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Re: Using + with strings considered bad

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Le mercredi 29 avril 2015 15:15:19 UTC+2, Steven D'Aprano a écrit :
> On Wed, 29 Apr 2015 06:29 pm, Cecil Westerhof wrote:
> 
> > Because I try to keep my lines (well) below 80 characters, I use the
> > following:
> >     print('Calculating fibonacci and fibonacci_memoize once for ' +
> >           str(large_fibonacci) + ' to determine speed increase')
> 
> That's perfectly fine, but these two alternatives may be better:
> 
>     print('Calculating fibonacci and fibonacci_memoize once for'
>            ' %s to determine speed increase' % large_fibonacci)
> 
>     print('Calculating fibonacci and fibonacci_memoize once for'
>            ' {} to determine speed increase'.format(large_fibonacci))
> 
> 
> 
> > But I was told that using + with strings was bad practice. Is this
> > true? If so, what is the better way to do this?
> 
> *Repeated* string concatenation is bad practice. Concatenating one or two
> strings is fine. Doing it in a loop to build up a big string is bad mojo.
> 
> 
> # Perfectly fine:
> message = prefix + "something or other" + suffix
> 
> 
> # Okay, but there are better alternatives:
> for item in things:
>     message = "something " + str(item)
>     print(message)
> 
> 
> # This is asking for trouble.
> # Use ''.join(substrings) instead.
> text = ''
> for s in substrings:
>     text = text + s
> 
> 
> The problem with the third one is that it has to make temporary strings
> which get thrown away, and that gets very expensive if there are many
> substrings. Suppose our substrings are "a", "bb", "ccc", "dddd", "eeeee",
> then the temporary strings that are made end up being:
> 
> text = "a"  # copies one character (maybe?)
> text = "abb"  # copies three characters
> text = "abbccc"  # copies six characters
> text = "abbcccdddd"  # copies ten characters
> text = "abbcccddddeeeee"  # copies fifteen characters
> 
> So to build a string of length 15, Python ends up copying 34 or 35
> characters. As the number of substrings increases, the amount of wasted
> copying blows out: repeated string concatenation behaves quadratically,
> which is very slow.
> 
> The tricky part is that Python starting from version 2.3 introduced an
> optimization that *may* avoid all those extra copying under *some*
> circumstances. So with casual testing, you might not notice the quadratic
> behaviour, and see linear behaviour.
> 
> Until you rely on it being fast, and it isn't.
> 

------

>>> timeit.repeat("''.join(['a', 'a']*1000)")
[35.80853889412481, 36.228519745690505, 36.15569110606032]
>>> timeit.repeat("''.join(['a', '\u3456']*1000)")
[38.041631106320324, 38.031590190424936, 38.020335857707664]
>>> 

-----------

>>> timeit.repeat("''.join(['a', 'a']*1000)")
[38.94708620458107, 38.91509429875259, 38.914195952835485]
>>> timeit.repeat("''.join(['a', '\u3456']*1000)")
[58.284382998616366, 58.23354945884603, 58.27770782766095]
>>> 

with a memory gain = 0

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Using + with strings considered bad Cecil Westerhof <Cecil@decebal.nl> - 2015-04-29 10:29 +0200
  Re: Using + with strings considered bad Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-04-29 10:08 +0100
  Re: Using + with strings considered bad Peter Otten <__peter__@web.de> - 2015-04-29 11:24 +0200
    Re: Using + with strings considered bad Cecil Westerhof <Cecil@decebal.nl> - 2015-04-29 13:17 +0200
    Re: Using + with strings considered bad Cecil Westerhof <Cecil@decebal.nl> - 2015-04-29 14:23 +0200
      Re: Using + with strings considered bad Chris Angelico <rosuav@gmail.com> - 2015-04-29 22:55 +1000
  Re: Using + with strings considered bad Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-04-29 23:15 +1000
    Re: Using + with strings considered bad wxjmfauth@gmail.com - 2015-04-29 08:16 -0700
  Re: Using + with strings considered bad Andrew Berg <aberg010@my.hennepintech.edu> - 2015-04-29 06:40 -0500

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