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4.3. Threads and Python

In this section, we discuss how to use threads in Python. This includes the limitations of threads due to the global interpreter lock and a quick demo script.

4.3.1. Global Interpreter Lock

Execution of Python code is controlled by the Python Virtual Machine (a.k.a. the interpreter main loop). Python was designed in such a way that only one thread of control may be executing in this main loop, similar to how multiple processes in a system share a single CPU. Many programs can be in memory, but only one is live on the CPU at any given moment. Likewise, although multiple threads can run within the Python interpreter, only one thread is being executed by the interpreter at any given time.

Access to the Python Virtual Machine is controlled by the global interpreter lock (GIL). This lock is what ensures that exactly one thread is running. The Python Virtual Machine executes in the following manner in an MT environment:

  1. Set the GIL
  2. Switch in a thread to run
  3. Execute either of the following:
    1. For a specified number of bytecode instructions, or
    2. If the thread voluntarily yields control (can be accomplished time.sleep(0))
  4. Put the thread back to sleep (switch out thread)
  5. Unlock the GIL
  6. Do it all over again (lather, rinse, repeat)

When a call is made to external code—that is, any C/C++ extension built-in function—the GIL will be locked until it has completed (because there are no Python bytecodes to count as the interval). Extension programmers do have the ability to unlock the GIL, however, so as the Python developer, you shouldn’t have to worry about your Python code locking up in those situations.

As an example, for any Python I/O-oriented routines (which invoke built-in operating system C code), the GIL is released before the I/O call is made, allowing other threads to run while the I/O is being performed. Code that doesn’t have much I/O will tend to keep the processor (and GIL) for the full interval a thread is allowed before it yields. In other words, I/O-bound Python programs stand a much better chance of being able to take advantage of a multithreaded environment than CPU-bound code.

Those of you who are interested in the source code, the interpreter main loop, and the GIL can take a look at the Python/ceval.c file.

4.3.2. Exiting Threads

When a thread completes execution of the function it was created for, it exits. Threads can also quit by calling an exit function such as thread.exit(), or any of the standard ways of exiting a Python process such as sys.exit() or raising the SystemExit exception. You cannot, however, go and “kill” a thread.

We will discuss in detail the two Python modules related to threads in the next section, but of the two, the thread module is the one we do not recommend. There are many reasons for this, but an obvious one is that when the main thread exits, all other threads die without cleanup. The other module, threading, ensures that the whole process stays alive until all “important” child threads have exited. (For a clarification of what important means, read the upcoming Core Tip, “Avoid using the thread module.”)

Main threads should always be good managers, though, and perform the task of knowing what needs to be executed by individual threads, what data or arguments each of the spawned threads requires, when they complete execution, and what results they provide. In so doing, those main threads can collate the individual results into a final, meaningful conclusion.

4.3.3. Accessing Threads from Python

Python supports multithreaded programming, depending on the operating system on which it’s running. It is supported on most Unix-based platforms, such as Linux, Solaris, Mac OS X, *BSD, as well as Windows-based PCs. Python uses POSIX-compliant threads, or pthreads, as they are commonly known.

By default, threads are enabled when building Python from source (since Python 2.0) or the Win32 installed binary. To determine whether threads are available for your interpreter, simply attempt to import the thread module from the interactive interpreter, as shown here (no errors occur when threads are available):

>>> import thread

If your Python interpreter was not compiled with threads enabled, the module import fails:

>>> import thread
Traceback (innermost last):
  File "<stdin>", line 1, in ?
ImportError: No module named thread

In such cases, you might need to recompile your Python interpreter to get access to threads. This usually involves invoking the configure script with the --with-thread option. Check the README file for your distribution to obtain specific instructions on how to compile Python with threads for your system.

4.3.4. Life Without Threads

For our first set of examples, we are going to use the time.sleep() function to show how threads work. time.sleep() takes a floating point argument and “sleeps” for the given number of seconds, meaning that execution is temporarily halted for the amount of time specified.

Let’s create two time loops: one that sleeps for 4 seconds (loop0()), and one that sleeps for 2 seconds (loop1()), respectively. (We use the names “loop0” and “loop1” as a hint that we will eventually have a sequence of loops.) If we were to execute loop0() and loop1() sequentially in a one-process or single-threaded program, as onethr.py does in Example 4-1, the total execution time would be at least 6 seconds. There might or might not be a 1-second gap between the starting of loop0() and loop1() as well as other execution overhead which can cause the overall time to be bumped to 7 seconds.

Example 4-1. Loops Executed by a Single Thread (onethr.py)

This script executes two loops consecutively in a single-threaded program. One loop must complete before the other can begin. The total elapsed time is the sum of times taken by each loop.

1    #!/usr/bin/env python
3    from time import sleep, ctime
5    def loop0():
6        print 'start loop 0 at:', ctime()
7        sleep(4)
8        print 'loop 0 done at:', ctime()
10   def loop1():
11       print 'start loop 1 at:', ctime()
12       sleep(2)
13       print 'loop 1 done at:', ctime()
15   def main():
16       print 'starting at:', ctime()
17       loop0()
18       loop1()
19       print 'all DONE at:', ctime()
21   if __name__ == '__main__':
22       main()

We can verify this by executing onethr.py, which renders the following output:

$ onethr.py
starting at: Sun Aug 13 05:03:34 2006
start loop 0 at: Sun Aug 13 05:03:34 2006
loop 0 done at: Sun Aug 13 05:03:38 2006
start loop 1 at: Sun Aug 13 05:03:38 2006
loop 1 done at: Sun Aug 13 05:03:40 2006
all DONE at: Sun Aug 13 05:03:40 2006

Now, assume that rather than sleeping, loop0() and loop1() were separate functions that performed individual and independent computations, all working to arrive at a common solution. Wouldn’t it be useful to have them run in parallel to cut down on the overall running time? That is the premise behind MT programming that we now introduce.

4.3.5. Python Threading Modules

Python provides several modules to support MT programming, including the thread, threading, and Queue modules. Programmers can us the thread and threading modules to create and manage threads. The thread module provides basic thread and locking support; threading provides higher-level, fully-featured thread management. With the Queue module, users can create a queue data structure that can be shared across multiple threads. We will take a look at these modules individually and present examples and intermediate-sized applications.

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