Wolfgang Maier added the comment:
> -----Ursprüngliche Nachricht-----
> Von: Steven D'Aprano [mailto:[email protected]]
> Gesendet: Sonntag, 2. Februar 2014 12:55
> An: [email protected]
> Betreff: [issue20479] Efficiently support weight/frequency mappings in the
> statistics module
>
>
> Steven D'Aprano added the comment:
>
> Off the top of my head, I can think of three APIs:
>
> (1) separate functions, as Nick suggests:
> mean vs weighted_mean, stdev vs weighted_stdev
>
> (2) treat mappings as an implied (value, frequency) pairs
>
(2) is clearly my favourite. (1) may work well, if you have a module with a
small fraction of functions, for which you need an alternate API.
In the statistics module, however, almost all of its current functions could
profit from having a way to treat mappings specially.
In such a case, (1) is prone to create lots of redundancies.
I do not share Oscar's opinion that
> apart from mode() the implementation of each function on
> map-format data will be completely different from the iterable version
> so you'd want to have it as a separate function at least internally
> anyway.
Consider _sum's current code (docstring omitted for brevity):
def _sum(data, start=0):
n, d = _exact_ratio(start)
T = type(start)
partials = {d: n} # map {denominator: sum of numerators}
# Micro-optimizations.
coerce_types = _coerce_types
exact_ratio = _exact_ratio
partials_get = partials.get
# Add numerators for each denominator, and track the "current" type.
for x in data:
T = _coerce_types(T, type(x))
n, d = exact_ratio(x)
partials[d] = partials_get(d, 0) + n
if None in partials:
assert issubclass(T, (float, Decimal))
assert not math.isfinite(partials[None])
return T(partials[None])
total = Fraction()
for d, n in sorted(partials.items()):
total += Fraction(n, d)
if issubclass(T, int):
assert total.denominator == 1
return T(total.numerator)
if issubclass(T, Decimal):
return T(total.numerator)/total.denominator
return T(total)
all you'd have to do to treat mappings as proposed here is to add a check
whether we are dealing with a mapping, then in this case, instead of the for
loop:
for x in data:
T = _coerce_types(T, type(x))
n, d = exact_ratio(x)
partials[d] = partials_get(d, 0) + n
use this:
for x,m in data.items():
T = _coerce_types(T, type(x))
n, d = exact_ratio(x)
partials[d] = partials_get(d, 0) + n*m
and no other changes (though I haven't tested this carefully).
Wolfgang
----------
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<http://bugs.python.org/issue20479>
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