Why Are Queues Preferred Over Lists When Using Multiple Threads?

Why Use Queues with Multiple Threads?

When working with multiple threads, it's common advice to utilize queues over lists. This raises the question of whether lists are inherently unsafe when accessed concurrently.

Thread-Safety of Lists

Contrary to popular belief, lists themselves are thread-safe in Python. Implementations such as CPython ensure that access to lists is protected by the GIL (Global Interpreter Lock), while other implementations employ fine-grained locks or synchronized datatypes. Therefore, lists themselves cannot become corrupt due to concurrent access.

Data Integrity with Lists

However, while lists as data structures are protected, the data within them is not. Consider the following operation:

L[0] += 1

This increment operation is not atomic, meaning that if multiple threads attempt to perform it simultaneously, they may not all increment the value correctly. This is because updates to the list's contents are not synchronized.

Queues for Concurrent Access

To address this issue, queues are used instead of lists. Queues inherently provide atomic operations for adding and removing elements, ensuring that modifications are handled correctly even when accessed concurrently by multiple threads.

Using queues helps avoid race conditions and ensures that the correct item is retrieved or removed from the list, preventing data corruption.

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