Thanks for the suggestion. I've thought about it and I think I still prefer
the lambda form here. My reasoning inasmuch as I can articulate it:
1) The cited reason to prefer other forms over lambda is that naming functions
makes them easier to debug. While that's fair in general, I have difficulty
seeing it coming into play here: the return value is slightly interesting, but
not particularly the execution of the function, and the former is very
straightforwardly visible from the caller anyway.
2) I tend to feel that a useful barometer for this kind of thing is to
consider adding additional levels. Before I noticed that the suite was
redundant, I had a three-level defaultdict instead, so it was "overrides =
defaultdict(lambda: defaultdict(lambda: defaultdict(set)))". Now, one might
argue about whether this is readable enough (I think that simplifying the
dictionary population code is worth it, but it's certainly up for debate).
The alternative would be "overrides =
defaultdict(defaultdict(defaultdict(set).copy).copy)". Admittedly that's a bit
more symmetrical, but I find it harder to follow; constructs where you just
close all the parentheses at the end are easier to parse visually because you
can see at a glance that you don't need to keep as careful track of how many
levels deep you are. Also, I had to test this to reassure myself that it
actually did the right thing rather than copying a partially-populated
defaultdict, whereas the correctness of the lambda form is clear once you have
the structure.
3) (weak) There's precedent for the lambda form:
lib/lp/bugs/model/structuralsubscription.py:793: statuses =
defaultdict(lambda: defaultdict(list))
--
https://code.launchpad.net/~cjwatson/launchpad/unnecessary-publisher-work/+merge/120356
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