Hello Alex,
On Sun, Oct 26, 2003 at 07:20:52PM +0100, Alex Martelli wrote:
> > def sum(seq, start=0):
> > for item in seq:
> > start = start + seq
> > return start
>
> It IS equivalent to that -- plus an explicit typetest to raise if start is an
> instance of str or unicode.
Yes, it is what I'm saying: it is what we expect it to be, but there is an
exception for no real reason apart from "don't do it like this, buddy, there
is a faster version out there".
I tend to regard this kind of exceptions as very bad, because if you write a
generic algorithm using sum(), even if you don't really see why someone would
think about using your algorithm with strings one day, chances are that
someone will.
Raising a Warning instead of an exception would have had the same result
without the generality problem.
> > reduce(operator.add, seq, start)
>
> sum doesn't reproduce reduce's quirk of using the first item of seq if start
> is not given. So, the def version is closer.
I was thinking about:
def sum(seq, start=0):
return reduce(operator.add, seq, start)
which is the same as the previous one.
> Admittedly the latter version may accept a few more cases, e.g.
> both versions would accept:
> sum([ range(3), 'foo' ], [])
> because [] is copyable, []+range(3) is fine, and list.__iadd__ is
> more permissive than list.__add__; however, the first version
> would fail on:
> sum([ 'foo', range(3) ], [])
> because []+'foo' fails, while the second version would be fine
> because [] is _still_ copyable and __iadd__ is still permissive:-).
These cases all show that we have a surprize problem (although probably not a
big one). The user will expect sum() to have a clean definition, and because
the += one doesn't work, it must be +. To my opinion, sum() should be
strictly equivalent to the naive + version and try to optimize common cases
under the hood.
Admittedly, this is not obvious, because of precisely all these strange mixed
type cases which could be user-defined classes with __add__ or __radd__
operators...
I'm sure someone will design a class
class x:
def __add__(self, other):
return other
so that x() can be used as a trivial starting point for sum() -- and
then sum(["abc", "def"], x()) works :-)
Armin
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