How to let Pool.map take a lambda function

Question

I have the following function:

def copy_file(source_file, target_dir):
    pass

Now I would like to use multiprocessing to execute this function at once:

p = Pool(12)
p.map(lambda x: copy_file(x,target_dir), file_list)

The problem is, lambda's can't be pickled, so this fails. What is the most neat (pythonic) way to fix this?

Answer 1

Use a function object:

class Copier(object):
    def __init__(self, tgtdir):
        self.target_dir = tgtdir
    def __call__(self, src):
        copy_file(src, self.target_dir)

To run your Pool.map:

p.map(Copier(target_dir), file_list)