Multithreading: why aren't generators thread safe? What happens when it is split between threads?

Question

I read this question that asks if the generators are thread safe, and one answer said:

It is not thread safe; simultaneous calls can alternate, and local variables.

Another answer shows that you can use locking to ensure that only one thread uses the generator at a time.

I am new to multithreading. Can someone come up with an example to show what exactly happens when you use a generator without locking?

For example, I have no problem if I do this:

import threading

def generator():
    for i in data:
        yield i

class CountThread(threading.Thread):
    def __init__(self, name):
        threading.Thread.__init__(self)
        self.name = name

    def run(self):
        for i in gen():
            print '{0} {1}'.format(self.name, i)

data = [i for i in xrange(100)]
gen = generator()
a = CountThread('a')
b = CountThread('b')
a.start()
b.start()

Answer 1

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import threading

class CountThread(threading.Thread):
  def __init__(self, gen):
      threading.Thread.__init__(self)
      self.gen = gen
      self.numbers_seen = 0

  def run(self):
      for i in self.gen:
          self.numbers_seen += 1


def generator(data):
    for _ in data:
        yield data

gen = generator(xrange(10000))

a = CountThread(gen)
b = CountThread(gen)

a.start()
b.start()

a.join()
b.join()

print "Numbers seen in a", a.numbers_seen
print "Numbers seen in b", b.numbers_seen

, , Python ( , 10000, 10000000), :

Exception in thread Thread-2:
Traceback (most recent call last):
  File "/usr/local/Cellar/python/2.7.5/Frameworks/Python.framework/Versions/2.7/lib/python2.7/threading.py", line 808, in __bootstrap_inner
    self.run()
  File "test.py", line 10, in run
    for i in self.gen:
ValueError: generator already executing