On Fri, Oct 21, 2011 at 1:57 AM, raymond.hettinger wrote:

+   If *typed* is set to True, function arguments of different types will be +   cached separately.  For example, ``f(3)`` and ``f(3.0)`` will be treated +   as distinct calls with distinct results.

I've been pondering this one a bit since reviewing it on the tracker, and I'm wondering if we have the default behaviour the wrong way around. For "typed=True": - never results in accidental type coercion and potentially wrong answers* (see below) - cache uses additional memory (each entry is larger, more entries may be stored) - additional cache misses - differs from current behaviour For "typed=False" (current default): - matches current (pre-3.3) behaviour - can lead to accidental type coercion and incorrect answers I only just realised this morning that the existing (untyped) caching behaviour can give answers that are *numerically* wrong, not just of the wrong type. This becomes clear once we use division as our test operation rather than multiplication and bring Decimal into the mix:

...

...

from functools import lru_cache @lru_cache() ... def divide(x, y): ... return x / y ... from decimal import Decimal 10 / 9 1.1111111111111112 Decimal(10) / Decimal(9) Decimal('1.111111111111111111111111111') divide(10, 9) 1.1111111111111112 divide(Decimal(10), Decimal(9)) 1.1111111111111112

At the very least, I think lru_cache should default to typed behaviour in 3.3, with people being able to explicitly switch it off as a cache optimisation technique when they know it doesn't matter. You could even make the case that making the cache type aware under the hood in 3.2 would be a bug fix, and the only new feature in 3.3 would be the ability to switch off the type awareness to save memory. Regards, Nick. -- Nick Coghlan   |   ncoghlan@gmail.com   |   Brisbane, Australia