Initial commit.

.gitignore

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+/target
+/Cargo.lock
+build/

Cargo.lock

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+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "rust_add_lib"
+version = "0.1.0"

Cargo.toml

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+[package]
+name = "rust_add_lib"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+
+[lib]
+crate-type = ["cdylib"]

README.md

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+# Calling a rust function from Python
+
+- `cargo build` builds a `./target/debug/librust_add_lib.so` with a `rust_add` function in it.
+- `LDFLAGS='-L target/debug/' python setup.py build` builds a Python module using `rust_add_lib`.
+
+Then the module can be imported and used from Python:
+
+```bash
+$ LD_LIBRARY_PATH=target/debug PYTHONPATH='./build/lib.linux-x86_64-cpython-311/' python
+Python 3.11.6 (main, Oct  8 2023, 05:06:43) [GCC 13.2.0] on linux
+Type "help", "copyright", "credits" or "license" for more information.
+>>> import add
+>>> add.add(.1, .2)
+[src/lib.rs:3] left = 0.1
+[src/lib.rs:3] right = 0.2
+[src/lib.rs:4] left + right = 0.30000000000000004
+0.30000000000000004
+>>>
+```

python_add.c

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+#define PY_SSIZE_T_CLEAN
+#include <Python.h>
+
+double rust_add(double, double); // from rust, from librust_lib.so
+
+
+static PyObject *python_add(PyObject *self, PyObject *args)
+{
+    double float_left;
+    double float_right;
+    double result;
+
+    if (!PyArg_ParseTuple(args, "dd", &float_left, &float_right))
+        return NULL;
+
+    result = rust_add(float_left, float_right);
+
+    return PyFloat_FromDouble(result);
+}
+
+static PyMethodDef AddMethods[] = {
+    {"add",  python_add, METH_VARARGS,
+     "Just add from rust."},
+    {NULL, NULL, 0, NULL}        /* Sentinel */
+};
+
+static struct PyModuleDef addmodule = {
+    PyModuleDef_HEAD_INIT,
+    "add",   /* name of module */
+    NULL   , /* module documentation, may be NULL */
+    -1,      /* size of per-interpreter state of the module,
+                or -1 if the module keeps state in global variables. */
+    AddMethods
+};
+
+PyMODINIT_FUNC PyInit_add(void)
+{
+    return PyModule_Create(&addmodule);
+}

setup.py

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+from distutils.core import setup
+from distutils.extension import Extension
+
+setup(name='add',
+      version='1.0',
+      ext_modules=[Extension('add', ['python_add.c'], libraries=['rust_add_lib'])],
+      )

src/lib.rs

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+#[no_mangle]
+pub extern "C" fn rust_add(left: f64, right: f64) -> f64 {
+    dbg!(left, right);
+    dbg!(left + right)
+}