[Wes Turner <wes.tur...@gmail.com>]
> The use cases these TypeErrors would exclude include weightings (whether
> from a generator without an intermediate tuple/list or from a dict) where
> the need is to do elementwise multiplication:
Yes. That's exactly what I had in mind when I wrote:
>> What we're trying to stop is things like "Counter * Counter" for now,
>> because the obvious implementation(*) of Counter.__mul__ would do a
>> strange thing with that, where a quite different thing is plausibly
>> wanted (and may - or may not - be added later - but, due to backward
>> compatibility, cannot be added later if the initial implementation
>> does the strange thing).
The obvious implementation (already given (*)) of Counter.__mul__
would NOT AT ALL do elementwise multiplication. Nobody has asked for
that yet either, so nobody is proposing to add it now either. But
it's predictable that someone _will_ ask for it when __mul__ is
defined for "scalars". It may or may not be added at that time. But
it will be flat-out impossible to add it later (because of backward
compatibility) _if_ the initial implementation does something entirely
different. So, at first, we want to raise an exception for a
non-'scalar" argument, so that it remains _possible_ to do something
sane with it later.
...
>> (*) The obvious implementation:
>>
>> def __mul__(self, other):
>> if isinstance(other, Sized):
>> raise TypeError("cannot multiply Counter by Sized type %s"
>> % type(other))
>> result = Counter()
>> for k, v in self.items():
>> result[k] = v * other
>> return result
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