(Paper) Python Interview Questions And Answers Set -5

Python Interview Questions And Answers Set - 5

 

How do I send mail from a Python script?
Use the standard library module smtplib.

Here's a very simple interactive mail sender that uses it. This method will work on any host that supports an SMTP listener.

import sys, smtplib

fromaddr = raw_input("From: ")
toaddrs = raw_input("To: ").split(',')
print "Enter message, end with ^D:"
msg = ''
while 1:
line = sys.stdin.readline()
if not line:
break
msg = msg + line

# The actual mail send
server = smtplib.SMTP('localhost')
server.sendmail(fromaddr, toaddrs, msg)
server.quit()

A Unix-only alternative uses sendmail. The location of the sendmail program varies between systems; sometimes it is /usr/lib/sendmail, sometime /usr/sbin/sendmail. The sendmail manual page will help you out. Here's some sample code:

SENDMAIL = "/usr/sbin/sendmail" # sendmail location
import os
p = os.popen("%s -t -i" % SENDMAIL, "w")
p.write("To: receiver@example.com\n")
p.write("Subject: test\n")
p.write("\n") # blank line separating headers from body
p.write("Some text\n")
p.write("some more text\n")
sts = p.close()
if sts != 0:
print "Sendmail exit status", sts

How do I avoid blocking in the connect() method of a socket?
The select module is commonly used to help with asynchronous I/O on sockets.

Are there any interfaces to database packages in Python?
Yes.

Python 2.3 includes the bsddb package which provides an interface to the BerkeleyDB library. Interfaces to disk-based hashes such as DBM and GDBM are also included with standard Python.

How do I generate random numbers in Python?
The standard module random implements a random number generator. Usage is simple:

import random
random.random()

This returns a random floating point number in the range [0, 1).

Can I create my own functions in C?
Yes, you can create built-in modules containing functions, variables, exceptions and even new types in C.

Can I create my own functions in C++?
Yes, using the C compatibility features found in C++. Place extern "C" { ... } around the Python include files and put extern "C" before each function that is going to be called by the Python interpreter. Global or static C++ objects with constructors are probably not a good idea.

How can I execute arbitrary Python statements from C?
The highest-level function to do this is PyRun_SimpleString() which takes a single string argument to be executed in the context of the module __main__ and returns 0 for success and -1 when an exception occurred (including SyntaxError). If you want more control, use PyRun_String(); see the source for PyRun_SimpleString() in Python/pythonrun.c.

How can I evaluate an arbitrary Python expression from C?
Call the function PyRun_String() from the previous question with the start symbol Py_eval_input; it parses an expression, evaluates it and returns its value.

How do I extract C values from a Python object?
That depends on the object's type. If it's a tuple, PyTupleSize(o) returns its length and PyTuple_GetItem(o, i) returns its i'th item. Lists have similar functions, PyListSize(o) and PyList_GetItem(o, i).

For strings, PyString_Size(o) returns its length and PyString_AsString(o) a pointer to its value. Note that Python strings may contain null bytes so C's strlen() should not be used.

To test the type of an object, first make sure it isn't NULL, and then use PyString_Check(o), PyTuple_Check(o), PyList_Check(o), etc.

There is also a high-level API to Python objects which is provided by the so-called 'abstract' interface -- read Include/abstract.h for further details. It allows interfacing with any kind of Python sequence using calls like PySequence_Length(), PySequence_GetItem(), etc.) as well as many other useful protocols.

How do I call an object's method from C?
The PyObject_CallMethod() function can be used to call an arbitrary method of an object. The parameters are the object, the name of the method to call, a format string like that used with Py_BuildValue(), and the argument values:

PyObject *
PyObject_CallMethod(PyObject *object, char *method_name,
char *arg_format, ...);

This works for any object that has methods -- whether built-in or user-defined. You are responsible for eventually Py_DECREF'ing the return value.

To call, e.g., a file object's "seek" method with arguments 10, 0 (assuming the file object pointer is "f"):

res = PyObject_CallMethod(f, "seek", "(ii)", 10, 0);
if (res == NULL) {
... an exception occurred ...
}
else {
Py_DECREF(res);
}

Note that since PyObject_CallObject() always wants a tuple for the argument list, to call a function without arguments, pass "()" for the format, and to call a function with one argument, surround the argument in parentheses, e.g. "(i)".

How do I catch the output from PyErr_Print() (or anything that prints to stdout/stderr)?
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):
... self.data = ''
... def write(self, stuff):
... self.data = self.data + stuff
...
>>> import sys
>>> sys.stdout = StdoutCatcher()
>>> print 'foo'
>>> print 'hello world!'
>>> sys.stderr.write(sys.stdout.data)
foo
hello world!

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

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

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

PyNode_Free(n);
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;
}
else
{
i = strlen (line);

if (i > 0)
add_history (line);
/* save non-empty lines */

if (NULL == code)
/* nothing in code yet */
j = 0;
else
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;
}
else
/* some other syntax error */
{
PyErr_Restore (exc, val, trb);
PyErr_Print ();
free (code);
code = NULL;
prompt = ps1;
}
}
else
/* some non-syntax error */
{
PyErr_Print ();
free (code);
code = NULL;
prompt = ps1;
}

free (line);
}
}

Py_XDECREF(glb);
Py_XDECREF(loc);
Py_Finalize();
exit(0);
}