关于python中线程输出的困惑

Question

I am currently working with python v.2.7 on windows 8.

My programme is using threads. I am providing a name to these threads during their creation. The first thread is named First-Thread and second one is named Second-Thread . The threads execute a method named as getData() that does the following:

• makes the current thread to sleep for some time
• calls the compareValues()
• retrieve the information from the compareValues () and adds them to a list called myList

The compareValues() does the following:

• generates a random number
• checks if it is less than 5 or if it is greater than or equal to 5 and yields the result along with the current thread's name

I save the results of these threads to a list named as myList and then finally print this myList .

Problem : Why I never see the Second-Thread in myList ? I don't understand this behavior. Please try to execute this code to see the output in order to understand my problem.

Code :

import time
from random import randrange
import threading

myList = []
def getData(i):
        print "Sleep for %d"%i
        time.sleep(i)
        data = compareValues()
        for d in list(data):
            myList.append(d)

def compareValues():
        number = randrange(10)
        if number >= 5:
             yield "%s: Greater than or equal to 5: %d  "%(t.name, number)
        else:
             yield "%s: Less than 5: %d  "%(t.name, number)

threadList = []
wait = randrange(10)+1
t = threading.Thread(name = 'First-Thread', target = getData, args=(wait,))
threadList.append(t)
t.start()
wait = randrange(3)+1
t = threading.Thread(name = 'Second-Thread', target = getData, args=(wait,))
threadList.append(t)
t.start()
for t in threadList:
    t.join()
print
print "The final list"
print myList

Sample output :

Sleep for 4Sleep for 1

The final list
['First-Thread: Greater than or equal to 5: 7  ', 'First-Thread: Greater than or equal to 5: 8  ']

Thank you for your time.

Answer 1

def compareValues():
        number = randrange(10)
        if number >= 5:
             yield "%s: Greater than or equal to 5: %d  "%(t.name, number)
        else:
             yield "%s: Less than 5: %d  "%(t.name, number)

In the body of compareValues the code refers to t.name . By the time compareValues() gets called by the threads, t , which is looked up according to the LEGB rule and found in the global scope , references the first thread because the t.join() is waiting on the first thread. t.name thus has the value First-Thread .

To get the current thread name, use threading.current_thread().name :

def compareValues():
        number = randrange(10)
        name = threading.current_thread().name
        if number >= 5:
             yield "%s: Greater than or equal to 5: %d  "%(name, number)
        else:
             yield "%s: Less than 5: %d  "%(name, number)

Then you will get output like

Sleep for 4
Sleep for 2

The final list
['Second-Thread: Less than 5: 3  ', 'First-Thread: Greater than or equal to 5: 5  ']