Python Concurrent (thread-safe) collections

tl;dr

Despite what many people think, Python's built-in list, dict, and deque are NOT thread-safe.
They may be thread safe for some operations, but not all.
This created a lot of confusion in the Python community.
Google style-guide recommends to not rely on atomicity of built-in collections.

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.

Why use these collections?

There is a lot of confusion on whether Python collections are thread-safe or not^{1, 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.

Even Google recommends to not rely on atomicity of built-in collections.

This concurrent_collections library provides drop-in replacements that handle locking for you.
Suggestions and feedbacks are welcome.

1. Are lists thread-safe?

2. Google style guide advises against relying on Python's assignment atomicity

3. What kind of "thread safe" are deque's actually?

Installation

Pip:

pip install concurrent_collections

My 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_collections

Collections

ConcurrentBag

A 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]

ConcurrentDictionary

A thread-safe dictionary. It has several atomic methods for safe concurrent operations:

• assign_atomic() - Atomically assign a value to a key
• update_atomic() - Atomically update a value using a function
• remove_atomic() - Atomically remove a key and return its value
• put_if_absent() - Atomically put a value only if the key doesn't exist
• replace_if_present() - Atomically replace a value only if the key exists
• replace_if_equal() - Atomically replace a value only if it equals the expected value
• remove_if_exists() - Atomically remove a key if it exists
• get_and_remove() - Atomically get and remove a value
• get_locked() - Context manager for safe read-modify-write operations

ConcurrentDictionary's assign_atomic()

Assigns a dictionary value under a key in a thread-safe way. While dict["somekey"] = value is allowed, it's best to use assign_atomic() for clarity of intent. Using normal assignment will work but raise a UserWarning.

ConcurrentDictionary's remove_atomic()

Atomically removes a key from the dictionary and returns its value, or None if the key doesn't exist.

from concurrent_collections import ConcurrentDictionary

d = ConcurrentDictionary({'x': 1, 'y': 2})
value = d.remove_atomic('x')  # Returns 1, removes 'x'

ConcurrentDictionary's put_if_absent()

Atomically puts a value for a key only if the key is not already present. Returns the existing value if the key exists, None if the key was added.

from concurrent_collections import ConcurrentDictionary

d = ConcurrentDictionary({'x': 1})
existing = d.put_if_absent('x', 2)  # Returns 1, no change
existing = d.put_if_absent('y', 3)  # Returns None, adds 'y': 3

ConcurrentDictionary's replace_if_present()

Atomically replaces the value for a key only if the key exists. Returns True if the key was replaced, False if the key doesn't exist.

from concurrent_collections import ConcurrentDictionary

d = ConcurrentDictionary({'x': 1})
replaced = d.replace_if_present('x', 2)  # Returns True
replaced = d.replace_if_present('y', 3)  # Returns False

ConcurrentDictionary's replace_if_equal()

Atomically replaces the value for a key only if the current value equals the expected value. Returns True if the value was replaced, False otherwise.

from concurrent_collections import ConcurrentDictionary

d = ConcurrentDictionary({'x': 1})
replaced = d.replace_if_equal('x', 1, 2)  # Returns True
replaced = d.replace_if_equal('x', 1, 3)  # Returns False (current value is 2)

ConcurrentDictionary's get_locked()

When working with ConcurrentDictionary, you should use the get_locked method to safely read or update the value for a specific key in a multi-threaded environment. This ensures that only one thread can access or modify the value for a given key at a time, preventing race conditions.

from concurrent_collections import ConcurrentDictionary

d = ConcurrentDictionary({'x': "some value" })

# Safely read and update the value for 'x'
with d.get_locked('x') as value:
    # value is locked for this thread
    d['x'] = "new value"

ConcurrentDictionary's update_atomic()

Performs a thread-safe, in-place update to an existing value under a key.

d = ConcurrentDictionary({'x': 1 })
d.update_atomic("x", lambda v: v + 1) # d now contains 2 under the 'x' key.

ConcurrentQueue

For thread-safe queues, Python offers already a lot of alternatives, even too many, so I'm not going to add another. Please refer to the following.

In the queue module, there are the following thread-safe queue classes:

• Queue
• SimpleQueue
• LifoQueue,
• PriorityQueue

⚠️ Note these queue collections are thread-safe, although it isn't explicitly clear from their type name, making it dangerously confusing for people mistakenly thinking that thread-safety applies also to e.g. the deque, which is absolutely not thread-safe.

Additionally, there are other queue classes in the multiprocessing module, which makes it even more confusing due to the redundancy with the above queue classes. This defines:

• JoinableQueue
• Queue (again)
• SimpleQueue (again)

Equality and Identity Semantics

ConcurrentBag Equality

ConcurrentBag compares as a multiset - order doesn't matter, but element frequency does:

from concurrent_collections import ConcurrentBag

# These are equal (same elements, same frequencies)
bag1 = ConcurrentBag([1, 2, 2, 3])
bag2 = ConcurrentBag([2, 1, 3, 2])
assert bag1 == bag2  # True

# These are not equal (different frequencies)
bag3 = ConcurrentBag([1, 2, 3, 3])
assert bag1 != bag3  # True

ConcurrentQueue Equality

ConcurrentQueue compares elements in order, taking snapshots for consistency during concurrent operations:

from concurrent_collections import ConcurrentQueue

# These are equal (same elements, same order)
queue1 = ConcurrentQueue([1, 2, 3])
queue2 = ConcurrentQueue([1, 2, 3])
assert queue1 == queue2  # True

# These are not equal (different order)
queue3 = ConcurrentQueue([3, 2, 1])
assert queue1 != queue3  # True

ConcurrentDictionary Equality

ConcurrentDictionary compares key-value pairs, order doesn't matter:

from concurrent_collections import ConcurrentDictionary

# These are equal (same key-value pairs)
dict1 = ConcurrentDictionary({'a': 1, 'b': 2})
dict2 = ConcurrentDictionary({'b': 2, 'a': 1})
assert dict1 == dict2  # True

# These are not equal (different values)
dict3 = ConcurrentDictionary({'a': 1, 'b': 3})
assert dict1 != dict3  # True

Thread Safety Guarantees

All collections provide the following guarantees:

1. Atomic Operations: All individual operations (append, remove, get, set) are atomic
2. Consistent Snapshots: Iteration and equality comparisons take consistent snapshots
3. No Race Conditions: Multiple threads can safely access and modify the collections
4. Identity Consistency: Hash values and equality comparisons are consistent within a single operation

Note: While individual operations are thread-safe, compound operations (like checking length then conditionally modifying) should use the provided atomic methods or context managers to ensure consistency.

License

MIT License