如何访问while循环中正在更新的变量

Question

I am trying to access a variable that is being constantly updated in a while loop in a different file. Here is the code I used for testing:

# file1    
import time
x = 0
while True:
    x += 1
    time.sleep(2.0)

# file2
from file1 import x
print x

When I run file2, it starts the while loop from the beginning. I would like to access one instance of x. For example, if x=10, I would want file2 to print 10. Is this possible?

Answer 1

I'm not quite sure what you mean. Are you wanting file1 to run, and be incrementing the value of "x" every 2 seconds, indefinitely and that when you run file2 at any time, it pulls the current value of "x" from the program/python instance running "file1"?

If so, this is not how you would approach it. With file2 you are pulling the set variable x=0 from file1. What you need to do is have some form of IPC (Inter-Process Communication) so that file2 can access the value of "x" from file1. You can do this a multitude of ways, including shared memory, a key/value store program like redis or memcached, a database, etc.

If you want to do it via redis or memcached, simply run redis, use the redis library for Python, and call the .incr method for the key "x" every 2 seconds. Then, when you run file2, call the .get method for key "x" and you will get the current value. When file1 is running, it will continue to increment x; when it's not, it won't and will effectively freeze. However, redis will keep the last known value in memory for the key "x".

To do it with a database, you can implement a mySQL database/table and increase the value of "x" in a key column in a table every 2 seconds. You'd have to look at the mySQL libraries for Python.

To do it with shared memory, look at the shared memory functions for Python.

There are also many other ways to share data. You could, simply, write the value of "x" to a file every 2 seconds by opening it, writing the new value, flushing and closing it. Then simply have file2 read that file. Of course with this you then have the issue of race conditions where it reads the file before its updated and gets a stale value, all dependent on the priority of the OS' filesystem writes for that file at that time from that process.

Answer 2

You can try the following. First, as there's and infinite loop, importing file1 will block, so you should run the loop in a thread. And second you can wrap the integer being incremented in a list (or any other kind of objects), so you can use the reference to its current value (otherwise you will be importing a value not a reference):

# file1    
import time
import threading

x = [0]

def update_var(var):
    while True:
        var[0] += 1
        time.sleep(2.0)

threading.Thread(target=update_var, args=(x,)).start()

# file2
from file1 import x
print x[0]