Ulrich Eckhardt wrote: > Hi! > > 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 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.
No, it is not. In a quick timeit test it takes 5 to 10 times as long as the original. Remember that function calls are costly, and that with s[:-1] you are trading the extra string for an extra list. Also, you are doubling the loop implicit in str.join() with the explicit one in your oh-so-efficient generator. > However, readability counts and > that is where this version fails and that is the reason why I'm writing > this message. So, dear fellow Pythonistas, any ideas to improve the > original versions efficiency while preserving its expressiveness? > > Oh, for all those that are tempted to tell me that this is not my > bottleneck unless it's called in a very tight loop, you're right. > Indeed, the overhead of the communication channel TCP between the two > programs is by far dwarving the few microseconds I could save here. I'm > still interested in learning new and better solutions though. Even if it were the bottleneck the helper generator approach would still be unhelpful. -- http://mail.python.org/mailman/listinfo/python-list