Why there is space in the code example in python threading

Question

I am reading the PyMOTW threading post

The first example:

import threading

def worker():
    """thread worker function"""
    print 'Worker'
    return

threads = []
for i in range(5):
    t = threading.Thread(target=worker)
    threads.append(t)
    t.start()

I run it, and get the result:

Worker
-Worker
Worker
-Worker
Worker

The - is a space, and the format will be difference every time

But I don't know why there is space?

Some times it will output empty line also, why?

Answer 1

If you examine the byte code for a print statement, it looks like this:

>>> def f():
...  print "Worker"
...
>>> dis.dis(f)
  2           0 LOAD_CONST               1 ('Worker')
              3 PRINT_ITEM
              4 PRINT_NEWLINE
              5 LOAD_CONST               0 (None)
              8 RETURN_VALUE

In fact, printing a number of items results in a number of PRINT_ITEM byte codes:

>>> def g():
...  print "Worker", "Hello"
...
>>> dis.dis(g)
  2           0 LOAD_CONST               1 ('Worker')
              3 PRINT_ITEM
              4 LOAD_CONST               2 ('Hello')
              7 PRINT_ITEM
              8 PRINT_NEWLINE
              9 LOAD_CONST               0 (None)
             12 RETURN_VALUE

To make multiple items have a space between them, the file object has a flag called softspace that indicates whether a space should be output before the next item. The code for PRINT_ITEM and PRINT_NEWLINE looks roughly like this:

def PRINT_ITEM(f, item):
    if f.softspace:
        f.write(' ')
    f.write(str(item))
    f.softspace = True

def PRINT_NEWLINE(f):
    f.write('\n')
    f.softspace = False

When you write to stdout from a number of threads at once, these operations become interleaved in random ways. So instead of getting PRINT_ITEM and PRINT_NEWLINE alternating, you can have two PRINT_ITEMs in a row. If that happens, you will have extra spaces in your output, because the two PRINT_ITEMs and the softspace flag will produce an extra space.

Answer 2

try using sys.stdout.write("Worker\n")