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Title: expose name, args, and kwargs from methodcaller
Type: enhancement Stage: patch review
Components: Library (Lib) Versions: Python 3.6
Status: open Resolution:
Dependencies: Superseder:
Assigned To: Nosy List: abarry, gvanrossum, llllllllll, martin.panter, matrixise, ncoghlan, rhettinger, serhiy.storchaka
Priority: normal Keywords: patch

Created on 2015-12-01 02:07 by llllllllll, last changed 2022-04-11 14:58 by admin.

File name Uploaded Description Edit
methodcaller-attrs.patch llllllllll, 2015-12-01 02:07 review
methodcaller-attrs-1.patch llllllllll, 2015-12-01 23:05 review
methodcaller-attrs-2.patch llllllllll, 2015-12-01 23:28 review
Messages (19)
msg255636 - (view) Author: Joe Jevnik (llllllllll) * Date: 2015-12-01 02:07
This patch adds 3 properties to methodcaller objects for inspecting the object at runtime:

1. 'name': the name of the method to call
2. 'args': the position arguments to pass to the method
3. 'keywords': the keyword arguments to pass to the method

args and keywords act like functools.partial (that is why I did not name it kwargs).

I noticed that recently the repr changed to expose this information which helps in the debugging use case; however, this allows us to use the methodcaller to maybe construct a new methodcaller with different args, or call a different method with the same args.
msg255644 - (view) Author: Stéphane Wirtel (matrixise) * (Python committer) Date: 2015-12-01 09:09

I just tried your patch with the last revision and I have an error with the tests.

stephane@sg1 ~/s/h/cpython> ./python -m test test_operator
[1/1] test_operator
Fatal Python error: ./Modules/_operator.c:975 object at 0x7ff804c2e3d8 has negative ref count -1

Current thread 0x00007ff806ee8700 (most recent call first):
  File "/home/stephane/src/", line 600 in run
  File "/home/stephane/src/", line 648 in __call__
  File "/home/stephane/src/", line 122 in run
  File "/home/stephane/src/", line 84 in __call__
  File "/home/stephane/src/", line 122 in run
  File "/home/stephane/src/", line 84 in __call__
  File "/home/stephane/src/", line 122 in run
  File "/home/stephane/src/", line 84 in __call__
  File "/home/stephane/src/", line 1679 in run
  File "/home/stephane/src/", line 1780 in _run_suite
  File "/home/stephane/src/", line 1814 in run_unittest
  File "/home/stephane/src/", line 161 in test_runner
  File "/home/stephane/src/", line 162 in runtest_inner
  File "/home/stephane/src/", line 126 in runtest
  File "/home/stephane/src/", line 295 in run_tests_sequential
  File "/home/stephane/src/", line 356 in run_tests
  File "/home/stephane/src/", line 392 in main
  File "/home/stephane/src/", line 433 in main
  File "/home/stephane/src/", line 455 in main_in_temp_cwd
  File "/home/stephane/src/", line 3 in <module>
  File "/home/stephane/src/", line 85 in _run_code
  File "/home/stephane/src/", line 170 in _run_module_as_main
fish: “./python -m test test_operator” terminated by signal SIGABRT (Abort)
stephane@sg1 ~/s/h/cpython> 

stephane@sg1 ~/s/h/cpython> hg tip
changeset:   99407:734247d5d0f9
tag:         tip
parent:      99404:ac0f7ed0e94d
parent:      99406:fee19d2d7713
user:        Zachary Ware <>
date:        Mon Nov 30 22:57:22 2015 -0600
summary:     Closes #25767: Merge with 3.5
msg255657 - (view) Author: Joe Jevnik (llllllllll) * Date: 2015-12-01 16:25
Thanks for review, looking into the reference count issue.
msg255680 - (view) Author: Martin Panter (martin.panter) * (Python committer) Date: 2015-12-01 22:08
Left a review comment which may help you chase that refleak

Also, as a new feature, surely it should be documented?
msg255682 - (view) Author: Joe Jevnik (llllllllll) * Date: 2015-12-01 23:05
Thanks for pointing me at the refleak, uploading an update
msg255684 - (view) Author: Joe Jevnik (llllllllll) * Date: 2015-12-01 23:28
Added a test case for the mutation of keywords.
msg255688 - (view) Author: Martin Panter (martin.panter) * (Python committer) Date: 2015-12-02 01:18
This is a bit odd. You can mutate the keyword arguments, but you are not allowed to change the positional arguments (“args” is readonly). If you want this to be part of the supported API, it should be documented, but it seems like bad design IMO.
msg255689 - (view) Author: Joe Jevnik (llllllllll) * Date: 2015-12-02 03:32
I only want this feature to keep the usage close to functools.partial. I was actually sort of surprised to see that mutation of the dict held in partial, but I would rather be consistent.
msg255694 - (view) Author: Serhiy Storchaka (serhiy.storchaka) * (Python committer) Date: 2015-12-02 07:39
"keywords" is unusual name. The most used name for the dict of keyword arguments is "kwargs".

$ find Lib/ -name '*.py' -exec egrep -ho '\*\*[a-zA-Z_0-9]+' '{}' + | sort | uniq -c | sort -nr | head
    803 **kwargs
    442 **kw
    244 **kwds

If you want to have keyword arguments non-mutable, you can use types.MappingProxyType.
msg255695 - (view) Author: Joe Jevnik (llllllllll) * Date: 2015-12-02 07:52
I agree that it is a strange name and I also think that it could be immutable or a copy of the internal dict; however, I think that consistency with existing APIs in the standard library is more important. 'keywords' is still very clear in context and is used in 'functools.partial'. This feature is very similar to 'partial' so I would like to follow the example set there.

I would not mind changing 'partial' to reflect this feedback; but, I imagine that will break people's code and be a harder sell. I would find it frustrating if 'partial' and 'methodcaller' spelled 'keywords' differently or had slightly different behavior when it comes to the keyword argument dictionary. Allowing the mutation is not that unintuitive if you look at the simple python translation I added to the docs.
msg255696 - (view) Author: Serhiy Storchaka (serhiy.storchaka) * (Python committer) Date: 2015-12-02 08:00
functools.partial() is unique in it's usage of name "keywords".
msg255725 - (view) Author: Joe Jevnik (llllllllll) * Date: 2015-12-02 16:57
partial's unique usage is why I feel like it would be so jarring for them do be named differently. I would be happiest having this feature at all so I will change the name to 'kwargs' if you would like. I just want to be sure that my reasons for choosing this name in the first place ere understood.
msg256894 - (view) Author: Joe Jevnik (llllllllll) * Date: 2015-12-23 02:34
Is there a decision on the name? I can update the patch if needed.
msg260425 - (view) Author: Joe Jevnik (llllllllll) * Date: 2016-02-18 04:10
ping: any decision on this?
msg260764 - (view) Author: Serhiy Storchaka (serhiy.storchaka) * (Python committer) Date: 2016-02-24 08:03
I don't like the name "keywords". May be I'm not right. Needed opinions of other core developers.
msg260841 - (view) Author: Raymond Hettinger (rhettinger) * (Python committer) Date: 2016-02-25 06:26
Now that partial() has started down the path of using "keywords", it seems reasonable to stick with that name.  In pure python code, we mostly use **kwds or **kwargs but there are other variants and not much consistency, so spelling out keywords isn't unreasonable in that regard.  That said, if it feels weird to you, ask Guido what his thoughts are.
msg260842 - (view) Author: Serhiy Storchaka (serhiy.storchaka) * (Python committer) Date: 2016-02-25 07:11
functools.partial is not the only precedence.

The "kwargs" attribute is used in following classes:


The "kwds" attribute is used in following class: contextlib._GeneratorContextManager.
msg264874 - (view) Author: Joe Jevnik (llllllllll) * Date: 2016-05-05 03:42
people have had some time to think about this, whats should the name be so we can merge this?
msg264927 - (view) Author: Guido van Rossum (gvanrossum) * (Python committer) Date: 2016-05-05 23:15
To me, the best rhythm has always been (*args, **kwds).
Date User Action Args
2022-04-11 14:58:24adminsetgithub: 69956
2016-05-05 23:15:28gvanrossumsetmessages: + msg264927
2016-05-05 03:42:04llllllllllsetmessages: + msg264874
2016-02-25 07:11:41serhiy.storchakasetnosy: + gvanrossum
messages: + msg260842
2016-02-25 06:26:51rhettingersetmessages: + msg260841
2016-02-24 08:03:01serhiy.storchakasetmessages: + msg260764
2016-02-18 04:10:28llllllllllsetmessages: + msg260425
2015-12-23 02:34:10llllllllllsetmessages: + msg256894
2015-12-02 16:57:24llllllllllsetmessages: + msg255725
2015-12-02 08:00:32serhiy.storchakasetnosy: + rhettinger, ncoghlan
messages: + msg255696
2015-12-02 07:52:37llllllllllsetmessages: + msg255695
2015-12-02 07:39:30serhiy.storchakasetnosy: + serhiy.storchaka
messages: + msg255694
2015-12-02 03:32:04llllllllllsetmessages: + msg255689
2015-12-02 01:18:36martin.pantersetmessages: + msg255688
2015-12-01 23:28:52llllllllllsetfiles: + methodcaller-attrs-2.patch

messages: + msg255684
2015-12-01 23:05:18llllllllllsetfiles: + methodcaller-attrs-1.patch

messages: + msg255682
2015-12-01 22:08:38martin.pantersetnosy: + martin.panter
messages: + msg255680
2015-12-01 16:25:48llllllllllsetmessages: + msg255657
2015-12-01 09:09:42matrixisesetnosy: + matrixise
messages: + msg255644
2015-12-01 02:17:09abarrysetnosy: + abarry

stage: patch review
2015-12-01 02:07:11llllllllllcreate