From teh docs:
http://docs.python.org/c-api/veryhigh.html

int PyRun_SimpleStringFlags(const char *command, PyCompilerFlags *flags)¶
    Executes the Python source code from command in the __main__ module
according to the flags argument. If __main__ does not already exist, it
is created. Returns 0 on success or -1 if an exception was raised. If
there was an error, there is no way to get the exception information.
For the meaning of flags, see below.

In case of a SystemExit it will not return. This detail, along another
issue with Py_Main() and SystemExit, made me google around for a while.