[Python-Dev] Intricacies of calling __eq__

[Steven D'Aprano]

On Tue, Mar 18, 2014 at 01:21:05PM +0200, Maciej Fijalkowski wrote:

> note that this is specifically about dicts, where __eq__ will be
> called undecided number of times anyway (depending on collisions in
> hash/buckets which is implementation specific to start with)

Exactly. Using a __eq__ method with side-effects is a good way to find
out how many collisions your dict has :-)

But specifically with your example,

    if x in d:
        return d[x]

my sense of this is that it falls into the same conceptual area as the 
identity optimization for checking list or set containment: slightly 
unclean, but justified. Provided d is an actual built-in dict, and it 
hasn't been modified between one call and the next, I think it would be 
okay to optimize the second lookup d[x].

A question: how far away will this optimization apply?

    if x in d:
        do_this()
        do_that()
        do_something_else()
        spam = d[x]

Assuming no modifications to d, will the second lookup still be 
optimized?



-- 
Steven