In the code below, child processes see changes to global variables caused by
other child processes. That is, pool.map is producing a list where the value of
``lst`` is being (non-deterministically) shared across child processes.
Standard advice is that, "Processes have independent memory space" and "each
process works with its own copy" of global variables, so I thought this was not
supposed to happen. In the end, the globals are not changed, which matches my
expectation. Still, clearly I have misunderstood what seemed to be standard
advice. Can someone clarify for me?
Expected result from pool.map: [[0],[1],[2]]
Usual result: [[0],[0,1],[0,1,2]]
Occasional result: [[0],[1],[0,2]]
Thanks,
Alan Isaac
#--------------- temp.py -------------------------
#run at Python 2.7 command prompt
import time
import multiprocessing as mp
lst = []
lstlst = []
def alist(x):
lst.append(x)
lstlst.append(lst)
print "a"
return lst
if __name__=='__main__':
pool = mp.Pool(3)
print pool.map(alist,range(3)) #UNEXPECTED RESULTS
print "b"
time.sleep(0.1)
print "c"
print lst
print lstlst
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