Issues with using C code in Python | Set 1
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 = 0
divide( 10 , 3 , x)
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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)
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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.
_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
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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.
Last Updated :
20 Mar, 2019
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