Define function in Python as it defined in C

Question

I have function in C that reads byte by byte from a given buffer and returns the result of a mathematical formula.
I need to write the same function in Python

The buffer in C is struct and in python i used ctypes Structure class
my prototype in c is int calc_formula(char *buff,int len)
so calling the function in c is staright forward but how i define such function in Python?

I try to define the following and have some questions

def calc_formula(buff,len):
    some code

In C I called the function with pointer to the strcut first char. How do I do it in Python? is buff passed as pointer? My buffer is very large and if it can't be done, I will use global variable (which is less preferred).
I need to read the buffer byte by byte, so in c I simply increment the buffer pointer. What's the way to do it in python? I read about ctypes union class that I can define over the Structure and go over it byte by byte. Do you have a better solution?

UPDATE

i tried bbrame solution :

def calc_formula(buff, len):
    sum = 0     
    for curChar in buff: 
        numericByteValue = ord(curChar) 
        sum += numericByteValue     
    return sum

with When i try its code with calc_formula(input_buff,len) , i get the following:
"*error:TypeError: 't_input_buff' object is not iterable*" - input_buff is instance of t_input_buff that is Class(Structure) . what can be the problem? (it give me the error when it try to do the for command)

ctypes is used to call C functions from Python. But you also talk about rewriting a C function in Python. Which of these do you need to do? — interjay
– interjay, Commented Nov 30, 2011 at 14:21
Just thought to check: are you aware of the existence of NumPy and SciPy, the high-performance numeric extensions of python? I'm asking because from the look of it it seems that using a numpy.ndarray would allow for a straightforward solution... Again: I'm not suggesting you are doing it wrong... just checking if you knew this other possibility! :) — mac
– mac, Commented Nov 30, 2011 at 14:23
@interjay , i used ctypes in order to define memory layout as same as my C program that is embedded software. i used this same memory layout to read the memory as whole block from the board and easily access each member in Python. so my question is how to write the function in python — Gil.I
– Gil.I, Commented Nov 30, 2011 at 14:38
thanks @mac , i'm not aware of these extensions (i'm newbie in python) — Gil.I
– Gil.I, Commented Nov 30, 2011 at 14:55

bbrame · Accepted Answer · 2011-11-30 14:36:58Z

1

In c, try using the type c_char_p rather than char* (see the ctypes documentation).

In python the parameter (buff) will be a python string. Loop through it as follows:

def calc_formula(buff, len):
    sum = 0
    for curChar in buff:
        numericByteValue = ord(curChar)
        sum += numericByteValue
    return sum

edited Nov 30, 2011 at 14:36

answered Nov 30, 2011 at 14:26

bbrame

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2 Comments

Gil.I Over a year ago

what you meant by "In c, try using the type c_char_p rather than char* ? ctypes c_char_p is ctypes type not C type (that's why i used char*)

Gil.I Over a year ago

,When i try your code with calc_formula(input_buff,len) , i get the following "error:TypeError: 't_input_buff' object is not iterable" i' - input_buff is instance of t_input_buff that is Class(Structure) . what can be the problem?

Community · Accepted Answer · 2017-05-23 12:27:37Z

0

UPDATE i solve it with ctypes union class
for answer look in this question

edited May 23, 2017 at 12:27

CommunityBot

11 silver badge

answered Dec 4, 2011 at 12:28

Gil.I

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