Keyword only argument default values are evaluated before other defaults

[kayhayen]

BPO	16967
Nosy	@birkenfeld, @ncoghlan, @benjaminp

^{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/benjaminp'
closed_at = <Date 2013-02-10.14:32:36.689>
created_at = <Date 2013-01-14.19:37:14.319>
labels = ['interpreter-core']
title = 'Keyword only argument default values are evaluated before other defaults'
updated_at = <Date 2014-06-21.04:10:09.514>
user = 'https://bugs.python.org/KayHayen'

bugs.python.org fields:

activity = <Date 2014-06-21.04:10:09.514>
actor = 'Kay.Hayen'
assignee = 'benjamin.peterson'
closed = True
closed_date = <Date 2013-02-10.14:32:36.689>
closer = 'python-dev'
components = ['Interpreter Core']
creation = <Date 2013-01-14.19:37:14.319>
creator = 'Kay.Hayen'
dependencies = []
files = []
hgrepos = []
issue_num = 16967
keywords = []
message_count = 6.0
messages = ['179970', '179972', '181783', '181808', '181809', '221151']
nosy_count = 6.0
nosy_names = ['georg.brandl', 'ncoghlan', 'benjamin.peterson', 'Arfrever', 'python-dev', 'Kay.Hayen']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue16967'
versions = ['Python 3.2']

[kayhayen]

Suprisingly, keyword only arguments become evaluated first:

>>> def f(a=undefined1,*,b=undefined2):pass
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'undefined2' is not defined

It should be "undefined1".

I am sure, this is going to surprise developers and breaks assumptions, people tend to make. So far (to my knowledge) nothing that was separated by a "," could be evaluated in a mixed order.

Please consider to change this around.

[birkenfeld]

Looks like a bug to me, although it isn't likely to cause great harm.

[ncoghlan]

Agreed on it being a bug that we do it the wrong way around, but "Yikes!" at the idea of code where it makes a significant difference.

[python-dev]

New changeset d296cf1600a8 by Benjamin Peterson in branch 'default':
evaluate positional defaults before keyword-only defaults (closes bpo-16967)
http://hg.python.org/cpython/rev/d296cf1600a8

[python-dev]

New changeset 6917402c6191 by Benjamin Peterson in branch 'default':
evaluate lambda keyword-only defaults after positional defaults (bpo-16967 again)
http://hg.python.org/cpython/rev/6917402c6191

[kayhayen]

I can confirm that Python3.4 is not affected. Python 3.3 and 3.2 still are.