multithreading - Python's threads block on IO operation -

i have following problem. whenever child thread wants perform io operation (writing file, downloading file) program hangs. in following example program hangs on opener.retrieve. if execute python main.py program blocked on retrieve function. if execute python ./src/tmp.py fine. don't understand why. can explain me happening?

i using python2.7 on linux system (kernel 3.5.0-27).

file ordering:

main.py ./src     __init__.py     tmp.py 

main.py

import src.tmp 

tmp.py

import threading import urllib  class downloaderthread(threading.thread):     def __init__(self, pool_sema, i):         threading.thread.__init__(self)         self.pool_sema  = pool_sema         self.daemon     = true         self.i =      def run(self):         try:             opener = urllib.fancyurlopener({})              opener.retrieve("http://www.greenteapress.com/thinkpython/thinkcspy.pdf", "/tmp/" + str(self.i) + ".pdf")         finally:             self.pool_sema.release()  class downloader(object):     def __init__(self):         maxthreads             = 1         self.pool_sema         = threading.boundedsemaphore(value=maxthreads)      def download_folder(self):         in xrange(20):             self.pool_sema.acquire()             print "downloading",             t = downloaderthread(self.pool_sema,i)             t.start()  d = downloader() d.download_folder() 

i managed work hacking urllib.py - if inspect see many import statements dispersed within code - i.e. uses imports stuff 'on fly' , not when module loads.

so, real reason still unknown - not worth investigating - deadlock in python's import system. shouldn't run nontrivial code during import - that's asking trouble.

if insist, can work if move these weird import statements beginning of urllib.py.