Manager in Python Multiprocessing

What is a Manager?

• A Manager in Python's multiprocessing module provides a way to create and manage shared objects between processes.
• Shared objects include lists, dictionaries, Events, Locks, and more.

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Why Use a Manager?

• Processes in Python have separate memory spaces.
• Without a Manager, objects created in one process are not automatically shared with others.
• A Manager allows processes to interact with shared objects safely.

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Common Use Cases

1. Sharing data structures like lists or dictionaries between processes.
2. Synchronizing processes using shared objects like Event or Lock.

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How to Use a Manager?

Basic Syntax

from multiprocessing import Manager

with Manager() as manager:
    shared_list = manager.list()  # Create a shared list
    shared_dict = manager.dict()  # Create a shared dictionary

Example: Shared Dictionary

from multiprocessing import Process, Manager

def worker(shared_dict):
    shared_dict["key"] = "value"  # Update the shared dictionary

if __name__ == "__main__":
    with Manager() as manager:
        shared_dict = manager.dict()  # Create a shared dictionary
        p = Process(target=worker, args=(shared_dict,))
        p.start()
        p.join()
        print(shared_dict)  # Output: {'key': 'value'}

Example: Shared Event

from multiprocessing import Process, Manager

def worker(event):
    print("Waiting for event...")
    event.wait()  # Wait for the event to be set
    print("Event is set!")

if __name__ == "__main__":
    with Manager() as manager:
        event = manager.Event()  # Create a shared Event
        p = Process(target=worker, args=(event,))
        p.start()
        event.set()  # Set the event
        p.join()

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Advantages

• Provides easy sharing of objects across processes.
• Manages synchronization safely and avoids manual complexity.

Disadvantages

• Slower than using shared memory (e.g., multiprocessing.Value or multiprocessing.Array) because communication is handled via a server process.
• Overhead for small or frequent updates.

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When to Use a Manager?

• Use a Manager when you need to share complex data structures or synchronization primitives like Event or Lock.
• For performance-critical tasks with simple data, prefer shared memory objects.