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apply Pool map with a method of a class and a list in pythob

+2 votes
547 views

I would like to apply the Pool.map method to a member of a class. Here is a small example that shows what I would like to do:

from multiprocessing import Pool

class A(object):
 def __init__(self,x):
 self.value = x
 def fun(self,x):
 return self.value**x

l = range(10)

p = Pool(4)

op = p.map(A.fun,l)

using this with the normal map doesn't cause any problem

This fails because it says that the methods can't be pickled. (I assume it has something to do with the note in the documentation: "functionality within this package requires that the __main__ module be importable by the children.", which is obscure to me).

I would like to understand two things: why my code fails and when I can expect it to fail? what is a possible workaround?

posted Aug 6, 2013 by Deepak Dasgupta

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2 Answers

0 votes

Do you ever instantiate any A() objects? You're attempting to call an unbound method without passing it a 'self'.

You may find the results completely different in Python 2 vs Python 3, and between bound and unbound methods. In Python 3, an unbound method is simply a function. In both versions, a bound method carries its
first argument around, so it has to be something different. Play around with it a bit.

answer Aug 6, 2013 by Salil Agrawal
0 votes

I have tried a lot of variations, instantiating the object, creating lambda functions that use the unbound version of fun (A.fun.__func__) etc etc.. I have played around it quite a bit before posting.

As far as I have understood the problem is due to the fact that Pool pickle the function and copy it in the various pools.. But since the methods cannot be pickled this fails..

The same example I posted won't run in Python 3.2 neither (I am mostly interested in a solution for Python 2.7, sorry I forgot to mention that).

answer Aug 6, 2013 by anonymous
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