Three Standard Streams: stdin, stdout, stderr

First, you shoud keep in mind that standard input/output is one of the best way to check bug. Because the data they offer is raw.

These are actually files (or file-like objects), and you can apply most of what you learn about files to them.

sys.stdin:

A standard source of data input is sys.stdin. When a program reads from standard input, you can supply text by typing it, or you can link it with the standard output of another program, using a pipe(This is a standard UNIX concept.)

sys.stdout:

The text you give to print appears in sys.stdout. The prompts for input and raw_input also go there. Data written to sys.stdout typically appears on your screen, but can be rerouted to the standard input of another program with a pipe, as mentioned.

sys.stderr:

Error messages (such as stack traces) are written to sys.stderr. In many ways, it is similar to sys.stdout

Simple Script That Counts the Words in sys.stdin:

# somescript.py
import sys
text = sys.stdin.read()
words = text.split()
wordcount = len(words)
print 'Wordcount:', wordcount 

file somefile.txt contain text like: 1 2 3 4 5 6 

Here are the results of linux command:  $cat somefile.txt | python somescript.py:

Wordcount: 6