how to use While loops in python programming language

Here we present our first control structure. Ordinarily the computer starts with the first line
and then goes down from there. Control structures change the order that statements are
executed or decide if a certain statement will be run. Here's the source for a program that
uses the while control structure:
a = 0
while a < 10:
a = a + 1
print a
And here is the extremely exciting output:
1
2
3
4
5
6
7
8
9
10
And you thought it couldn't get any worse after turning your computer into a five dollar
calculator?
So what does the program do? First it sees the line a = 0 and sets a to zero. Then it sees
while a < 10: and so the computer checks to see if a < 10. The first time the computer
sees this statement a is zero so it is less than 10. In other words as long as a is less than ten
the computer will run the tabbed in statements. This eventually makes a equal to ten (by
adding one to a again and again), and the while a < 10 is not true any longer. Reaching
that point the program will not run the indented lines any longer.
Always remember to put a colon ":" after the "while" statement!
Here is another example of the use of while:

a = 1
s = 0
print ‚Enter Numbers to add to the sum.‚
print ‚Enter 0 to quit.‚
while a != 0:
print ‚Current Sum:‚, s
a = input(‚Number? ‚)
s = s + a
print ‚Total Sum =‚, s
Enter Numbers to add to the sum.
Enter 0 to quit.
Current Sum: 0
Number? 200
Current Sum: 200
Number? -15.25
Current Sum: 184.75
Number? -151.85
Current Sum: 32.9
Number? 10.00
Current Sum: 42.9
Number? 0
Total Sum = 42.9
Notice how print 'Total Sum =', s is only run at the end. The while statement only
affects the lines that are indented with whitespace. The != means "does not equal" so "while
a != 0:" means: "until a is zero, run the tabbed statements that follow."
Infinite loops
Now that we have while loops, it is possible to have programs that run forever. An easy
way to do this is to write a program like this:
while 1 == 1:
print "Help, I‚m stuck in a loop."
The "==" operator is used to test equality of the expressions on the two sides of the operator, just as "<" was used for "less than" before (you will get a complete list of all comparison
operators in the next chapter).
This program will output Help, I'm stuck in a loop. until the heat death of the universe or until you stop it, because 1 will forever be equal to 1. The way to stop it is to hit the Control (or Ctrl) button and C (the letter) at the same time. This will kill the program.
(Note: sometimes you will have to hit enter after the Control-C.)

check out this example.
Password.py
# Waits until a password has been entered. Use Control-C to break
out without
# the password
# Note that this must not be the password so that the
# while loop runs at least once.
password = "no password"
# note that != means not equal
while password != "unicorn":
password = raw_input("Password: ")
print "Welcome in"
Sample run:
Password: auo
Password: y22
Password: password
Password: open sesame
Password: unicorn
Welcome in