为什么全局变量在Python中的不同函数中不起作用？

Question

I'm beginner in Python. I wrote the code like as below just for practice.. Please look on it

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

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

def Main():
   wrte()
   appnd()

Main()

Output is like as below

  1 .Line..

    2 .Line..

    3 .Line..

    4 .Line..

**Traceback (most recent call last):
  Line 18, in <module>
    Main()
  Line 16, in Main
    appnd()
  Line 9, in appnd
    j=i
UnboundLocalError: local variable 'i' referenced before assignment**

Expected Result:: The next sequence is should be appended like

5. Line..
6. Line..
7. Line..

Please help me on this..

Answer 1

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.

Answer 2

adding

global i

before

j=i

should solve the problem

Answer 3

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.

Answer 4

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.

Answer 5

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

Answer 6

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()