Set - 7

Question 1 :

How do I catch the output from PyErr_Print() (or anything that prints to stdout/stderr)?

Answer :

In Python code, define an object that supports the write() method. Assign this object to sys.stdout and sys.stderr. Call print_error, or just allow the standard traceback mechanism to work. Then, the output will go wherever your write() method sends it. 
The easiest way to do this is to use the StringIO class in the standard library. 
Sample code and use for catching stdout:

>>> class StdoutCatcher:
... def __init__(self):
... = ''
... def write(self, stuff):
... = + stuff
>>> import sys
>>> sys.stdout = StdoutCatcher()
>>> print 'foo'
>>> print 'hello world!'
>>> sys.stderr.write(

hello world!

Question 2 :

How do I access a module written in Python from C?

Answer :

You can get a pointer to the module object as follows:

module = PyImport_ImportModule("<modulename>"); 

If the module hasn't been imported yet (i.e. it is not yet present in sys.modules), this initializes the module; otherwise it simply returns the value of sys.modules["<modulename>"]. Note that it doesn't enter the module into any namespace -- it only ensures it has been initialized and is stored in sys.modules. 
You can then access the module's attributes (i.e. any name defined in the module) as follows:

attr = PyObject_GetAttrString(module, "<attrname>");

Calling PyObject_SetAttrString() to assign to variables in the module also works.

Question 3 :

How do I interface to C++ objects from Python?

Answer :

Depending on your requirements, there are many approaches. To do this manually, begin by reading the "Extending and Embedding" document. Realize that for the Python run-time system, there isn't a whole lot of difference between C and C++ -- so the strategy of building a new Python type around a C structure (pointer) type will also work for C++ objects.

Question 4 :

How do I tell "incomplete input" from "invalid input"?

Answer :

Sometimes you want to emulate the Python interactive interpreter's behavior, where it gives you a continuation prompt when the input is incomplete (e.g. you typed the start of an "if" statement or you didn't close your parentheses or triple string quotes), but it gives you a syntax error message immediately when the input is invalid. 

In Python you can use the codeop module, which approximates the parser's behavior sufficiently. IDLE uses this, for example. 

The easiest way to do it in C is to call PyRun_InteractiveLoop() (perhaps in a separate thread) and let the Python interpreter handle the input for you. You can also set the PyOS_ReadlineFunctionPointer to point at your custom input function. See Modules/readline.c and Parser/myreadline.c for more hints. 

However sometimes you have to run the embedded Python interpreter in the same thread as your rest application and you can't allow the PyRun_InteractiveLoop() to stop while waiting for user input. The one solution then is to call PyParser_ParseString() and test for e.error equal to E_EOF, which means the input is incomplete). Here's a sample code fragment, untested, inspired by code from Alex Farber:

#include <Python.h>
#include <node.h>
#include <errcode.h>
#include <grammar.h>
#include <parsetok.h>
#include <compile.h>
int testcomplete(char *code)
/* code should end in \n */
/* return -1 for error, 0 for incomplete,
1 for complete */
    node *n;
    perrdetail e;
    n = PyParser_ParseString(code, &_PyParser_Grammar,
    Py_file_input, &e);
    if (n == NULL) {
        if (e.error == E_EOF) 
        return 0;
        return -1;
    return 1;	

Another solution is trying to compile the received string with Py_CompileString(). If it compiles without errors, try to execute the returned code object by calling PyEval_EvalCode(). Otherwise save the input for later. If the compilation fails, find out if it's an error or just more input is required - by extracting the message string from the exception tuple and comparing it to the string "unexpected EOF while parsing". Here is a complete example using the GNU readline library (you may want to ignore SIGINT while calling readline()):

#include <stdio.h>
#include <readline.h>
#include <Python.h>
#include <object.h>
#include <compile.h>
#include <eval.h>
int main (int argc, char* argv[])
    int i, j, done = 0; /* lengths of line, code */
    char ps1[] = ">>> ";
    char ps2[] = "... ";
    char *prompt = ps1;
    char *msg, *line, *code = NULL;
    PyObject *src, *glb, *loc;
    PyObject *exc, *val, *trb, *obj, *dum;
    Py_Initialize ();
    loc = PyDict_New ();
    glb = PyDict_New ();
    PyDict_SetItemString (glb, "__builtins__", 
    PyEval_GetBuiltins ());
	while (!done){
		line = readline (prompt);
		if (NULL == line) /* CTRL-D pressed */{
			done = 1;
			i = strlen (line);
			if (i > 0)
                add_history (line);
                /* save non-empty lines */
            if (NULL == code) 
            /* nothing in code yet */
            j = 0;
            j = strlen (code);

            code = realloc (code, i + j + 2);
            if (NULL == code) 
            /* out of memory */
            exit (1);

        if (0 == j) 
        /* code was empty, so */
        code[0] = '\0'; 
        /* keep strncat happy */
        strncat (code, line, i); 
        /* append line to code */
        code[i + j] = '\n'; 
        /* append '\n' to code */
        code[i + j + 1] = '\0';

    src = Py_CompileString (code, " <stdin>", Py_single_input); 

if (NULL != src) 
/* compiled just fine - */
if (ps1 == prompt || 
/* ">>> " or */
'\n' == code[i + j - 1]) 
/* "... " and double '\n' */
/* so execute it */
dum = PyEval_EvalCode ((PyCodeObject *)src, glb, loc);
Py_XDECREF (dum);
Py_XDECREF (src);
free (code);
code = NULL;
if (PyErr_Occurred ())
PyErr_Print ();
prompt = ps1;
/* syntax error or E_EOF? */

else if (PyErr_ExceptionMatches (PyExc_SyntaxError)) 
PyErr_Fetch (&exc, &val, &trb); 
/* clears exception! */

if (PyArg_ParseTuple (val, "sO", &msg, &obj) &&
!strcmp (msg, "unexpected EOF while parsing")) /* E_EOF */
Py_XDECREF (exc);
Py_XDECREF (val);
Py_XDECREF (trb);
prompt = ps2;
/* some other syntax error */
PyErr_Restore (exc, val, trb);
PyErr_Print ();
free (code);
code = NULL;
prompt = ps1;
/* some non-syntax error */
PyErr_Print ();
free (code);
code = NULL;
prompt = ps1;

free (line);



Question 5 :

How do I run a Python program under Windows?

Answer :

This is not necessarily a straightforward question. If you are already familiar with running programs from the Windows command line then everything will seem obvious; otherwise, you might need a little more guidance. There are also differences between Windows 95, 98, NT, ME, 2000 and XP which can add to the confusion. 

Unless you use some sort of integrated development environment, you will end up typing Windows commands into what is variously referred to as a "DOS window" or "Command prompt window". Usually you can create such a window from your Start menu; under Windows 2000 the menu selection is "Start | Programs | Accessories | Command Prompt". You should be able to recognize when you have started such a window because you will see a Windows "command prompt", which usually looks like this: 


The letter may be different, and there might be other things after it, so you might just as easily see something like: 


depending on how your computer has been set up and what else you have recently done with it. Once you have started such a window, you are well on the way to running Python programs. 

You need to realize that your Python scripts have to be processed by another program called the Python interpreter. The interpreter reads your script, compiles it into bytecodes, and then executes the bytecodes to run your program. So, how do you arrange for the interpreter to handle your Python? 

First, you need to make sure that your command window recognises the word "python" as an instruction to start the interpreter. If you have opened a command window, you should try entering the command python and hitting return. You should then see something like: 

Python 2.2 (#28, Dec 21 2001, 12:21:22) [MSC 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.

You have started the interpreter in "interactive mode". That means you can enter Python statements or expressions interactively and have them executed or evaluated while you wait. This is one of Python's strongest features. Check it by entering a few expressions of your choice and seeing the results: 

>>> print "Hello"
>>> "Hello" * 3

Many people use the interactive mode as a convenient yet highly programmable calculator. When you want to end your interactive Python session, hold the Ctrl key down while you enter a Z, then hit the "Enter" key to get back to your Windows command prompt. 

You may also find that you have a Start-menu entry such as "Start | Programs | Python 2.2 | Python (command line)" that results in you seeing the >>> prompt in a new window. If so, the window will disappear after you enter the Ctrl-Z character; Windows is running a single "python" command in the window, and closes it when you terminate the interpreter. 

If the python command, instead of displaying the interpreter prompt >>>, gives you a message like: 

'python' is not recognized as an internal or external command,
operable program or batch file.


Bad command or filename 

then you need to make sure that your computer knows where to find the Python interpreter. To do this you will have to modify a setting called PATH, which is a list of directories where Windows will look for programs. You should arrange for Python's installation directory to be added to the PATH of every command window as it starts. If you installed Python fairly recently then the command 

dir C:\py* 

will probably tell you where it is installed; the usual location is something like C:\Python23. Otherwise you will be reduced to a search of your whole disk ... use "Tools | Find" or hit the "Search" button and look for "python.exe". Supposing you discover that Python is installed in the C:\Python23 directory (the default at the time of writing), you should make sure that entering the command 


starts up the interpreter as above (and don't forget you'll need a "CTRL-Z" and an "Enter" to get out of it). Once you have verified the directory, you need to add it to the start-up routines your computer goes through. For older versions of Windows the easiest way to do this is to edit the C:\AUTOEXEC.BAT file. You would want to add a line like the following to AUTOEXEC.BAT: 

PATH C:\Python23;%PATH% 

For Windows NT, 2000 and (I assume) XP, you will need to add a string such as 


to the current setting for the PATH environment variable, which you will find in the properties window of "My Computer" under the "Advanced" tab. Note that if you have sufficient privilege you might get a choice of installing the settings either for the Current User or for System. The latter is preferred if you want everybody to be able to run Python on the machine. 

If you aren't confident doing any of these manipulations yourself, ask for help! At this stage you may want to reboot your system to make absolutely sure the new setting has taken effect. You probably won't need to reboot for Windows NT, XP or 2000. You can also avoid it in earlier versions by editing the file C:\WINDOWS\COMMAND\CMDINIT.BAT instead of AUTOEXEC.BAT. 

You should now be able to start a new command window, enter python at the C:> (or whatever) prompt, and see the >>> prompt that indicates the Python interpreter is reading interactive commands. 

Let's suppose you have a program called in directory C:\Steve\Projects\Python. A session to run that program might look like this: 

C:\> cd \Steve\Projects\Python
C:\Steve\Projects\Python> python

Because you added a file name to the command to start the interpreter, when it starts up it reads the Python script in the named file, compiles it, executes it, and terminates, so you see another C:\> prompt. You might also have entered 

C:\> python \Steve\Projects\Python\ 

if you hadn't wanted to change your current directory. 

Under NT, 2000 and XP you may well find that the installation process has also arranged that the command (or, if the file isn't in the current directory, C:\Steve\Projects\Python\ will automatically recognize the ".py" extension and run the Python interpreter on the named file. Using this feature is fine, but some versions of Windows have bugs which mean that this form isn't exactly equivalent to using the interpreter explicitly, so be careful. 

The important things to remember are:
1. Start Python from the Start Menu, or make sure the PATH is set correctly so Windows can find the Python interpreter. 


should give you a '>>>" prompt from the Python interpreter. Don't forget the CTRL-Z and ENTER to terminate the interpreter (and, if you started the window from the Start Menu, make the window disappear). 

2. Once this works, you run programs with commands:

python {program-file}

3. When you know the commands to use you can build Windows shortcuts to run the Python interpreter on any of your scripts, naming particular working directories, and adding them to your menus. Take a look at 

python --help 

if your needs are complex.
4. Interactive mode (where you see the >>> prompt) is best used for checking that individual statements and expressions do what you think they will, and for developing code by experiment.