[Python-Dev] RFC: PEP 509: Add a private version to dict

Ethan Furman ethan at stoneleaf.us
Fri Apr 15 17:16:22 EDT 2016

On 04/15/2016 01:41 PM, Victor Stinner wrote:
> 2016-04-15 19:54 GMT+02:00 Jim J. Jewett:

>> (2)  Why *promise* not to update the version_tag when replacing a
>> value with itself?
> It's an useful property. For example, let's say that you have a guard
> on globals()['value']. The guard is created with value=3. An unit test
> replaces the value with 50, but then restore the value to its previous
> value (3). Later, the guard is checked to decide if an optimization
> can be used.

I don't understand -- shouldn't the version be incremented with the 
value was replaced with 50, and again when re-replaced with 3?

>> (6)  I'm also not sure why version_tag *doesn't* solve the problem
>> of dicts that fool the iteration guards by mutating without changing
>> size ( https://bugs.python.org/issue19332 ) ... are you just saying
>> that the iterator views aren't allowed to rely on the version-tag
>> remaining stable, because replacing a value (as opposed to a
>> key-value pair) is allowed?
> If the dictionary values are modified during the loop, the dict
> version is increased. But it's allowed to modify values when you
> iterate on *keys*.

I don't understand.  Could you provide a small example?


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