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

Because print write to stdout text first, then end string. Pseudo-code to explain:

def print(*args, **kwargs):
    write_to_stdout(to_single_string(args))
    write_to_stdout(end)  # usually a newline "\n"

so, in multithreading first string of both threads executes before second, so two newlines printed at the same time. But why lines don't on same line? I don't know. Needs to check realization of python print more deeply.

Answer 2

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: <Thread(Thread-1, initial)>.start(): starting thread
Thread-1: <Thread(Thread-1, started 6204)>.__bootstrap(): thread started
MainThread: <Thread(Thread-2, initial)>.start(): starting thread
Thread-2: <Thread(Thread-2, started 3752)>.__bootstrap(): thread started
MainThread: <Thread(Thread-3, initial)>.start(): starting thread
Thread-3: <Thread(Thread-3, started 4412)>.__bootstrap(): thread started
MainThread: <Thread(Thread-2, started 3752)>.join(): waiting until thread stops
a
b
Thread-1: <Thread(Thread-1, started 6204)>.__bootstrap(): normal return
Thread-2: <Thread(Thread-2, started 3752)>.__bootstrap(): normal return
MainThread: <Thread(Thread-2, stopped 3752)>.join(): thread stopped
MainThread: <Thread(Thread-3, started 4412)>.join(): waiting until thread stops
Thread-3: <Thread(Thread-3, started 4412)>.__bootstrap(): normal return
MainThread: <Thread(Thread-3, stopped 4412)>.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.