Initialization of variables using no-arg constructor

Chris Mellon arkanes at gmail.com
Tue Oct 10 00:21:49 CEST 2006


On 10/9/06, Edward Waugh <edward_waugh at hotmail.com> wrote:
> Consider the following (working) Python code:
>
> import sys
>
> def sum(list):
>     # total = 0 does not work for non-numeric types
>     total = list[0].__class__()
>     for v in list:
>         total += v
>     return total
>
> l = [1, 2, 3]
> print sum(l)
>
> l = [1.1, 2.2, 3.3]
> print sum(l)
>
> l = ["a", "b", "c"]
> print sum(l)
>
> In order for sum() to be generic I initialize total to the value of
> list[0].__class__().  This works but I would like to know if this is the
> correct or preferred way of doing it.  It means that sum() must be given a
> list whose elements are types or classes that have a no-arg constructor
> (through this is probably almost always the case).
>

I'd do this:

>>> def sum(list):
...     total = list[0]
...     for v in list[1:]:
...         total += v
...     return total

I'm not sure if regular slice notation makes a copy of the list or
not, if it does you can use itertools:

>>> def sum(list):
...     total = list[0]
...     for v in itertools.islice(list, 1, len(list)):
...         total += v
...     return total


> Thanks,
> Edward
>



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