Thread safe python dictionaries?

Question

I have a function call that starts 10 threads. Before starting these threads, I have

from collections import defaultdict output = defaultdict(dict) 

and the output is empty.

Each thread will generate data for writing to the dictionary.

Sort of:

 output['water'] = 'h20' output['fire'] = 'delta of oxygen' .... 

Only elements will be added to the streams, and they will not iterate over any other elements or change any other elements. output['water'] is an element other than output['fire'] . I can also guarantee that none of the two threads will create the same element. That is, each thread T has a unique i. In the code: output [i] is unique for each stream.

Is this vocabulary safe in this regard?

Answer 1

Yes.

If you use CPython and strings as keys, then yes. Gil in CPython ensures that only one thread executes the bytecode at a time, and the key is set to dict in the same STORE_SUBSCR operation STORE_SUBSCR . If you are not using CPython or using a key with custom methods __hash__ , __eq__ or __cmp__ , all bets are disabled. If I had a soapbox, I would jump on it and warn you of evil, relying on implementation details like this for the sake of correctness. It is more pythonic of you to write something that works only for the case and in the environment where it will be used, since otherwise it could be considered as premature optimization. Enjoy your working code!

 >>> from dis import dis >>> dis(compile('output = defaultdict(dict); output["water"] = "H2O"', 'example', 'exec')) 1 0 LOAD_NAME 0 (defaultdict) 3 LOAD_NAME 1 (dict) 6 CALL_FUNCTION 1 (1 positional, 0 keyword pair) 9 STORE_NAME 2 (output) 12 LOAD_CONST 0 ('H2O') 15 LOAD_NAME 2 (output) 18 LOAD_CONST 1 ('water') 21 STORE_SUBSCR 22 LOAD_CONST 2 (None) 25 RETURN_VALUE 

This has been discussed elsewhere .