On Mar 24, 4:19 pm, Paul Rubin <no.em...@nospam.invalid> wrote:
> kj <no.em...@please.post> writes:
> > Is there a sequence-oriented equivalent to the sum built-in? E.g.:
> > seq_sum(((1, 2), (5, 6))) --> (1, 2) + (5, 6) --> (1, 2, 5, 6)
>
> use itertools.chain for this. A few people have mentioned that sum will
> also work, but I think for that purpose it could have O(n**2)
> complexity.
I agree on the practical matter that itertools.chain and other
solutions are usually the way to go for most tasks that involve
iterating through several lists.
>From a purely academic standpoint, I'm not convinced that sum() is
inefficient in terms of big-O complexity, though.
show...@showell-laptop:~$ python
Python 2.6.2 (release26-maint, Apr 19 2009, 01:56:41)
[GCC 4.3.3] on linux2
>>> class StupidList:
... def __init__(self, lst):
... print 'creating', lst
... self.lst = lst
... def __add__(self, other):
... self.lst += '|'
... self.lst.extend(other.lst)
... return self
...
>>> result = sum([StupidList([1, 2]), StupidList([3,4]),
StupidList([5,6])], StupidList([0]))
creating [1, 2]
creating [3, 4]
creating [5, 6]
creating [0]
>>> result.lst
[0, '|', 1, 2, '|', 3, 4, '|', 5, 6]
If I'm interpreting the above program correctly, then sum() is doing
the most efficient thing under the hood--it appears to do the
equivalent of += without creating unnecessary objects for intermediate
sums.
I think the special-case error message might be a case where
practicality simply beats out purity. It would be nice if sum() were
completely duck-typed-let-you-shoot-yourself-in-foot-if-you-know-what-
you-are-doing, but maybe this was such a pitfall at one time, that
extra safeguards were put into sum(). I wonder how severely sum(),
without the restriction, would underperform join() on modern versions
of Python, though.
>>> sum('1', '2')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: sum() can't sum strings [use ''.join(seq) instead]
Note that you can easily fake out sum() to get duck typing.
>>> class EmptyStringStarter:
... def __add__(self, other): return other
...
>>> empty = EmptyStringStarter()
>>> sum(['hello ', 'world'], empty)
'hello world'
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