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

[Victor Stinner]

Hi,

2016-01-20 22:18 GMT+01:00 Glenn Linderman <v+python at g.nevcal.com>:
> On 1/20/2016 12:50 PM, Brett Cannon wrote:
>>
>> A global (shared between all dicts) unit64 ma_version is actually quite
>> reliable -- if a program does 1,000,000 dict modifications per second,
>> it would take it 600,000 years till wrap-around.

I think that Yury found a bug in FAT Python. I didn't test the case
when the builtins dictionary is replaced after the definition of the
function. To be more concrete: when a function is executed in a
different namespace using exec(code, namespace). That's why I like the
PEP process, it helps to find all issues before going too far :-)

I like the idea of global counter for dictionary versions. It means
that the dictionary constructor increases this counter instead of
always starting to 0.

FYI a fat.GuardDict keeps a strong reference to the dictionary. For
some guards, I hesitated to store the object identifier and/or using a
weak reference. An object identifier is not reliable because the
object can be destroyed and a new object, completly different, or of
the same type, can get the same identifier.

> But would invalidate everything, instead of just a fraction of things, on
> every update to anything that is monitored...

I don't understand this point.

In short, the guard only has to compare two 64 bit integers in the
fast-path, when nothing changed. For a namespace, it means that no
value was replaced in this namespace.

If a different namespace is modified, the version of the watched
namespace does not change, so we are still in the fast-path.

If a value is replaced in the watched namespace, but not the watched
variable, we have to take a slow-path, hopefully only once.

The worst case is when a value different than the watched value is
modified between each guard check. In this case, we always need a dict
lookup. An heuristic can be chosen to decide to give up after N tries.
Currently, fat.GuardDict always retries.

Victor