What is wrong with the code?

[dullboy]

Here is the code,

class A():
   target = open("test.log", "a")

   def test(self):
       print "test"
       target.write("test\n")
       target.close()

def main():
   a = A()
   a.test()

if __name__ == "__main__":
   main() 
 

There is an error: NameError: global name 'target' is not defined. Why? Thanks.