Python sum, why not strings?

Asked 2010-08-19T19:13:30.977

Python has a built in function sum, which is effectively equivalent to:

def sum2(iterable, start=0):
    return start + reduce(operator.add, iterable)

for all types of parameters except strings. It works for numbers and lists, for example:

 sum([1,2,3], 0) = sum2([1,2,3],0) = 6    #Note: 0 is the default value for start, but I include it for clarity
 sum({888:1}, 0) = sum2({888:1},0) = 888

Why were strings specially left out?

 sum( ['foo','bar'], '') # TypeError: sum() can't sum strings [use ''.join(seq) instead]
 sum2(['foo','bar'], '') = 'foobar'

I seem to remember discussions in the Python list for the reason, so an explanation or a link to a thread explaining it would be fine.

Edit: I am aware that the standard way is to do "".join. My question is why the option of using sum for strings was banned, and no banning was there for, say, lists.

Edit 2: Although I believe this is not needed given all the good answers I got, the question is: Why does sum work on an iterable containing numbers or an iterable containing lists but not an iterable containing strings?

Edit Report

2 Answers

Python tries to discourage you from "summing" strings. You're supposed to join them:

"".join(list_of_strings)

It's a lot faster, and uses much less memory.

A quick benchmark:

$ python -m timeit -s 'import operator; strings = ["a"]*10000' 'r = reduce(operator.add, strings)'
100 loops, best of 3: 8.46 msec per loop
$ python -m timeit -s 'import operator; strings = ["a"]*10000' 'r = "".join(strings)'
1000 loops, best of 3: 296 usec per loop

Edit (to answer OP's edit): As to why strings were apparently "singled out", I believe it's simply a matter of optimizing for a common case, as well as of enforcing best practice: you can join strings much faster with ''.join, so explicitly forbidding strings on sum will point this out to newbies.

BTW, this restriction has been in place "forever", i.e., since the sum was added as a built-in function (rev. 32347)

answered 2010-08-19T19:15:55.930

You can in fact use sum(..) to concatenate strings, if you use the appropriate starting object! Of course, if you go this far you have already understood enough to use "".join(..) anyway..

>>> class ZeroObject(object):
...  def __add__(self, other):
...   return other
...
>>> sum(["hi", "there"], ZeroObject())
'hithere'

answered 2010-08-19T20:01:06.497

Python sum, why not strings?

2 Answers

Your Answer