How to generate character array in ctypes and pass its pointer as function parameter

I want to use ctypes to use some information in C++ which is contained in a serialized buffer in a list of Python characters. I have tried the following but I don't understand why my parameter type is throwing an error.

I have a C wrapper for some C++ code in the form

extern "C" {
    special_object *special_object_new() { return new special_object(); }

    int function(special_object *O, int buffer_size, char *buffer) {
        char *buf = (char*) malloc(buffer_size);
        for(int i = 0; i < buffer_size; i++) buf[i] = buffer[i];
        O->do_something_in_cpp_with_buf(buf);
        free(buf);
        buf = NULL;
        return 0;
    }
}

I want to pass functiona character buffer from Python ctypes buf. This char buffer originally came as a list of chars in Python, so I first converted it to a C char array using

import ctypes
import cdll
buf_c = (ctypes.c_char*len(buf))(*buf)

then use

buf_p = ctypes.cast(buf_c, ctypes.POINTER(ctypes.c_char))

has a type

>>> <class 'ctypes.LP_c_char'>

I already know the size ( integer) of this buffer .buf_size

I do the following (load the library and declare function parameters),

lib = cdll.LoadLibrary('PATH_TO_LIB/lib.so')
lib.function.restype = ctypes.c_int
lib.function.argtypes = [ctypes.c_int, ctypes.POINTER(ctypes.c_char)]

Then, I create the class that contains the function,

class Special(object):
    def __init__(self):
        self.obj = lib.special_object_new()

    def py_function(self, buffer_size, buffer):
        res = lib.function(self.obj, buffer_size, buffer)

Finally, I try to call it as

s = Special()
s.py_function(ctypes.c_int(buf_size), buffer)

but i get the error

ArgumentError: argument 1: <type 'exceptions.TypeError'>: wrong type

I can't figure out what I'm doing wrong when doing pointers? Any help would be greatly appreciated!