lib2to3 fails on a trailing comma after **kwargs in a function signature

[ambv]

BPO	33064
Nosy	@ned-deily, @ambv, @serhiy-storchaka
PRs	bpo-33064: lib2to3: support trailing comma after **kwargs #6096 bpo-33064: lib2to3: support trailing comma after **kwargs #6096 [3.7] bpo-33064: lib2to3: support trailing comma after *args and **kwargs (GH-6096) #6097 [3.6] bpo-33064: lib2to3: support trailing comma after *args and **kwargs (GH-6096) #6098 bpo-33064: lib2to3: Add more tests #6101

^{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 = 'https://github.com/ambv'
closed_at = <Date 2018-03-13.22:19:55.107>
created_at = <Date 2018-03-13.02:50:30.090>
labels = ['3.8', 'type-bug', '3.7']
title = 'lib2to3 fails on a trailing comma after **kwargs in a function signature'
updated_at = <Date 2018-03-13.22:19:55.106>
user = 'https://github.com/ambv'

bugs.python.org fields:

activity = <Date 2018-03-13.22:19:55.106>
actor = 'lukasz.langa'
assignee = 'lukasz.langa'
closed = True
closed_date = <Date 2018-03-13.22:19:55.107>
closer = 'lukasz.langa'
components = []
creation = <Date 2018-03-13.02:50:30.090>
creator = 'lukasz.langa'
dependencies = []
files = []
hgrepos = []
issue_num = 33064
keywords = ['patch', 'patch']
message_count = 9.0
messages = ['313718', '313719', '313722', '313723', '313724', '313725', '313726', '313728', '313729']
nosy_count = 3.0
nosy_names = ['ned.deily', 'lukasz.langa', 'serhiy.storchaka']
pr_nums = ['6096', '6096', '6097', '6098', '6101']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue33064'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8']

[ambv]

Title says all. I have a patch.

[ambv]

Ned, I'd like this to make into 3.6.5 if at all possible.

[serhiy-storchaka]

Does it support a trailing comma after *args or name=value?

[serhiy-storchaka]

I'm not sure that a trailing comma after **kwargs in a function signature should be allowed in Python grammar. Reasons for allowing it after other arguments don't work here since **kwargs is always a last item in a signature.

But this ship perhaps is sailed.

[ambv]

New changeset b51f5de by Łukasz Langa in branch 'master':
bpo-33064: lib2to3: support trailing comma after *args and **kwargs (bpo-6096)
b51f5de

[ambv]

Yes, this ship has sailed with Python 3.6.

I understand your logic and this is indeed why we didn't support this for a long time. However, people tend to grow muscle memory to add trailing commas and that was tripping them over for no real gain on our part. So this was added. The reason I found out about lib2to3 not following suit was that 3.6 users quickly started to put those trailing commas in and tools using lib2to3 were crashing on such files :) This proves it was worth it.

[ambv]

Oh, and in terms of *calling* functions, now we can specify multiple unpacks, like:

  >>> f(
  ...   **d,
  ...   **e,
  ... )

so the extra comma for signatures simply makes it symmetrical in terms of appearance.

[ambv]

New changeset b4c8871 by Łukasz Langa (Miss Islington (bot)) in branch '3.7':
bpo-33064: lib2to3: support trailing comma after *args and **kwargs (GH-6096) (bpo-6097)
b4c8871

[ambv]

New changeset 6a526f6 by Łukasz Langa (Miss Islington (bot)) in branch '3.6':
bpo-33064: lib2to3: support trailing comma after *args and **kwargs (GH-6096) (bpo-6098)
6a526f6