Why the global variable is not working in the different functions in Python?

You need the global definition in each function in which you use that variable.

def appnd():
   global i

Note: If possible, move global variables and related functions into a class.

The next definition will work:

def appnd():
  j=i
  while i in range(i,j+3):
     print "%s .Line..\n" %i

# it would print infinitely, but will work

At compile time Python looks at a way variables are used in the function to define a scope to look for them. In your definition of appnd it sees assign for i thus trying to threat it as local variable. In my code there are no assignments, so Python simply get the value for i from the parent scope - in this case i is neither local nor global, it called free variable. Execution model - Scopes and Binding - highly recommended reading.

I guess you know when you should use global, otherwise it wouldn't be in your write function. You can omit it if you only read the variable, which I think you want in your append function, but you've got an i+=1 in it, so you do modify it. Just change append to do:

for line in range(i, i + 3):
    print "%s .Line..\n" % line

adding

global i

before

j=i

should solve the problem

The scope of your definitions are local. If you declare a variable as global in a function, it doesn't mean that it would be applied to all the functions. You have to declare i as global in your appnd() function too. Having said that, it doesn't mean that your style is right. You would rather pass your variables to functions.

In the appnd function, you must make the global variable i.

i=1
def wrte():
 global i
 while i<5:
     print "%s .Line..\n" %i
     i+=1

def appnd():
    global i
    j=i
    while i in range(i,j+3):
        print "%s .Line..\n" %i
        i+=1

def Main():
   wrte()
   appnd()

Main()