I can't figure out why the threading object `threadObj` can't use `isAlive`, when I call `stop()`. The code is described below.

```
from threading import Thread

class workerThread(Thread):
  def __init__(self, _parent):
    Thread.__init__(self)
    self.parent = _parent
    self.active = False

  def run(self):
    while(self.active == False):
      print 'I am here'
    print 'and now I am here'

class imp():
  def __init__(self):
    self.threadObj = None

  def start(self):
    self.threadObj = workerThread(self)
    self.threadObj.start()

  def stop(self):
    if self.threadObj.isAlive() == True:
      print 'it is alive'

```

It says: AttributeError: 'NoneType' object has no attribute 'isAlive'

The calling code is descibed below:

```
from filename import imp

filename = imp()
if option == 'A':
  filename.start()
elif option == 'B':
  filename.stop()

```