Help on convert PyObject to string (c) Python 3.6

[Chris Angelico]

On Mon, Feb 5, 2018 at 3:52 AM, Jason Qian via Python-list
<python-list at python.org> wrote:
> Hi,
>
>    This is the case of calling python from c and the python function  will
> return a string.
>
>    It seems python been called correctly, but got error when convert the
> python string to c string.
>
> -- c --
>
>    PyObject* pValue = PyObject_CallObject(pFunc, pArgs);
>
>
> -- python --
>
> import string, random
> def StringGen(argv):
>     out_string_size=int(argv);
>     output_data=''.join(random.choice(string.ascii_lowercase) for x in
> range(out_string_size))
>     return output_data
>
>
> I try:
>
>  PyObject* pValue = PyObject_CallObject(pFunc, pArgs);
> const char* sp = 0;
> if (!PyArg_ParseTuple(pValue, "s", &sp)) {
> }
>
> and got
>
> "SystemError: new style getargs format but argument is not a tuple"
>

You're using something that is specifically for parsing *arguments*
(as indicated by the "PyArg" prefix). If you want to get a 'const char
*' from a Python string, you probably want to encode it UTF-8. There's
a convenience function for that:

https://docs.python.org/3/c-api/unicode.html#c.PyUnicode_AsUTF8AndSize

And if you know for sure that there won't be any \0 in the string, you
can use the next function in the docs, which doesn't bother returning
the size. (It'll always be null-terminated.)

ChrisA