Prerequisite: Using C codes in Python | Set 1, Set 2
Issue #1 : If using ctypes then there is a problem that the original C code may use language that don’t map cleanly to Python.
Let’s take the example of divide() function as it returns a value through one of its arguments. It is a common C technique but it is often not clear how it is supposed to work in Python.
divide = _mod.divide divide.argtypes = (ctypes.c_int, ctypes.c_int, ctypes.POINTER(ctypes.c_int)) x = 0divide(10, 3, x) |
Output :
Traceback (most recent call last): File "", line 1, in ctypes.ArgumentError: argument 3: : expected LP_c_int instance instead of int
Issue #2 :
Even if it works, then there is a violation of Python’s immutability of integers. For arguments involving pointers, a compatible ctypes object has to be constructed and is pass as in the given code below.
x = ctypes.c_int() print ("divide(10, 3, x) : ", divide(10, 3, x)) print ("x.value : ", x.value) |
Output :
divide(10, 3, x) : 3 x.value : 1
Issue #3 :
In the code below, an instance of a ctypes.c_int is created and is passed in as the pointer object .c_int object can be mutated unlike a normal Python integer. To either retrieve or change the value, .value attribute can be used.
# int divide(int, int, int *) _divide = _mod.divide _divide.argtypes = (ctypes.c_int, ctypes.c_int, ctypes.POINTER(ctypes.c_int)) _divide.restype = ctypes.c_int def divide(x, y): rem = ctypes.c_int() quot = _divide(x, y, rem) return quot, rem.value |
Issue #4 : The avg() function is a challenge.
The underlying C code expects a length representing an array and to receive a pointer. However, from the Python side in practice, a Python “array” can take many different forms, it can be array, list, tuple or numpy array and maybe you would like to support multiple possibilities.
