STINNER Victor <vstin...@redhat.com> added the comment:
I dislike adding a public API for an optimization. Would it be possible to make
it private? Would it make sense? tzidx => _tzidx.
> One other thing I might mention here is that I did explore the idea of
> storing this cache on the tzinfo implementation itself, but it is problematic
> for a number of reasons:
>
> 1. It would either need to use some sort of expiring cache (lru, ttl) or
> require a great deal of memory, greatly reducing the utility - the proposed
> implementation requires no additional memory.
In test of your PR, tzinfo allocates memory for its cache:
offsets = [timedelta(hours=0), timedelta(hours=1)]
names = ['+00:00', '+01:00']
dsts = [timedelta(hours=0), timedelta(hours=1)]
This memory isn't free. I don't see how using an index completely prevents the
need to allocate memory for a cache.
Somehow, we need a method to clear the cache and decide a caching policy. The
simplest policy is to have no limit. The re.compile() uses a cache of 512
entries. functools.lru_cache uses a default limit of 128 entries.
Instead of adding a new API, would it be possible to reuse functools.lru_cache
somehow?
> 2. Because the implementation of datetime.__hash__ invokes utcoffset(), it is
> impossible to implement utcoffset in terms of a dictionary of tz-aware
> datetimes. This means that you need to construct a new, naive datetime, which
> is a fairly slow operation and really puts a damper in the utility of the
> cache.
For special local timezones, would it be possible to explicitly exclude them,
and restrict the cache the simple timespace (fixed offset)?
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<https://bugs.python.org/issue35723>
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