Reposting because the original got bounced from Google Groups. ---------- Forwarded message --------- From: Tim Peters <tim.pet...@gmail.com> Date: Fri, Jun 29, 2018 at 5:54 PM Subject: Re: [Python-ideas] collections.Counter should implement fromkeys To: <abedil...@gmail.com> Cc: python-ideas <python-id...@googlegroups.com>
[Abe Dillon <abedil...@gmail.com>] > ... > I'm using the copy-constructor because I know Counter is a subclass of dict. > I'm using fromkeys because I know how that class method works. > So why does the subclass lack functionality that the superclass has? > Because programmers wouldn't be able to wrap their heads around it? > I don't buy it. This feels like nanny-design trumping SOLID design <https://en.wikipedia.org/wiki/SOLID>. More because Counter.fromkeys() could be incoherent. From the implementation (in your Lib/collections/__init__.py): @classmethod def fromkeys(cls, iterable, v=None): # There is no equivalent method for counters because setting v=1 # means that no element can have a count greater than one. raise NotImplementedError( 'Counter.fromkeys() is undefined. Use Counter(iterable) instead.') For a dict, a value appearing multiple times in the iterable doesn't matter. But a fundamental use case for Counters is to tally the _number_ of times duplicate keys appear. So, e.g., someone will be unpleasantly surprised no matter what: Counter.fromkeys("aaaaa", 2) returned. "It should set key 'a' to 2! that's what I said it should do!" "No! It should set key 'a' to 10! that's what a Counter _always_ does - sums the values associated with duplicate keys!" "You're both right - and wrong! It should raise an exception if there's a duplicate key, because there's no compelling answer to what it should do!" I expect Raymond called it NotImplementedError instead so he could release the code instead of waiting 3 years for that debate to end ;-)
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