negative "counts" in collections.Counter?
python at rcn.com
Mon Mar 8 23:44:38 CET 2010
> I think you should be providing two types: one is a
> multiset, which disallows negative counts altogether;
> the other behaves like a sparse vector with appropriate
> arithmetic operations.
That is pretty close to what I said above:
In this case, we have an indication that what you really want is a
separate class supporting elementwise binary and unary operations on
vectors (where the vector fields are accessed by a dictionary key
instead of a positional value).
The first and foremost goal of the class was to provide a simple
counting capability along the lines of:
def __init__(self, iterable=()):
for elem in iterable:
self[elem] += 1
def __missing__(self, key):
Note, there is no "disallowing negatives" or even any insistence
that the "counts" are actually numbers. In that respect, it was
a consenting adults class from the start. It is short, simple,
The other methods were mostly just "extras" to make the tool
more useful. Each adds its own minimal restrictions:
elements() requires integer counts
most_common() requires that counts be orderable
multiset operations are defined for non-negative values
We could have kept this same design and wrapped every dictionary
method so that it would raise an exception for any count that was not
a positive integer. That would have slowed down the class, increased
the code volume, and precluded some uses that are currently possible.
IMO, that would not have been a win and it would not have helped the
OP who was seeking a more vector-like tool for signed-number
Anyway, it is what it is. The tool is released and most
users seem to be happy with what we have.
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