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

[Victor Stinner]

2016-04-15 11:01 GMT+02:00 Antoine Pitrou <antoine at python.org>:
> Victor Stinner <victor.stinner <at> gmail.com> writes:
>> You're right that incrementing the global version is useless for these
>> specific cases, and using the version 0 should work. It only matters
>> that the version (version? version tag?) is different.
>
> Why do this? It's a nice property that two dicts always have different
> version tags, and now you're killing this property for... no obvious
> reason?

I guess that the reason is to reduce *a little bit* the risk of
integer overflow (especially the bug when a guard doesn't see a change
between new_version = old_version % 2**64).

> Do you really think dict.clear() is in need of micro-optimizing a
> couple CPU cycles away?

The advantage of having a different version for empty dict is to be
able to use the version to check that they are different. Using the
dictionary pointer is not enough, since it's common that a new
dictionary gets the address of a previously destroyed dictionary. This
case can be avoided if you keep dictionaries alive by keeping a strong
reference, but there are good reasons to not keep a strong reference.

Victor