16.04.18 08:07, Tim Peters пише:
Adding Counter * integer doesn't bother me a bit, but the definition
of what that should compute isn't obvious. In particular, that
implementation doesn't preserve that `x+x == 2*x` if x has any
negative values:
x = Counter(a=-1)
x
Counter({'a': -1})
x+x
Counter()
It would be strange if x+x != 2*x, and if x*-1 != -x:
y = Counter(a=1)
y
Counter({'a': 1})
-y
Counter()
Etc.
Then again, it's already the case that, e.g., x-y isn't always the
same as x + -y:
x = Counter(a=1)
y = Counter(a=2)
x - y
Counter()
x + -y
Counter({'a': 1})
So screw obvious formal identities ;-)
I'm not clear on why "+" and "-" discard keys with values <= 0 to
begin with. For "-" it's natural enough viewing "-" as being multiset
difference, but for "+"? That's just made up ;-)
In any case, despite the oddities, I think your implementation would
be least surprising overall (ignore the sign of the resulting values).
At least for Counters that actually make sense as multisets (have no
values <= 0), and for a positive integer multiplier `n > 0`, it does
preserve that `x*n` = `x + x + ... + x` (with `n` instances of `x`).
There are methods update() and subtract() which are similar to operators
"+" and "-", but don't discard non-positive values.
I expect that "*" and "/" discard non-positive values for consistency
with "+" and "-". And a new method should be added which does
multiplication without discarding non-positive values.
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