[issue20775] Modifications to global variables ignored after instantiating multiprocessing.Pool

[Naftali Harris]

New submission from Naftali Harris:

Hi everyone,

It appears that if you use a global variable in a function that you pass to Pool.map, but modify that global variable after instantiating the Pool, then the modification will not be reflected when Pool.map calls that function.

Here's a short script, (also attached), that demonstrates what I mean:

$ cat reproduces.py
from multiprocessing import Pool

name = "Not Updated"
def f(ignored):
    print(name)


def main():
    global name
    p = Pool(3)
    name = "Updated"
    p.map(f, range(3))

if __name__ == "__main__":
    main()
$ python reproduces.py 
Not Updated
Not Updated
Not Updated


If the `name = "Updated"' line is moved above the `p = Pool(3)' line, then the script will print "Updated" three times instead.

This behavior is present in versions 2.6, 2.7, 3.1, 3.2, 3.3, and 3.4. I run Linux Mint 14 (nadia), on an Intel i5-3210M processor (four cores).

Is this expected behavior?

Thanks very much,

Naftali

----------
components: Library (Lib)
files: reproduces.py
messages: 212221
nosy: Naftali.Harris
priority: normal
severity: normal
status: open
title: Modifications to global variables ignored after instantiating multiprocessing.Pool
type: behavior
versions: Python 2.7, Python 3.1, Python 3.2, Python 3.3, Python 3.4
Added file: http://bugs.python.org/file34223/reproduces.py

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<http://bugs.python.org/issue20775>
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