Why a script that uses threads prints extra lines occasionally?

Question

If print s is replaced by print >>sys.stderr, s then the effect vanishes.

import random, sys, time
import threading

lock = t         


        

Answer 1

Take a look at this stackoverflow thread: How do I get a thread safe print in Python 2.6?. Apparently, printing to sout is not thread-safe.

If you turn on verbose threading, you can see this better:

threading.Thread(target=echo, args=(c,), verbose=True).start()

I get output like this:

MainThread: .start(): starting thread
Thread-1: .__bootstrap(): thread started
MainThread: .start(): starting thread
Thread-2: .__bootstrap(): thread started
MainThread: .start(): starting thread
Thread-3: .__bootstrap(): thread started
MainThread: .join(): waiting until thread stops
a
b
Thread-1: .__bootstrap(): normal return
Thread-2: .__bootstrap(): normal return
MainThread: .join(): thread stopped
MainThread: .join(): waiting until thread stops
Thread-3: .__bootstrap(): normal return
MainThread: .join(): thread stopped
c

You can see that thread 3 is shown as finishing before printing the 'c' character. This clearly cannot be the case, so this leads me to assume that printing to the console is not thread-safe.

This, however, does not explain why printing to sys.stderr appears to work correctly.