Global name 'display' is not defined during "exec()"

I tried executing a simple line drawing program using exec(). It worked fine. But when I tried to execute the same program by sending it through a tcp/ip network(the server reads the program and sends it to the client which receives it to a variable(b) of string type) and then i use exec(b) in the client to execute it but it says: NameError: global name 'display' is not defined

The line drawing code is:

from OpenGL.GLUT import *
from OpenGL.GLU import *
from OpenGL.GL import *
import sys

name = 'line'

def display():
 glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT)
 glPushMatrix()
 glTranslatef(-1,-1,0)
 gluLookAt(
  0.1, 0.1, 0.3,
  0.0, 0.0, 0.0,
  0.0, 1.0, 0.0);

 glLineWidth(3.0)
 color = [1.,1.,1.,1.]
 glBegin(GL_LINES)
 glVertex3f(0,0,0) # origin of the line
 glVertex3f(.5,1.0,.9) # ending point of the line
 glEnd()
 glPopMatrix()
 glu开发者_JS百科tSwapBuffers()
 return
def main():
 glutInit(sys.argv)
 print 'hello'
 glutCreateWindow(name)
 glClearColor(0.4,0.5,0.3,1.0)
 glutDisplayFunc(display)
 glutMainLoop()
 return
main()

This part of the client code receives the program and stores it into the variable and then we use exec():

while f: 
   a = client.recv(1024)
   if a=="#p":
    f=0
    break
   b+=a

  print b

  exec(b) 

The code executes upto the part where print hello is given and then stops.

The error message:

hello
Exception in thread Thread-1:
Traceback (most recent call last):
  File "/usr/lib/python2.6/threading.py", line 532, in __bootstrap_inner
    self.run()
  File "r13client.py", line 31, in run
    exec(b)
  File "<string>", line 34, in <module>
  File "<string>", line 31, in main
NameError: global name 'display' is not defined

I am unable to understand what is going wrong here. If anyone could help I'd be grateful.

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What exactly is the string you send and then exec?

Your error sounds like you only send the main() function, which won't work.

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For an example of one approach to networking and graphics, take a look at https://launchpad.net/game