Are Lists Thread-Safe in Python?

Thread-Safety of Lists

Many discussions advocate the use of queues over lists with multiple threads. This raises the question: are lists inherently thread-unsafe?

List Thread-Safety

Contrary to popular belief, lists themselves are thread-safe. In the CPython implementation, the Global Interpreter Lock (GIL) prevents concurrent access to lists. Other implementations employ fine-grained locking or synchronized data structures for list operations.

Data Integrity Challenges

However, the thread-safety of lists solely protects the list data structure itself, not the data it contains. Consider the following code:

L[0] += 1

This operation is not guaranteed to increment L[0] by one in a multithreaded environment. The = operator is not atomic, meaning that concurrent executions of this statement could lead to data corruption.

Why Use Queues?

Queues are recommended for multithreaded data handling because they enforce a strict order of operations. Using an unprotected list can result in race conditions, where multiple threads attempt to access or modify the same list item simultaneously, leading to incorrect or unexpected results. Queues, on the other hand, guarantee that data will be accessed in a FIFO (first-in-first-out) manner, preventing such issues.

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