Pass Python object directly to C++ program without using subprocess

Question

I have a C++ program that, through the terminal, takes a text file as input and produces another text file. I'm executing this program from a Python script which first produces said text string, stores it to a file, runs the C++ program as a subprocess with the created file as input and parses the output text file back into a Python object.

Is it possible to do this without using a subprocess call? In other words: is it possible to avoid the reading and writing and just run the C++ program inside the Python environment with the text-string as input and then capture the output, again inside the Python environment?

For code I refer to the function community_detection_multiplex in this IPython notebook.

Do you have a C++ parser/interpreter written in Python? If yes, great. Otherwise, no. — Lightness Races in Orbit
– Lightness Races in Orbit, Commented Sep 5, 2016 at 16:34
spawning a subprocess isn't really a big deal is it? You could send the input/collect output via stdin/stdout pipe redirects if you want to avoid the security risk of the file. — Richard Hodges
– Richard Hodges, Commented Sep 5, 2016 at 16:35
@RichardHodges how would I pass the string to the C++ program from Python if it were never stored as a file? — Ulf Aslak
– Ulf Aslak, Commented Sep 5, 2016 at 16:45
@UlfAslak with a pipe. The subordinate program could just read data from stdin and write to stdout. These would map to the python program's pipe (which it would create). — Richard Hodges
– Richard Hodges, Commented Sep 5, 2016 at 16:48

galah92 · Accepted Answer · 2016-09-05 17:29:47Z

3

You can use ctypes.
It requires the C++ function to be wrapped with extern "c" and compiled as C code.

Say your C++ function looks like that:

char* changeString(char* someString)
{
    // do something with your string
    return someString;
}

You can call it from python like that:

import ctypes as ct

yourString = "somestring"
yourDLL = ct.CDLL("path/to/dll") # assign the dll to a variable
cppFunc = yourDLL.changeString # assign the cpp func to a variable
cppFunc.restype = ct.c_char_p # set the return type to a string
returnedString = cppfunc(yourString.encode('ascii')).decode()

Now returnedString will have the processed string.

edited Sep 5, 2016 at 17:29

answered Sep 5, 2016 at 16:34

galah92

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5 Comments

Ulf Aslak Over a year ago

Ideally I would want the file to not be stored, and it seems that your solution does this?

galah92 Over a year ago

My solution calls a C++ function and pass a string to it. If you want to open a file in Python and pass it (while open) to C++, then this is not the right solution for you.

Ulf Aslak Over a year ago

as I understand this solution has the string stored as a file

galah92 Over a year ago

yourPathToFile is a string variable contains a path to a file (ideally, I would use the os.path module to create a path string). You can do with open(yourPathToFile) as f: and read the file in python. In the solution I posted, it is expected that the c++ function will open the file within itself.

Ulf Aslak Over a year ago

I'm sorry I must not have explained it well enough. The python script generates the string which should be processed by the C++ program. That's why I want to avoid saving any files.

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