Are Python Lists Thread-Safe for Concurrent Data Access?

Can Lists Be Concurrently Accessed Safely?

Many developers recommend utilizing queues instead of lists and the .pop() method when working with multiple threads. This recommendation raises the question: Are lists intrinsically thread-unsafe or is there another underlying reason?

Thread Safety of Lists

In fact, lists are intrinsically thread-safe. In the CPython implementation, the Global Interpreter Lock (GIL) guards against simultaneous access to lists, effectively preventing data corruption. Other Python implementations implement fine-grained locking or synchronized data structures for their list implementations.

However, while lists themselves remain protected from corruption, the data they contain remains vulnerable to concurrency issues.

Data Protection in Lists

Operations such as:

L[0] += 1

are not guaranteed to atomically increment the value of L[0] if multiple threads attempt to perform the same operation simultaneously. This is because the = operation involves multiple steps that can be interrupted by other threads.

In summary, while lists themselves are thread-safe, their data is not. To ensure data integrity and prevent incorrect item retrieval or deletion due to race conditions, the use of queues is recommended in multi-threaded code access scenarios.

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