Question

I want to call a C library from a Python application. I don’t want to wrap the whole API, only the functions and datatypes that are relevant to my case. As I see it, I have three choices:

1. Create an actual extension module in C. Probably overkill, and I’d also like to avoid the overhead of learning extension writing.
2. Use Cython to expose the relevant parts from the C library to Python.
3. Do the whole thing in Python, using ctypes to communicate with the external library.

I’m not sure whether 2) or 3) is the better choice. The advantage of 3) is that ctypes is part of the standard library, and the resulting code would be pure Python – although I’m not sure how big that advantage actually is.

Are there more advantages / disadvantages with either choice? Which approach do you recommend?

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Edit: Thanks for all your answers, they provide a good resource for anyone looking to do something similar. The decision, of course, is still to be made for the single case—there’s no one “This is the right thing” sort of answer. For my own case, I’ll probably go with ctypes, but I’m also looking forward to trying out Cython in some other project.

With there being no single true answer, accepting one is somewhat arbitrary; I chose FogleBird’s answer as it provides some good insight into ctypes and it currently also is the highest-voted answer. However, I suggest to read all the answers to get a good overview.

Thanks again.

Answer 1

ctypes is your best bet for getting it done quickly, and it’s a pleasure to work with as you’re still writing Python!

I recently wrapped an FTDI driver for communicating with a USB chip using ctypes and it was great. I had it all done and working in less than one work day. (I only implemented the functions we needed, about 15 functions).

We were previously using a third-party module, PyUSB, for the same purpose. PyUSB is an actual C/Python extension module. But PyUSB wasn’t releasing the GIL when doing blocking reads/writes, which was causing problems for us. So I wrote our own module using ctypes, which does release the GIL when calling the native functions.

One thing to note is that ctypes won’t know about #define constants and stuff in the library you’re using, only the functions, so you’ll have to redefine those constants in your own code.

Here’s an example of how the code ended up looking (lots snipped out, just trying to show you the gist of it):

from ctypes import *

d2xx = WinDLL('ftd2xx')

OK = 0
INVALID_HANDLE = 1
DEVICE_NOT_FOUND = 2
DEVICE_NOT_OPENED = 3

...

def openEx(serial):
    serial = create_string_buffer(serial)
    handle = c_int()
    if d2xx.FT_OpenEx(serial, OPEN_BY_SERIAL_NUMBER, byref(handle)) == OK:
        return Handle(handle.value)
    raise D2XXException

class Handle(object):
    def __init__(self, handle):
        self.handle = handle
    ...
    def read(self, bytes):
        buffer = create_string_buffer(bytes)
        count = c_int()
        if d2xx.FT_Read(self.handle, buffer, bytes, byref(count)) == OK:
            return buffer.raw[:count.value]
        raise D2XXException
    def write(self, data):
        buffer = create_string_buffer(data)
        count = c_int()
        bytes = len(data)
        if d2xx.FT_Write(self.handle, buffer, bytes, byref(count)) == OK:
            return count.value
        raise D2XXException

Someone did some benchmarks on the various options.

I might be more hesitant if I had to wrap a C++ library with lots of classes/templates/etc. But ctypes works well with structs and can even callback into Python.