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Title: csv.DictWriter is slow when writing files with large number of columns
Type: performance Stage: resolved
Components: Library (Lib) Versions: Python 3.7, Python 3.6
Status: closed Resolution: fixed
Dependencies: Superseder:
Assigned To: Nosy List: Mariatta, hughdbrown, methane, mtraskin, peter.otten, python-dev, r.david.murray, serhiy.storchaka, terry.reedy, vstinner
Priority: normal Keywords: patch

Created on 2013-06-15 05:12 by mtraskin, last changed 2022-04-11 14:57 by admin. This issue is now closed.

File name Uploaded Description Edit
csvdictwriter.patch mtraskin, 2013-06-15 05:12 review
csvdictwriter.v2.patch mtraskin, 2013-06-16 06:19 review
csvdictwriter.v3.patch mtraskin, 2013-08-15 05:23 review
csvdictwriter.v4.patch mtraskin, 2013-09-03 03:30 review
issue18219.patch Mariatta, 2016-10-21 03:15 review
issue18219v2.patch Mariatta, 2016-10-21 04:29 review
issue18219v3.patch Mariatta, 2016-10-21 09:27 review
issue18219v4.patch Mariatta, 2016-10-21 09:44 review
issue18219v5.patch Mariatta, 2016-10-21 10:09 review
issue18219v6.patch Mariatta, 2016-10-21 10:11 review
issue18219v7.patch Mariatta, 2016-10-21 10:28 review
issue18219v8.patch Mariatta, 2016-10-21 10:48 review
issue18219v9.patch Mariatta, 2016-10-21 14:15 review
Pull Requests
URL Status Linked Edit
PR 552 closed dstufft, 2017-03-31 16:36
Messages (23)
msg191197 - (view) Author: Mikhail Traskin (mtraskin) * Date: 2013-06-15 05:12
_dict_to_list method of the csv.DictWriter objects created with extrasaction="raise" uses look-up in the list of field names to check if current row has any unknown fields. This results in O(n^2) execution time and is very slow if there are a lot of columns in a CSV file (in hundreds or thousands). Replacing look-up in a list with a look-up in a set solves the issue (see the attached patch).
msg191198 - (view) Author: Serhiy Storchaka (serhiy.storchaka) * (Python committer) Date: 2013-06-15 05:52
I think there is no need in public fieldset property. Just use private self._fieldset field in private _dict_to_list() method.
msg191263 - (view) Author: Mikhail Traskin (mtraskin) * Date: 2013-06-16 06:19
Any way is fine with me. If you prefer to avoid having public filedset property, please use the attached patch.
msg191604 - (view) Author: Terry J. Reedy (terry.reedy) * (Python committer) Date: 2013-06-21 19:05
What is the purpose in touching fieldnames, either in tuple-izing it or in making it private and wrapped with a property. If someone wants to modify it, that is up to them. In any case, this change is not germane to the issue and could break code, so I would not make it.

wrong_fields could be calculated with
any(k for k in rowdict if k not in self._fieldset)
to stop on the first extra, if any.

That said, in 3.x, replacing

  wrong_fields = <long expression>
  if wrong_fields:
  if rowdict.keys() - self._fieldset:
should be even faster because the iteration, which will nearly always go to completion, is entirely in C (or whatever).

Does test/text_cvs have tests for DictWriter, both with and without rowdict errors? If so, or if added, I would be willing to commit a patch that simply added ._fieldset and used it as above for a set difference.

Also, if you have not done so yet, please go to  and  new electronic form
and submit a contributor agreement. An '*' will appear after your name here when it is processed.
msg195233 - (view) Author: Mikhail Traskin (mtraskin) * Date: 2013-08-15 05:23
> What is the purpose in touching fieldnames [...]

Wrapping the fieldnames property and tupleizing it guarantees that fieldnames and _fieldset fields are consistent.
Otherwise, having a separate _fieldset field means that someone who is modifying the fieldnames field will not modify the _fieldset. This will result in inconsistent DictWriter behavior. Normal DictWriter users (ones that do not modify fieldnames after DictWriter was created) will not notice this wrapper. "Non-normal" DictWriter will have their code broken, but it is better than having inconsistent internal data structures since these errors are very hard to detect. If you insist on keeping the interface intact, then use the attached v3 of the patch: it creates a fieldset object every time the _dict_to_list method is executed. This does slow execution down, but performance is acceptable, just about 1.5 time slower than version with _fieldset field.

> wrong_fields could be calculated with [...]
I believe it is better to report all wrong fields at ones. In addition this optimization is meaningless, since usually, unless something is wrong, the field check will require full scan of the rowdict.

> That said, in 3.x, replacing [...]

In 2.x the list comprehension version is faster than the set difference version. In 3.x the set difference is slightly faster (maybe 10% faster). However, list comprehension works both in 2.x and 3.x, while set difference requires different code for them. Hence I prefer sticking with list comprehension.

> Does test/text_cvs have tests [...]
No there are no tests for wrong fields. Correct fields are already checked with standard writing tests. I do not know how you write tests for exception handling. If you provide a link with instructions, I can write the missing test part.

> Also, if you have not done so yet, please go to [...]
I have already done this.
msg195245 - (view) Author: Peter Otten (peter.otten) * Date: 2013-08-15 10:23
Note that set operations on dict views work with lists, too. So the only change necessary is to replace

wrong_fields = [k for k in rowdict if k not in self.fieldnames]


wrong_fields = rowdict.keys() - self.filenames

(A backport to 2.7 would need to replace keys() with viewkeys())
msg196821 - (view) Author: Mikhail Traskin (mtraskin) * Date: 2013-09-03 03:30
Peter, thank you for letting me know that views work with list, I was not aware of this. This is indeed the best solution and it also keeps the DictWriter interface unchanged.

Terry, attached patch contains the DictWriter change and a test case in
msg279058 - (view) Author: Hugh Brown (hughdbrown) Date: 2016-10-20 17:25
I came across this problem today when I was using a 1000+ column CSV from a client. It was taking about 15 minutes to process each file. I found the problem and made this change:

            # wrong_fields = [k for k in rowdict if k not in self.fieldnames]
            wrong_fields = set(rowdict.keys()) - set(self.fieldnames)

And my processing time went down to 12 seconds per file -- a 75x speedup.

It's kind of sad that this change has been waiting for over three years when it is so simple. Any chance we could make one of the acceptable code changes and release it?
msg279101 - (view) Author: Mariatta (Mariatta) * (Python committer) Date: 2016-10-21 03:15
Hello, please review my patch.

I used set subtraction to calculate wrong_fields, added more test cases, and clarify documentation with regards to extrasaction parameter.

Please let me know if this works.

Thanks :)
msg279105 - (view) Author: Hugh Brown (hughdbrown) Date: 2016-10-21 03:27
Fabulous. Looks great. Let's ship!

It is not the *optimal* fix for 3.x platforms. A better fix would calculate the set of fieldnames only once in __init__ (or only as often as fieldnames is changed).

But I stress that it is a robust change that works in versions 2.7 through 3.x for sure. And it is *way* better than the alternative of searching a list.
msg279107 - (view) Author: Mariatta (Mariatta) * (Python committer) Date: 2016-10-21 04:20
Thanks Hugh,

Are you thinking of something like the following?

class DictWriter:
    def __init__(self, f, fieldnames, restval="", extrasaction="raise",
                 dialect="excel", *args, **kwds):
        self._fieldnames = fieldnames    # list of keys for the dict
        self._fieldnames_set = set(self._fieldnames)

    def fieldnames(self):
        return self._fieldnames

    def fieldnames(self, value):
        self._fieldnames = value
        self._fieldnames_set = set(self._fieldnames)

    def _dict_to_list(self, rowdict):
        if self.extrasaction == "raise":
            wrong_fields = rowdict.keys() - self._fieldnames_set

If so, I can work on another patch.
msg279108 - (view) Author: Hugh Brown (hughdbrown) Date: 2016-10-21 04:24

Yes, that is what I was thinking of.

That takes my 12 execution time down to 10 seconds. (Or, at least, a fix I did of this nature had that effect -- I have not timed your patch but it should be the same.)
msg279109 - (view) Author: Mariatta (Mariatta) * (Python committer) Date: 2016-10-21 04:29
Thanks, Hugh.
Please check the updated patch :)
msg279115 - (view) Author: Inada Naoki (methane) * (Python committer) Date: 2016-10-21 10:19
LGTM, Thanks Mariatta.
(But one more LGTM from coredev is required for commit)
msg279116 - (view) Author: STINNER Victor (vstinner) * (Python committer) Date: 2016-10-21 10:22
issue18219v6.patch: LGTM, but I added a minor PEP 8 comment.

INADA Naoki: "LGTM, Thanks Mariatta. (But one more LGTM from coredev is required for commit)"

If you are confident (ex: if the change is simple, like this one), you can push it directly.
msg279117 - (view) Author: Mariatta (Mariatta) * (Python committer) Date: 2016-10-21 10:28
Inada-san, Victor, thank you.

Here is the updated patch.
msg279118 - (view) Author: Inada Naoki (methane) * (Python committer) Date: 2016-10-21 10:37
> If you are confident (ex: if the change is simple, like this one), you can push it directly.

My mentor (Yury) prohibit it while I'm beginner.
And as you saw, I missed PEP 8 violation :)
msg279119 - (view) Author: STINNER Victor (vstinner) * (Python committer) Date: 2016-10-21 10:45
> My mentor (Yury) prohibit it while I'm beginner.

Oh right, trust your mentor more than me ;-)
msg279120 - (view) Author: Roundup Robot (python-dev) (Python triager) Date: 2016-10-21 10:53
New changeset 1928074e6519 by INADA Naoki in branch '3.6':
Issue #18219: Optimize csv.DictWriter for large number of columns.

New changeset 6f1602dfa4d5 by INADA Naoki in branch 'default':
Issue #18219: Optimize csv.DictWriter for large number of columns.
msg279121 - (view) Author: Inada Naoki (methane) * (Python committer) Date: 2016-10-21 10:54
msg279124 - (view) Author: Serhiy Storchaka (serhiy.storchaka) * (Python committer) Date: 2016-10-21 13:13
Shouldn't docs changes and new tests be added to 3.5?
msg279128 - (view) Author: R. David Murray (r.david.murray) * (Python committer) Date: 2016-10-21 14:01
Serhiy: I know you prefer applying test changes to the maint version, and I don't disagree, but there are others who prefer not to and we really don't have an official policy on it at this point.  (We used to say no, a few years ago :)

The doc change looks wrong to me. It looks like a rst source paragraph was split into separate lines instead of being a flowed paragraph in the source?  I don't understand why that was done.
msg279129 - (view) Author: Mariatta (Mariatta) * (Python committer) Date: 2016-10-21 14:15
Thanks David. I uploaded patch to address your concern with the docs.
Can you please check?

Serhiy, with regards to applying docs and test to 3.5, does that require a different patch than what I have?

Date User Action Args
2022-04-11 14:57:46adminsetgithub: 62419
2017-03-31 16:36:23dstufftsetpull_requests: + pull_request969
2016-10-21 14:15:53Mariattasetfiles: + issue18219v9.patch

messages: + msg279129
2016-10-21 14:01:42r.david.murraysetnosy: + r.david.murray
messages: + msg279128
2016-10-21 13:13:08serhiy.storchakasetmessages: + msg279124
2016-10-21 10:54:34methanesetstatus: open -> closed
resolution: fixed
messages: + msg279121

stage: commit review -> resolved
2016-10-21 10:53:41python-devsetnosy: + python-dev
messages: + msg279120
2016-10-21 10:48:17Mariattasetfiles: + issue18219v8.patch
2016-10-21 10:45:16vstinnersetmessages: + msg279119
2016-10-21 10:37:59methanesetmessages: + msg279118
2016-10-21 10:28:01Mariattasetfiles: + issue18219v7.patch

messages: + msg279117
2016-10-21 10:22:26vstinnersetnosy: + vstinner
messages: + msg279116
2016-10-21 10:19:35methanesetnosy: + methane

messages: + msg279115
versions: - Python 3.5
2016-10-21 10:11:49Mariattasetfiles: + issue18219v6.patch
2016-10-21 10:09:58Mariattasetfiles: + issue18219v5.patch
2016-10-21 09:44:55Mariattasetfiles: + issue18219v4.patch
2016-10-21 09:27:07Mariattasetfiles: + issue18219v3.patch
2016-10-21 04:29:58Mariattasetfiles: + issue18219v2.patch

messages: + msg279109
2016-10-21 04:24:09hughdbrownsetmessages: + msg279108
2016-10-21 04:20:11Mariattasetmessages: + msg279107
2016-10-21 03:27:23hughdbrownsetmessages: + msg279105
2016-10-21 03:15:19Mariattasetfiles: + issue18219.patch
nosy: + Mariatta
messages: + msg279101

2016-10-20 18:05:29SilentGhostsetstage: commit review
versions: + Python 3.5, Python 3.6, Python 3.7, - Python 3.4
2016-10-20 17:25:36hughdbrownsetnosy: + hughdbrown
messages: + msg279058
2013-09-03 03:30:23mtraskinsetfiles: + csvdictwriter.v4.patch

messages: + msg196821
2013-08-15 10:23:17peter.ottensetnosy: + peter.otten
messages: + msg195245
2013-08-15 05:23:29mtraskinsetfiles: + csvdictwriter.v3.patch

messages: + msg195233
2013-06-21 19:05:40terry.reedysetnosy: + terry.reedy

messages: + msg191604
versions: + Python 3.4
2013-06-16 06:19:13mtraskinsetfiles: + csvdictwriter.v2.patch

messages: + msg191263
2013-06-15 05:52:23serhiy.storchakasetnosy: + serhiy.storchaka
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2013-06-15 05:12:39mtraskincreate