wrong error messages when too many kwargs are received

[orenmn]

BPO	31229
Nosy	@vstinner, @serhiy-storchaka, @orenmn
PRs	bpo-31229: fix wrong error messages when too many keyword arguments are received #3180
Files	issue31229_ver1.diff: diff file - ver1

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2017-08-23.18:19:03.308>
created_at = <Date 2017-08-17.18:02:16.188>
labels = ['interpreter-core', 'type-bug', '3.7']
title = 'wrong error messages when too many kwargs are received'
updated_at = <Date 2017-08-23.18:19:03.307>
user = 'https://github.com/orenmn'

bugs.python.org fields:

activity = <Date 2017-08-23.18:19:03.307>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = True
closed_date = <Date 2017-08-23.18:19:03.308>
closer = 'serhiy.storchaka'
components = ['Interpreter Core']
creation = <Date 2017-08-17.18:02:16.188>
creator = 'Oren Milman'
dependencies = []
files = ['47091']
hgrepos = []
issue_num = 31229
keywords = ['patch']
message_count = 6.0
messages = ['300451', '300454', '300617', '300619', '300761', '300762']
nosy_count = 3.0
nosy_names = ['vstinner', 'serhiy.storchaka', 'Oren Milman']
pr_nums = ['3180']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue31229'
versions = ['Python 3.7']

[orenmn]

Some functions produce wrong error messages in case they receive too many
keyword arguments:
- in Objects/exceptions.c - ImportError_init:
    >>> ImportError(1, 2, 3, 4, a=5, b=6, c=7)
    TypeError: ImportError() takes at most 2 arguments (3 given)
- in Python/bltinmodule.c - min_max:
    >>> min(1, 2, 3, 4, a=5, b=6, c=7)
    TypeError: function takes at most 2 arguments (3 given)
    >>> max(1, 2, 3, 4, a=5, b=6, c=7)
    TypeError: function takes at most 2 arguments (3 given)
- in Modules/itertoolsmodule.c - product_new:
    >>> itertools.product(0, a=1, b=2, c=3, d=4, e=5, f=6)
    TypeError: product() takes at most 1 argument (6 given)
- in Python/bltinmodule.c - builtin_print: 
    >>> print(0, a=1, b=2, c=3, d=4, e=5, f=6)
    TypeError: print() takes at most 4 arguments (6 given)

ISTM that changing these error messages to refer to 'keyword arguments' instead
of 'arguments' is a possible solution. (e.g. the last one would become
'print() takes at most 4 keyword arguments (6 given)'

To do that, I changed two 'takes at most' PyErr_Format calls in Python/getargs.c.
I ran the test suite, and it seems that this patch doesn't break anything.
The diff file is attached.
(I didn't open a PR before hearing your opinion, as ISTM that changing code in
getargs.c is a delicate thing.)

what do you think?

[serhiy-storchaka]

In all of these cases PyArg_ParseTupleAndKeywords() is used in uncommon way. These functions accept variable number of positional-only arguments, and PyArg_ParseTupleAndKeywords() is called with empty args tuple for parsing keyword arguments only. The patch uses a trick for detecting this case and generating more appropriate error message. Correct error message is generated in a normal case too.

LGTM. But it may be worth to add a short comment about the necessary of this special case. And please add tests for error messages in all of these functions. test_call looks appropriate place for them.

[serhiy-storchaka]

Could you please make a PR from your patch Oren?

[orenmn]

I already wrote a patch, but I thought it would be better to wait until
bpo-31236 is resolved.
this is because bpo-31236 would change the error messages of min() and max(), and test_call tests exact error messages in
CFunctionCallsErrorMessages, which is where I thought the tests of this
issue should be added.

[serhiy-storchaka]

New changeset bf9075a by Serhiy Storchaka (Oren Milman) in branch 'master':
bpo-31229: Fixed wrong error messages when too many keyword arguments are received. (bpo-3180)
bf9075a

[serhiy-storchaka]

Thank you Oren for your contribution.