Share a variable between workers with Python multiprocessing [duplicate]

Question

This question already has an answer here:

• Python multiprocessing and a shared counter 4 answers

How can I read and update a variable shared between multiple workers in Python?

For example, I'm scanning through a list of files using multiple processes in Python, and would like to check if the parent directory has been scanned or not.

def readFile(filename):
  """ Add the parent folder to the database and process the file
  """

  path_parts = os.path.split(filename)
  dirname = os.path.basename(path_parts[0])
  if dirname not in shared_variable:
    # Insert into the database


   #Other file functions


def main():
  """ Walk through files and pass each file to readFile()
  """
  queue = multiprocessing.Queue()
  pool = multiprocessing.Pool(None, init, [queue])

  for dirpath, dirnames, filenames in os.walk(PATH):

    full_path_fnames = map(lambda fn: os.path.join(dirpath, fn),
                           filenames)
    pool.map(readFile, full_path_fnames)

Answer 1

You can use multiprocessing.Manager to help with this. It allows you to create a list that can be shared between processes:

from functools import partial
import multiprocessing

def readFile(shared_variable, filename):
  """ Add the parent folder to the database and process the file
  """

  path_parts = os.path.split(filename)
  dirname = os.path.basename(path_parts[0])
  if dirname not in shared_variable:
    # Insert into the database


   #Other file functions


def main():
  """ Walk through files and pass each file to readFile()
  """
  manager = multiprocessing.Manager()
  shared_variable = manager.list()
  queue = multiprocessing.Queue()
  pool = multiprocessing.Pool(None, init, [queue])

  func = partial(readFile, shared_variable)
  for dirpath, dirnames, filenames in os.walk(PATH):

    full_path_fnames = map(lambda fn: os.path.join(dirpath, fn),
                           filenames)
    pool.map(func, full_path_fnames)

The partial is just used to make it easier to pass shared_variable to each call of readFile, along with each member of full_path_fnames via map.

Answer 2

Have a look at https://docs.python.org/2/library/multiprocessing.html#sharing-state-between-processes. You can use shared memory using Value or Array to share data between two or more threads.