How do I convert a Python list into a C array by using ctypes?

The following code works on arbitrary lists:

import ctypes
pyarr = [1, 2, 3, 4]
arr = (ctypes.c_int * len(pyarr))(*pyarr)

This is an explanation of the accepted answer.

ctypes.c_int * len(pyarr) creates an array (sequence) of type c_int of length 4 (python3, python 2). Since c_int is an object whose constructor takes one argument, (ctypes.c_int * len(pyarr)(*pyarr) does a one shot init of each c_int instance from pyarr. An easier to read form is:

pyarr = [1, 2, 3, 4]
seq = ctypes.c_int * len(pyarr)
arr = seq(*pyarr)

Use type function to see the difference between seq and arr.

From the ctypes tutorial:

>>> IntArray5 = c_int * 5
>>> ia = IntArray5(5, 1, 7, 33, 99)

import ctypes
import typing

def foo(aqs : typing.List[int]) -> ctypes.Array:
    array_type = ctypes.c_int64 * len(aqs)
    ans = array_type(*aqs)
    return ans

for el in foo([1,2,3]):
    print(el)

this will give:

1
2
3