[Python-Dev] Issue 14417: consequences of new dict runtime error (original) (raw)

Stefan Behnel stefan_ml at behnel.de
Thu Mar 29 23:00:20 CEST 2012

R. David Murray, 29.03.2012 22:31:

On Thu, 29 Mar 2012 13:09:17 -0700, Guido van Rossum wrote:

On Thu, Mar 29, 2012 at 12:58 PM, R. David Murray wrote:

Some of us have expressed uneasiness about the consequences of dict raising an error on lookup if the dict has been modified, the fix Victor made to solve one of the crashers.

I don't know if I speak for the others, but (assuming that I understand the change correctly) my concern is that there is probably a significant amount of threading code out there that assumes that dict lookup is a thread-safe operation. Much of that code will, if moved to Python 3.3, now be subject to random runtime errors for which it will not be prepared. Further, code which appears safe can suddenly become unsafe if a refactoring of the code causes an object to be stored in the dictionary that has a Python equality method. My original assessment was that this only affects dicts whose keys have a user-implemented hash or eq implementation, and that the number of apps that use this and assume the threadsafe property would be pretty small. This is just intuition, I don't have hard facts. But I do want to stress that not all dict lookups automatically become thread-unsafe, only those that need to run user code as part of the key lookup. You are probably correct, but the thing is that one still has to do the code audit to be sure...and then make sure that no one later introduces such an object type as a dict key.

The thing is: the assumption that arbitrary dict lookups are GIL-atomic has always been false. Only those that do not involve Python code execution for the hash key calculation or the object comparison are. That includes the built-in strings and numbers (and tuples of them), which are by far the most common dict keys. Looking up arbitrary user provided objects is definitely not guaranteed to be atomic.

Stefan

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