust:
 - How to call Python functions
 - How to execute existing Python code

## Calling Python functions

Any Python-native object reference (such as `&PyAny`, `&PyList`, or `&PyCell<MyClass>`) can be used to call Python functions.

PyO3 offers two APIs to make function calls:

* [`call`](https://docs.rs/pyo3/0.12.3/pyo3/struct.PyAny.html#method.call) - call any callable Python object.
* [`call_method`](https://docs.rs/pyo3/0.12.3/pyo3/struct.PyAny.html#method.call_method) - call a method on the Python object.

Both of these APIs take `args` and `kwargs` arguments (for positional and keyword arguments respectively). There are variants for less complex calls:

* [`call1`](https://docs.rs/pyo3/0.12.3/pyo3/struct.PyAny.html#method.call1) and [`call_method1`](https://docs.rs/pyo3/0.12.3/pyo3/struct.PyAny.html#method.call_method1) to call only with positional `args`.
* [`call0`](https://docs.rs/pyo3/0.12.3/pyo3/struct.PyAny.html#method.call0) and [`call_method0`](https://docs.rs/pyo3/0.12.3/pyo3/struct.PyAny.html#method.call_method0) to call with no arguments.

For convenience the [`Py<T>`](types.html#pyt-and-pyobject) smart pointer also exposes these same six API methods, but needs a `Python` token as an additional first argument to prove the GIL is held.

The example below shows a calling Python functions behind a `PyObject` (aka `Py<PyAny>`) reference:

```rust
use pyo3::prelude::*;
use pyo3::types::{PyDict, PyTuple};

struct SomeObject;
impl SomeObject {
    fn new(py: Python) -> PyObject {
        PyDict::new(py).to_object(py)
    }
}

fn main() {
    let arg1 = "arg1";
    let arg2 = "arg2";
    let arg3 = "arg3";

    let gil = Python::acquire_gil();
    let py = gil.python();

    let obj = SomeObject::new(py);

    // call object without empty arguments
    obj.call0(py);

    // call object with PyTuple
    let args = PyTuple::new(py, &[arg1, arg2, arg3]);
    obj.call1(py, args);

    // pass arguments as rust tuple
    let args = (arg1, arg2, arg3);
    obj.call1(py, args);
}
```

### Creating keyword arguments

For the `call` and `call_method` APIs, `kwargs` can be `None` or `Some(&PyDict)`. You can use the [`IntoPyDict`](https://docs.rs/pyo3/latest/pyo3/types/trait.IntoPyDict.html) trait to convert other dict-like containers, e.g. `HashMap` or `BTreeMap`, as well as tuples with up to 10 elements and `Vec`s where each element is a two-element tuple.

```rust
use pyo3::prelude::*;
use pyo3::types::{IntoPyDict, PyDict};
use std::collections::HashMap;

struct SomeObject;

impl SomeObject {
    fn new(py: Python) -> PyObject {
        PyDict::new(py).to_object(py)
    }
}

fn main() {
    let key1 = "key1";
    let val1 = 1;
    let key2 = "key2";
    let val2 = 2;

    let gil = Python::acquire_gil();
    let py = gil.python();

    let obj = SomeObject::new(py);

    // call object with PyDict
    let kwargs = [(key1, val1)].into_py_dict(py);
    obj.call(py, (), Some(kwargs));

    // pass arguments as Vec
    let kwargs = vec![(key1, val1), (key2, val2)];
    obj.call(py, (), Some(kwargs.into_py_dict(py)));

    // pass arguments as HashMap
    let mut kwargs = HashMap::<&str, i32>::new();
    kwargs.insert(key1, 1);
    obj.call(py, (), Some(kwargs.into_py_dict(py)));
}
```

## Executing existing Python code

If you already have some existing Python code that you need to execute from Rust, the following FAQs can help you select the right PyO3 functionality for your situation:

### Want to access Python APIs? Then use `PyModule::import`.

[`Pymodule::import`](https://docs.rs/pyo3/latest/pyo3/types/struct.PyModule.html#method.import) can
be used to get handle to a Python module from Rust. You can use this to import and use any Python
module available in your environment.

```rust
use pyo3::prelude::*;

fn main() -> PyResult<()> {
    Python::with_gil(|py| {
        let builtins = PyModule::import(py, "builtins")?;
        let total: i32 = builtins.call1("sum", (vec![1, 2, 3],))?.extract()?;
        assert_eq!(total, 6);
        Ok(())
    })
}
```

### Want to run just an expression? Then use `eval`.

[`Python::eval`](https://docs.rs/pyo3/latest/pyo3/struct.Python.html#method.eval) is
a method to execute a [Python expression](https://docs.python.org/3.7/reference/expressions.html)
and return the evaluated value as a `&PyAny` object.

```rust
use pyo3::prelude::*;
use pyo3::types::IntoPyDict;

# fn main() -> Result<(), ()> {
Python::with_gil(|py| {
    let result = py.eval("[i * 10 for i in range(5)]", None, None).map_err(|e| {
        e.print_and_set_sys_last_vars(py);
    })?;
    let res: Vec<i64> = result.extract().unwrap();
    assert_eq!(res, vec![0, 10, 20, 30, 40]);
    Ok(())
})
# }
```

### Want to run statements? Then use `run`.

[`Python::run`] is a method to execute one or more
[Python statements](https://docs.python.org/3.7/reference/simple_stmts.html).
This method returns nothing (like any Python statement), but you can get
access to manipulated objects via the `locals` dict.

You can also use the [`py_run!`] macro, which is a shorthand for [`Python::run`].
Since [`py_run!`] panics on exceptions, we recommend you use this macro only for
quickly testing your Python extensions.

```rust
use pyo3::prelude::*;
use pyo3::{PyCell, PyObjectProtocol, py_run};

# fn main() {
#[pyclass]
struct UserData {
    id: u32,
    name: String,
}

#[pymethods]
impl UserData {
    fn as_tuple(&self) -> (u32, String) {
        (self.id, self.name.clone())
    }
}

#[pyproto]
impl PyObjectProtocol for UserData {
    fn __repr__(&self) -> PyResult<String> {
        Ok(format!("User {}(id: {})", self.name, self.id))
    }
}

Python::with_gil(|py| {
    let userdata = UserData {
        id: 34,
        name: "Yu".to_string(),
    };
    let userdata = PyCell::new(py, userdata).unwrap();
    let userdata_as_tuple = (34, "Yu");
    py_run!(py, userdata userdata_as_tuple, r#"
assert repr(userdata) == "User Yu(id: 34)"
assert userdata.as_tuple() == userdata_as_tuple
    "#);
})
# }
```

## You have a Python file or code snippet? Then use `PyModule::from_code`.

[PyModule::from_code](https://docs.rs/pyo3/latest/pyo3/types/struct.PyModule.html#method.from_code)
can be used to generate a Python module which can then be used just as if it was imported with
`PyModule::import`.

```rust
use pyo3::{prelude::*, types::{IntoPyDict, PyModule}};

# fn main() -> PyResult<()> {
Python::with_gil(|py| {
    let activators = PyModule::from_code(py, r#"
def relu(x):
    """see https://en.wikipedia.org/wiki/Rectifier_(neural_networks)"""
    return max(0.0, x)

def leaky_relu(x, slope=0.01):
    return x if x >= 0 else x * slope
    "#, "activators.py", "activators")?;

    let relu_result: f64 = activators.call1("relu", (-1.0,))?.extract()?;
    assert_eq!(relu_result, 0.0);

    let kwargs = [("slope", 0.2)].into_py_dict(py);
    let lrelu_result: f64 = activators
        .call("leaky_relu", (-1.0,), Some(kwargs))?
        .extract()?;
    assert_eq!(lrelu_result, -0.2);
#    Ok(())
})
# }
```

[`Python::run`]: https://docs.rs/pyo3/latest/pyo3/struct.Python.html#method.run
[`py_run!`]: https://docs.rs/pyo3/latest/pyo3/macro.py_run.html