Using Pool map with a method of a class and a list
Peter Otten
__peter__ at web.de
Wed Aug 7 10:46:09 EDT 2013
Joshua Landau wrote:
> On 7 August 2013 11:10, Luca Cerone <luca.cerone at gmail.com> wrote:
>> I can't try it now, I'll let you know later if it works!
>> (Though just by reading I can't really understand what the code does).
>
> Well,
>
>>> from multiprocessing import Pool
>>> from functools import partial
>>>
>>> class A(object):
>>> def __init__(self,x):
>>> self.value = x
>>> def fun(self,x):
>>> return self.value**x
>
> This is all the same, as with
>
>>> l = range(100)
>>> p = Pool(4)
>
> You then wanted to do:
>
>> op = p.map(A(3).fun, l)
>
> but bound methods can't be pickled, it seems.
>
> However, A(3) *can* be pickled. So what we want is a function:
>
> def proxy(arg):
> A(3).fun(arg)
>
> so we can write:
>
>> op = p.map(proxy, l)
>
> To generalise you might be tempted to write:
>
> def generic_proxy(instance, name):
> def proxy(arg):
> # Equiv. of instance.name(arg)
> getattr(instance, name)(arg)
>
> but the inner function won't work as functions-in-functions can't be
> pickled either.
>
> So we use:
>
>>> def _getattr_proxy_partialable(instance, name, arg):
>>> return getattr(instance, name)(arg)
>
> Which takes all instance, name and arg. Of course we only want our
> function to take arg, so we partial it:
>
>>> def getattr_proxy(instance, name):
>>> """
>>> A version of getattr that returns a proxy function that can
>>> be pickled. Only function calls will work on the proxy.
>>> """
>>> return partial(_getattr_proxy_partialable, instance, name)
>
> partial objects are picklable, btw.
>
>>> op = p.map(getattr_proxy(A(3), "fun"), l)
>>> print(op)
>
> :)
There is also the copy_reg module. Adapting
<http://mail.python.org/pipermail/python-list/2008-July/469164.html>
you get:
import copy_reg
import multiprocessing
import new
def make_instancemethod(inst, methodname):
return getattr(inst, methodname)
def pickle_instancemethod(method):
return make_instancemethod, (method.im_self, method.im_func.__name__)
copy_reg.pickle(
new.instancemethod, pickle_instancemethod, make_instancemethod)
class A(object):
def __init__(self, a):
self.a = a
def fun(self, b):
return self.a**b
if __name__ == "__main__":
items = range(10)
pool = multiprocessing.Pool(4)
print pool.map(A(3).fun, items)
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