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

Victor Stinner victor.stinner at gmail.com
Fri Apr 15 17:24:21 EDT 2016 
2016-04-15 23:16 GMT+02:00 Ethan Furman <ethan at stoneleaf.us>:
>> 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?

Oh wait, I'm tired and you are right.

Not increasing the value only helps on this code:

dict[key] = value
dict[key] = value  # version doesn't change


>> 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?

For example, this loop is fine:

for key in dict:
   dict[key] = None

In this loop, the dict version is increased at each loop iteration.

For iter(dict), the check prevents a crash.

The following example raises a RuntimeError("dictionary changed size
during iteration"):

d={1:2}
for k in d:
    d[k+1] = None

Victor
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