Path: csiph.com!usenet.pasdenom.info!nntpfeed.proxad.net!proxad.net!feeder1-2.proxad.net!news.tele.dk!news.tele.dk!small.news.tele.dk!newsgate.cistron.nl!newsgate.news.xs4all.nl!post.news.xs4all.nl!not-for-mail
To: python-list@python.org
From: Peter Otten <__peter__@web.de>
Subject: Re: Using + with strings considered bad
Date: Wed, 29 Apr 2015 11:24:51 +0200
Organization: None
References: <878udbxrpg.fsf@Equus.decebal.nl>
Mime-Version: 1.0
Content-Type: text/plain; charset="ISO-8859-1"
Content-Transfer-Encoding: 7Bit
User-Agent: KNode/4.13.3
Precedence: list
Newsgroups: comp.lang.python
Message-ID: <mailman.79.1430299516.3680.python-list@python.org>
Lines: 67
NNTP-Posting-Host: 2001:888:2000:d::a6
Xref: csiph.com comp.lang.python:89536

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')