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Re: Using + with strings considered bad

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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')
> 
> But I was told that using + with strings was bad practice. Is this
> true? 

No. What was meant was probably that str.join() is preferred when you are to 
concat an arbitrary number of strings, i. e.

# wrong
s = ""
for item in items:
    s += " " + item.name # may be inefficient depending on implementation
s = s[1:]

# correct
s = " ".join(item.name for item in items)

(For more complex operations than just getting an attribute you may have to 
write a helper generator:

def bogus(items):
    prev = ""
    for item in items:
        yield str(len(prev) - len(item))
        prev = item

s = "*".join(bogus(items))
)

> If so, what is the better way to do this?
 
Python concats adjacent string constants implicitly

>>> "one" "two"
'onetwo'

but in CPython an extra + will be removed by the peephole optimiser:

>>> def f(): return "one" + "two"
... 
>>> import dis
>>> dis.dis(f)
  1           0 LOAD_CONST               3 ('onetwo')
              3 RETURN_VALUE


>     print('Calculating fibonacci and fibonacci_memoize once for ' +
>           str(large_fibonacci) + ' to determine speed increase')

You could write that as

print('Calculating fibonacci and fibonacci_memoize once for '
      '{} to determine speed increase'.format(large_fibonacci))

but in a simple case like yours I'd go with the obvious

print(
    'Calculating fibonacci and fibonacci_memoize once for',
    large_fibonacci,
    'to determine speed increase')

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Thread

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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