[Python-ideas] Revisiting Immutable Mappings

Steven D'Aprano steve at pearwood.info
Thu Oct 11 18:35:28 EDT 2018

On Thu, Oct 11, 2018 at 12:34:13PM -0700, Chris Barker via Python-ideas wrote:

> 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.

So you do care about what is and isn't a subclass :-)

It is 2018 and we've had isinstance and issubclass in the language since 
Python 2.2 in 2002, I really wish that we could get away from the idea 
that checking types is unPythonic and duck-typing is the One True Way to 
do things. The old, Python 1.5 form of type-checking:

    type(spam) is dict

was not so great, since it tested only for a specific type by identity. 
But with ABCs and virtual subclasses and inheritance, isinstance is a 
great way to check that your duck both quacks and swims, rather than 
watching it sink and drown in the middle of your function :-)


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