Szathmáry László honlapja @ DEIK | DLang / Calling D from Python

Suppose we use Python and we have a very slow function. Idea: implement that function in D and use it as a library from Python.

Makefile:

cat:
	cat Makefile

so:
	# -shared → build a shared library
	# -fPIC → position-independent code (needed on Linux for shared libs)
	dmd -shared -fPIC mylib.d -of=libmylib.so

run:
	./main.py

Write the D library

(1) D library

extern (C) auto twice(const int n)
{
return 2 * n;
}

Notes:

extern (C) doesn't mean that you have to write the code in C. It just means to use the C calling convention and symbol naming so other languages (like Python, Rust, etc.) can call this function.
I used the keyword auto, which doesn't exist in C. This is a D function.

Compile it with "make so" (see the Makefile above).

Write the main Python program

(2) the Python program

#!/usr/bin/env python3

from cffi import FFI

ffi = FFI()
lib = ffi.dlopen("./libmylib.so")
ffi.cdef("auto twice(const int n);") # function declaration

def main():
print(lib.twice(21)) # → 42

if __name__ == "__main__":
main()

Notes:

The module cffi is a 3rd-party library. This is the Foreign Function Interface for Python calling C code. Under Manjaro, I had to install the package python-cffi (with pacman).
You need to declare the functions that you want to use.
Instead of cffi, you could also use the ctypes module. It's in the stdandard library, but its usage is a bit more complicated.

After this, you can execute your Python program:

$ ./main.py
42