Despite what many people think, Python's built-in list, dict, and deque are thread-safe for some operations, but not all. This created a lot of confusion in the Python community.
concurrent_collections provides thread-safe alternatives by using locks internally to ensure safe concurrent access and mutation from multiple threads.
Inspired from the amazing C#'s concurrent collections.
There is a lot of confusion on whether Python collections are thread-safe or not1, 2, 3.
The bottom line is that Python's built-in collections are not fully thread-safe for all operations. While some simple operations (like list.append() or dict[key] = value) are thread-safe due to the Global Interpreter Lock (GIL), compound operations and iteration with mutation are not. This can lead to subtle bugs, race conditions, or even crashes in multi-threaded programs.
See the Python FAQ: "What kinds of global value mutation are thread-safe?" for details. The FAQ explains that only some (if common) operations are guaranteed to be atomic and thread-safe, but for anything more complex, you must use your own locking.
The docs even go as far as to say:
When in doubt, use a mutex!
Which is telling.
concurrent_collections provides drop-in replacements that handle locking for you, making concurrent programming safer and easier.
Suggestions and feedbacks are welcome.
Pip:
pip install concurrent_collectionsMy recommendation is to always use uv instead of pip – I personally think it's the best package and environment manager for Python.
uv add concurrent_collectionsA thread-safe, list-like collection.
from concurrent_collections import ConcurrentBag
bag = ConcurrentBag([1, 2, 3])
bag.append(4)
print(list(bag)) # [1, 2, 3, 4]A thread-safe dictionary. For atomic compound updates, use update_atomic.
from concurrent_collections import ConcurrentDictionary
d = ConcurrentDictionary({'x': 1})
d['y'] = 2 # Simple assignment is thread-safe
# For atomic updates:
d.update_atomic('x', lambda v: v + 1)
print(d['x']) # 2A thread-safe double-ended queue.
from concurrent_collections import ConcurrentQueue
q = ConcurrentQueue()
q.append(1)
q.appendleft(0)
print(q.pop()) # 1
print(q.popleft()) # 0MIT License