python中的全局变量混淆

我有一个全局变量,我将其用作默认变量.根据我的程序中发生的情况,我需要能够更改这些默认值,并通过我的代码的剩余操作保持更改.我希望它们在任何地方都有变化和定义,因此我使用了全局变量这是一些测试代码,显示了我如何修改这些变量.

当我这样做时,我有以下问题……

>该程序认为myGlobal尚未在main中定义.但
它有.为什么？
>当我改变myGlobal后调用子程序时.我不希望这发生.

完成我想要做的事情的正确方法是什么？例子？

#!/usr/bin/python

import sys

myGlobal = "foo"

print "********************"
print "MyGlobal %s" % myGlobal
print "********************"


def main(argv):

   #UnboundLocalError: local variable 'myGlobal' referenced before assignment
   print '1. Printing the global again: ' + myGlobal

   myGlobal = "bar"
   print "2. Change the global and print again: " + myGlobal

   # now call a subroutine
   mySub()

# Checks for output file,if it doesn't exist creates one
def mySub():
   # Why isn't the global "bar" not "foo"?
   print '3. Printing the global again: ' + myGlobal

   myGlobal = "black sheep"
   print "4. Change the global and print again: " + myGlobal




if __name__ == "__main__":
   main(sys.argv[1:])

def main(argv):
  global myGlobal
  #...
  myGlobal = "bar"

class MyApp(object):
  def main(self,argv):
    self.myVar = "bar"
    self.mySub()

  def mySub(self):
    print self.myVar

MyApp().main(sys.argv[1:])