CODEWITHSUNDEEP
pythonmultiprocessing
Monica
Monica

Reputation: 1070

List is not modified, multiprocessing Python

I am trying to figure out how to properly modify a list in multiprocessing using Python. I have written a toy code:

 input_list = ["A", "B", "C", "D", "E", "F"]
  
  manager = Manager()
  shared_list = manager.list()
  shared_list = input_list[:]
  
  
  def do_stuff(element, i):
      global shared_list
      shared_list[i] = element[i] * 2
      print(shared_list[i])
  
  
  processes = []
  for i in range(len(input_list)):
      p = Process(target=do_stuff, args=(input_list, i))
      p.start()
      processes.append(p)
  
  for p in processes:
      p.join()
  
  print(shared_list)

I had created a shared list by means of Manager and subsequently I tried to utilize it inside the do_stuff function. The problem is that once the multiprocessing is over, shared_list is not modify. Could you please help me to understand what I am doing wrong?

Upvotes: 0

Views: 149

Answers (1)

larsks
larsks

Reputation: 312323

Your problem is here:

shared_list = input_list[:]

With this statement, you are replacing the value of shared_list with the value of input_list[:] (that is, you are discarding your manager.list instance).

You're probably looking for:

shared_list[:] = input_list[:]

By using a slice assignment like this, you are setting the contents of shared_list to the contents of input_list. This is called "slice assignment" and there are a variety of tutorials out there that discuss it in more detail.

Upvotes: 3

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