How to convert a C string (char array) into a Python string?

Question

I have embedded a Python interpreter in a C program. Suppose the C program reads some bytes from a file into a char array and learns (somehow) that the bytes represent text with a certain encoding (e.g., ISO 8859-1, Windows-1252, or UTF-8). How do I decode the contents of this char array into a Python string?

The Python string should in general be of type unicode—for instance, a 0x93 in Windows-1252 encoded input becomes a u'\u0201c'.

I have attempted to use PyString_Decode, but it always fails when there are non-ASCII characters in the string. Here is an example that fails:

#include <Python.h>
#include <stdio.h>

int main(int argc, char *argv[])
{
     char c_string[] = { (char)0x93, 0 };
     PyObject *py_string;

     Py_Initialize();

     py_string = PyString_Decode(c_string, 1, "windows_1252", "replace");
     if (!py_string) {
          PyErr_Print();
          return 1;
     }
     return 0;
}

The error message is UnicodeEncodeError: 'ascii' codec can't encode character u'\u201c' in position 0: ordinal not in range(128), which indicates that the ascii encoding is used even though we specify windows_1252 in the call to PyString_Decode.

The following code works around the problem by using PyString_FromString to create a Python string of the undecoded bytes, then calling its decode method:

#include <Python.h>
#include <stdio.h>

int main(int argc, char *argv[])
{
     char c_string[] = { (char)0x93, 0 };
     PyObject *raw, *decoded;

     Py_Initialize();

     raw = PyString_FromString(c_string);
     printf("Undecoded: ");
     PyObject_Print(raw, stdout, 0);
     printf("\n");
     decoded = PyObject_CallMethod(raw, "decode", "s", "windows_1252");
     Py_DECREF(raw);
     printf("Decoded: ");
     PyObject_Print(decoded, stdout, 0);
     printf("\n");
     return 0;
}

Answer 1

You don't want to decode the string into a Unicode representation, you just want to treat it as an array of bytes, right?

Just use PyString_FromString:

char *cstring;
PyObject *pystring = PyString_FromString(cstring);

That's all. Now you have a Python str() object. See docs here: http://www.python.org/doc/2.5.2/api/stringObjects.html

I'm a little bit confused about how you say you want a "str or unicode." They're very different if you have non-ASCII characters. If you want to decode a C string and you know exactly what character set it's in then, yes, PyString_DecodeString is a good place to start.

Answer 2

Try calling PyErr_Print() in the "if (!py_string)" clause. Perhaps the python exception will give you some more information.

Answer 3

PyString_Decode does this:

PyObject *PyString_Decode(const char *s,
        Py_ssize_t size,
        const char *encoding,
        const char *errors)
{
    PyObject *v, *str;

    str = PyString_FromStringAndSize(s, size);
    if (str == NULL)
    return NULL;
    v = PyString_AsDecodedString(str, encoding, errors);
    Py_DECREF(str);
    return v;
}

IOW, it does basically what you're doing in your second example - converts to a string, then decode the string. The problem here arises from PyString_AsDecodedString, rather than PyString_AsDecodedObject. PyString_AsDecodedString does PyString_AsDecodedObject, but then tries to convert the resulting unicode object into a string object with the default encoding (for you, looks like that's ASCII). That's where it fails.

I believe you'll need to do two calls - but you can use PyString_AsDecodedObject rather than calling the python "decode" method. Something like:

#include <Python.h>
#include <stdio.h>

int main(int argc, char *argv[])
{
     char c_string[] = { (char)0x93, 0 };
     PyObject *py_string, *py_unicode;

     Py_Initialize();

     py_string = PyString_FromStringAndSize(c_string, 1);
     if (!py_string) {
          PyErr_Print();
          return 1;
     }
     py_unicode = PyString_AsDecodedObject(py_string, "windows_1252", "replace");
     Py_DECREF(py_string);

     return 0;
}

I'm not entirely sure what the reasoning behind PyString_Decode working this way is. A very old thread on python-dev seems to indicate that it has something to do with chaining the output, but since the Python methods don't do the same, I'm not sure if that's still relevant.