@lru_cache on functions with no arguments

[Matt Wheeler]

On Tue, 1 Aug 2017 at 02:32 Terry Reedy <tjreedy at udel.edu> wrote:

> On 7/31/2017 7:31 PM, tom at tomforb.es wrote:
> > As part of the Python 3 cleanup in Django there are a fair few uses of
> @functools.lru_cache on functions that take no arguments.
>
> This makes no sense to me.  If the function is being called for
> side-effects, then it should not be cached.  If the function is being
> called for a result, different for each call, calculated from a changing
> environment, then it should not be cached.  (Input from disk is an
> example.) If the function returns a random number, or a non-constant
> value from an oracle (such as a person), it should not be cached.  If
> the function returns a constant (possible calculated once), then the
> constant should just be bound to a name (which is a form of caching)
> rather than using the overkill of an lru cache.  What possibility am I
> missing?
>

A function which is moderately expensive to run, that will always return
the same result if run again in the same process, and which will not be
needed in every session.

Also particularly in the context of a large framework it may be neater &
easier to expose a constant-like thing as a function to be called rather
than directly bound to a name; if there are several implementations of a
particular function, to be chosen based on configuration, and *some* of
them are pure functions while others may change depending on the
environment during a single run, maintaining the interface would trump
simplicity for the simple case).

I've not investigated the Django codebase, so I don't know if my guesses
line up with it exactly, but it should be quite easy to construct a grep or
sed script to scan the source to find out :)
-- 

--
Matt Wheeler
http://funkyh.at