better way for ' '.join(args) + '\n'?
steve+comp.lang.python at pearwood.info
Fri Oct 26 11:36:22 CEST 2012
On Fri, 26 Oct 2012 09:49:50 +0200, Ulrich Eckhardt wrote:
> General advise when assembling strings is to not concatenate them
> repeatedly but instead use string's join() function, because it avoids
> repeated reallocations and is at least as expressive as any alternative.
> What I have now is a case where I'm assembling lines of text for driving
> a program with a commandline interface. In this scenario, I'm currently
> doing this:
> args = ['foo', 'bar', 'baz']
> line = ' '.join(args) + '\n'
> So, in other words, I'm avoiding all the unnecessary copying, just to
> make another copy to append the final newline.
The difference between ' '.join(sequence) and (' '.join(sequence) + '\n')
is, in Big Oh analysis, insignificant. The first case does O(N)
operations, the second does O(N) + O(N) = 2*O(N) operations, which is
still O(N). In effect, the two differ only by an approximately constant
If you really care, and you don't mind ending your last line with a
space, just append '\n' to the sequence before calling join.
> The only way around this that I found involves creating an intermediate
> sequence like ['foo', ' ', 'bar', ' ', 'baz', '\n']. This can be done
> rather cleanly with a generator:
> def helper(s):
> for i in s[:-1]:
> yield i
> yield ' '
> yield s[-1]
> yield '\n'
> line = ''.join(tmp(args))
> Efficiency-wise, this is satisfactory.
Have you actually tested this? I would not be the least surprised if
that's actually less efficient than the (' '.join(seq) + '\n') version.
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