CODEWITHSUNDEEP
pythonpython-2.7python-multithreading
Carsten S
Carsten S

Reputation: 205

Is Python's `list(some_dictionary)` thread-safe?

If d is a dictionary, then apparently CPython does not like it if another thread modifies d while

[k for k in d]

is evaluated. How about using

list(d)

instead? Is this thread-safe?

Upvotes: 2

Views: 163

Answers (1)

user395760
user395760

Reputation:

I'm fully with @user2357112, this is an implementation detail and you should not rely on it. But for the sake of answering the question: The relevant list and dict methods are written in C, hence the call is a single bytecode operation, and the code does not release the GIL directly or indirectly, as far as I can tell. Therefore in current CPython no other Python thread can interrupt the list call and modify d.

Note that this breaks down if either list has been replaced with something else, or d is not an actual honest-to-god dictobject written in C. Also note that "is written in C" is not a sufficient condition, for example many collection operations call comparison/hashing methods and thus execute arbitrary Python code, allowing the GIL to be released.

Upvotes: 6

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