Using + with strings considered bad

Peter Otten __peter__ at web.de
Wed Apr 29 11:24:51 CEST 2015


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')





More information about the Python-list mailing list