Python sum, why not strings? [closed]

Answer 1

The Reason Why

@dan04 has an excellent explanation for the costs of using sum on large lists of strings.

The missing piece as to why str is not allowed for sum is that many, many people were trying to use sum for strings, and not many use sum for lists and tuples and other O(n**2) data structures. The trap is that sum works just fine for short lists of strings, but then gets put in production where the lists can be huge, and the performance slows to a crawl. This was such a common trap that the decision was made to ignore duck-typing in this instance, and not allow strings to be used with sum.

Answer 2

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'

Answer 3

Here's the source: http://svn.python.org/view/python/trunk/Python/bltinmodule.c?revision=81029&view=markup

In the builtin_sum function we have this bit of code:

     /* reject string values for 'start' parameter */
        if (PyObject_TypeCheck(result, &PyBaseString_Type)) {
            PyErr_SetString(PyExc_TypeError,
                "sum() can't sum strings [use ''.join(seq) instead]");
            Py_DECREF(iter);
            return NULL;
        }
        Py_INCREF(result);
    }

So.. that's your answer.

It's explicitly checked in the code and rejected.

Answer 4

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)

Answer 5

I dont know why, but this works!

import operator
def sum_of_strings(list_of_strings):
    return reduce(operator.add, list_of_strings)

Answer 6

From the docs:

The preferred, fast way to concatenate a sequence of strings is by calling ''.join(sequence).

By making sum refuse to operate on strings, Python has encouraged you to use the correct method.