Title: Start should be a keyword argument of the built-in sum
Type: enhancement Stage: resolved
Components: ctypes Versions: Python 3.7
Status: closed Resolution: duplicate
Dependencies: Superseder: Make *start* usable as a keyword argument for sum().
View: 34637
Assigned To: lisroach Nosy List: Mark.Bell, cheryl.sabella, lisroach, rhettinger, serhiy.storchaka, steven.daprano, vstinner
Priority: normal Keywords:

Created on 2017-08-08 11:52 by Mark.Bell, last changed 2018-09-13 06:20 by vstinner. This issue is now closed.

Pull Requests
URL Status Linked Edit
PR 3022 closed python-dev, 2017-08-08 11:53
Messages (10)
msg299908 - (view) Author: Mark Bell (Mark.Bell) * Date: 2017-08-08 11:52
The built-in function sum takes an optional argument "start" to specify what value to start adding from (defaults to 0). This argument should be a keyword argument in order to match the other built-in functions such as:

    enumerate(range(10), start=5)

This patch allows users to write:

    sum(range(10), start=5)

which previously raised "TypeError: sum() takes no keyword arguments". Since the only change is making an optional positional argument into a keyword argument, this has no effect on any existing code using the current convention of:

    sum(range(10), 5)
msg299957 - (view) Author: Steven D'Aprano (steven.daprano) * (Python committer) Date: 2017-08-09 00:33
This seems like a reasonable enhancement to `sum` to me.

Since 2.7 is in feature freeze, this can only apply to 3.7.
msg299968 - (view) Author: Raymond Hettinger (rhettinger) * (Python committer) Date: 2017-08-09 04:11
Lisa, would you like to take this one?
msg299973 - (view) Author: Serhiy Storchaka (serhiy.storchaka) * (Python committer) Date: 2017-08-09 07:08
Adding this feature is so easy as moving '/' in Argument Clinic declaration one line up. I don't think it is worth to allow passing the first argument as a keyword argument.

Check what performance effect of this change on simple calls sum(()), sum((), 0).
msg300356 - (view) Author: Mark Bell (Mark.Bell) * Date: 2017-08-16 12:52
I ran some timing tests of the patch I submitted to compare it to the current build of Python. Using timit on the current master branch I got:

    python.exe -m timeit "sum(())"    .... 1.12 usec per loop
    python.exe -m timeit "sum((), 0)" .... 1.22 usec per loop

And for the patched version:

    python.exe -m timeit "sum(())"    .... 1.46 usec per loop
    python.exe -m timeit "sum((), 0)" .... 1.57 usec per loop

However my patch wasn't just the simple argument clinic change suggested by serhiy.storchaka, so maybe that would be more efficient and easier to understand.
msg300420 - (view) Author: Serhiy Storchaka (serhiy.storchaka) * (Python committer) Date: 2017-08-17 13:13
Your tests show that there is a performance regression of getting rid of Argument Clinic (in addition to increasing the maintenance cost of the code that was generated previously). Try to use the simple Argument Clinic change (it can has non-zero cost too, but I expect that its penalty is much smaller).
msg315790 - (view) Author: Cheryl Sabella (cheryl.sabella) * (Python committer) Date: 2018-04-26 12:21
Hi Mark,

Are you able to make the Argument Clinic change the Serhiy suggested to come up with new benchmarks?  

msg315791 - (view) Author: Serhiy Storchaka (serhiy.storchaka) * (Python committer) Date: 2018-04-26 12:43
First than to allow this argument be passes by keyword, we mast choose its name. See the discussion "Start argument for itertools.accumulate()" on Python-ideas (
msg315793 - (view) Author: STINNER Victor (vstinner) * (Python committer) Date: 2018-04-26 12:55
> I don't think it is worth to allow passing the first argument as a keyword argument.

I concur. Would you mind to add a test to make sure that passing the first argument as the "iterable" keyword doesn't work?

"iterable" name comes from the Doc/library/functions.rst documentation and from the docstring.
msg315794 - (view) Author: Serhiy Storchaka (serhiy.storchaka) * (Python committer) Date: 2018-04-26 13:06
In 2.6 it was "sequence".
Date User Action Args
2018-09-13 06:20:51vstinnersetresolution: out of date -> duplicate
2018-09-12 17:59:37rhettingersetstatus: open -> closed
superseder: Make *start* usable as a keyword argument for sum().
resolution: out of date
stage: resolved
2018-04-26 13:06:34serhiy.storchakasetmessages: + msg315794
2018-04-26 12:55:35vstinnersetnosy: + vstinner
messages: + msg315793
2018-04-26 12:43:34serhiy.storchakasetmessages: + msg315791
2018-04-26 12:21:26cheryl.sabellasetnosy: + cheryl.sabella
messages: + msg315790
2017-08-17 13:13:16serhiy.storchakasetmessages: + msg300420
2017-08-16 12:52:16Mark.Bellsetmessages: + msg300356
2017-08-09 07:08:11serhiy.storchakasetnosy: + serhiy.storchaka
messages: + msg299973
2017-08-09 04:11:49rhettingersetassignee: lisroach

messages: + msg299968
nosy: + lisroach, rhettinger
2017-08-09 00:33:03steven.dapranosetversions: - Python 2.7
nosy: + steven.daprano

messages: + msg299957

type: behavior -> enhancement
2017-08-08 12:00:27Mark.Bellsettype: behavior
2017-08-08 11:53:21python-devsetpull_requests: + pull_request3055
2017-08-08 11:52:48Mark.Bellcreate