Writing factorial function in Python - newbie

The recursive version:

import sys

Usage = """
Usage:
$ python factorial.py 
"""

def fact(x):
    if x == 0:
        return 1
    else:
        return x * fact(x-1)

if (len(sys.argv)>1) :
    print fact(int(sys.argv[1]))
else:
    print Usage

Executing:

$ python factorial.py 6
720

A shorter version of the above:

import sys

Usage = """
Usage:
$ python factorial.py 
"""

def fact(x):
    return (1 if x==0 else x * fact(x-1))

if (len(sys.argv)>1) :
    print fact(int(sys.argv[1]))
else:
    print Usage

Or a non-recursive version

def fact(x):
    f = 1
    while (x > 0):
        f = f * x
        x = x - 1
    return f

And in python 2.6, Math module (Mathematical functions) provides factorial function (math.factorial(x))

$ /jks/bin/python2.6
Python 2.6.2 (r262:71600, Jun 17 2009, 22:31:41)
[GCC 3.3.3 (Debian 20040306)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import math
>>> math.factorial(6)
720
>>>

Related concepts:
- Python math module

Performing multiple split in python

This post is mainly for python newbies.
Input file:

$ cat data.txt
File Start
#Comment
Gid034:s9823,I1290,s9034,s1230
Gid309:s9034,I5678,s1293,s4590
Gid124:s2145,K9008,s2381,s0234
Gid213:s9012,N9034,s8913,s9063
#End

Required: Extract the 3 rd field (colored blue) from the above file. i.e. required output:

I1290
I5678
K9008
N9034

Here we would need to split the required lines twice (one for field separator : and then for comma) to extract the desired column.

Using bash cut command or awk, the solution would be:
$ grep ^Gid data.txt | cut -d":" -f2 | cut -d"," -f2
$ awk -F"[:,]" '/^Gid/{print $3}' data.txt

The python code:

for line in open("data.txt"):
    if line.startswith("Gid"):
        print line.split(":")[1].split(",")[1]

Get your ip address in python

Python socket module provides the following functions you can get the IP address of your machine.

>>> import socket
>>> print socket.gethostname()
k172-16-0-12.heo.unstableme.com
>>> print socket.gethostbyname(socket.gethostname())
172.16.0.12
>>> socket.gethostbyaddr(socket.gethostbyname(socket.gethostname()))
('k172-16-0-12.heo.unstableme.com', ['k172-16-0-12'], ['172.16.0.12'])

Few definitions:

gethostbyname (hostname)
Translate a host name to IP address format. The IP address is returned as a string

gethostname ()
Return a string containing the hostname of the machine where the Python interpreter is currently executing. If you want to know the current machine's IP address, use socket.gethostbyname(socket.gethostname()). Note: gethostname() doesn't always return the fully qualified domain name; use socket.gethostbyaddr(socket.gethostname())

gethostbyaddr (ip_address)
Return a triple (hostname, aliaslist, ipaddrlist) where hostname is the primary host name responding to the given ip_address, aliaslist is a (possibly empty) list of alternative host names for the same address, and ipaddrlist is a list of IP addresses for the same interface on the same host (most likely containing only a single address). To find the fully qualified domain name, check hostname and the items of aliaslist for an entry containing at least one period.

Read more about Built-in Module socket here

Another link to "Determine the IP address of an eth interface"