[Python-Dev] RFC: PEP 509: Add a private version to dict (original) (raw)

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

More information about the Python-Dev mailing list