On Sep 4, 2019, at 15:25, r...@hwyl.org wrote:
>
> ..."skip" being a unique object defined in and accessible from dict (there
> may be better places to make it available). The dict insertion code then
> simply ignores those entries when creating the dict.
I’m a bit confused about exactly when this does and doesn’t come into play.
You say here “the dict insertion code” does it. Would they mean d[key]=d.skip
doesn’t insert anything? If so, then how could I put skip in a dict at all? If
there’s already a d[key] does that get left alone, or deleted? Wouldn’t this
even affect dict inserts that happen behind the scenes, like any time you tried
to assign dict.skip to a global variable or a class or instance attribute, or
pass it as a keyword argument? If an implementation doesn’t use dict.insert to
build dict literals, does that mean it doesn’t work in dict literals on that
implementation, even though that was the whole motivation for the feature?
Maybe instead of starting with how it could be implemented, start with where
you ideally do and don’t want it to take effect, and then we can try to work
out how it could be implemented to do that, and how to specify it to require
such an implementation.
* Obviously you want it in non-comprehension dict displays, because that’s
your example.
* What about comprehensions?
* Constructor calls with the keyword-arg API like dict(a=1, b=dict.skip,
**d2}?
* Constructor calls from iterables of pairs, like dict((1, 2), (3,
dict.skip))?
* Subscription assignments?
* Calls to insert?
* Calls to update?
* Capturing ** keyword args?
* Assignments to globals, attributes, etc. when those use a dict under the
covers (as they usually do!?
* Assignments to such things when they don’t use a dict (as with a slots
class instance)?
Also, what should happen with this case:
flag0, flag1 = True, False
d = {'k': 0 if flag0 else dict.skip,
'k': 1 if flag1 else dict.skip}
Do we keep the last value for 'k' as usual, and because that’s dict.skip,
therefore d['k'] ends up as a KeyError? Or do we keep the last non-skip value
and therefore d['k'] ends up as 0?
What about this:
class D(dict):
skip = 0
d = D()
d.skip = 1
d[0] = 0
d[1] = 1
d[dict.skip] = dict.skip
Do we skip the one that uses d.skip, the one that uses type(d).skip (like a
special method lookup), or the one that uses dict.skip?
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