BTW: In [7]: issubclass(set, frozenset) Out[7]: False
In [8]: issubclass(frozenset, set) Out[8]: False no reason to do anything different here. and: In [13]: issubclass(MappingProxyType, abc.Hashable) Out[13]: False so yes, there is a need for something different -CHB On Thu, Oct 11, 2018 at 12:34 PM, Chris Barker <chris.bar...@noaa.gov> wrote: > On Thu, Oct 11, 2018 at 9:54 AM, Jonathan Fine <jfine2...@gmail.com> > wrote: > >> Summary: Long post. Because of LSP, neither dict nor frozendict are a >> subclass of the other. > > > given Python's dynamic typing, these issues are kinda academic :-) > > > Intuition tells me that a frozen dictionary is a form of dictionary >> > that adds restrictions, not that a dictionary is a frozen dictionary >> > that you left out to thaw. >> > > well, IIUC the Liskov principle correctly then a subclass is never a > version of a class that does less, but rather always one that does more. > > Think you intuition may be driven by the choice of names and history: yes, > a frozen dict sounds like a regular dict that has been altered > (specifically frozen) -- but if we called them "hash_table" and > "mutable_has_table", then your intuition may be different :-) > > As for subclassing or not, for most Python code is makes no difference -- > polymorphism is not achieved through subclassing. and the "Pythonic" way to > test for type is through ABCs, and we already have Mapping and > MutableMapping, which kind of surprised me, as there is no builtin Mapping > that isn't also a MutableMapping. > > Notice that I said USEFUL property P. The negation Q of property P is >> also a property. > > > well, yeah, but I think the concept of "useful" is pretty vague. > > in this case, is "imutablilty" the "useful" property -- or is > "hashability"?, which would want us to use abc.Hashable. > > So: > > I don't care what is or isn't a subclass of what -- I don't think that's a > Pythonic question. But I do think : > > issubclass(frozendict, abc.Mapping) and issubclass(frozendict, > abc.Hashable) > > would be useful. > > BTW, I just noticed that: > > A tuple, as in ``issubclass(x, (A, B, ...))``, may be given as the target > to check against. This is equivalent to ``issubclass(x, A) or issubclass(x, > B) or ...`` etc. > > which seems less useful than "and" -- at least for ABCs > > I suppose that API pre-dates ABCs.... > > One way to fix this in Python 4 would be to create a common ancestor, >> which has only the shared methods of dict and frozendict. > > > which would be an immutable, but not hashable, mapping ?!? > > Or do >> something similar with abstract base classes. >> > > already done -- see above. > > -CHB > > > -- > > Christopher Barker, Ph.D. > Oceanographer > > Emergency Response Division > NOAA/NOS/OR&R (206) 526-6959 voice > 7600 Sand Point Way NE (206) 526-6329 fax > Seattle, WA 98115 (206) 526-6317 main reception > > chris.bar...@noaa.gov > -- Christopher Barker, Ph.D. Oceanographer Emergency Response Division NOAA/NOS/OR&R (206) 526-6959 voice 7600 Sand Point Way NE (206) 526-6329 fax Seattle, WA 98115 (206) 526-6317 main reception chris.bar...@noaa.gov
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