Empty Python lists [] don't know the type of the items it will
contain, so this sounds strange:
>>> sum([])
0
Because that [] may be an empty sequence of someobject:
>>> sum(s for s in ["a", "b"] if len(s) > 2)
0
In a statically typed language in that situation you may answer the
initializer value of the type of the items of the list, as I do in the
sum() in D.
This sounds like a more correct/clean thing to do:
>>> max([])
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: max() arg is an empty sequence
So it may be better to make the sum([]) too raise a ValueError, in
Python 3/3.1 (if this isn't already true). On the other hand often
enough I have code like this:
>>> max(fun(x) for x in iterable if predicate(x))
This may raise the ValueError both if iterable is empty of if the
predicate on its items is always false, so instead of catching
exceptions, that I try to avoid, I usually end with a normal loop,
that's readable and fast:
max_value = smallvalue
for x in iterable:
if predicate(x):
max_value = max(max_value, fun(x))
Where running speed matters, I may even replace that max(max_value,
fun(x)) with a more normal if/else.
A possible alternative is to add a default to max(), like the next()
built-in of Python 2.6:
>>> max((fun(x) for x in iterable if predicate(x)), default=smallvalue)
This returns smallvalue if there are no items to compute the max of.
Bye,
bearophile
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