[3.5] Wrong order of stack for CALL_FUNCTION_VAR and CALL_FUNCTION_VAR_KW

[mvaled]

BPO	33216
Nosy	@vstinner, @larryhastings, @ericsnowcurrently, @serhiy-storchaka, @miss-islington, @mvaled
PRs	[3.5] bpo-33216: Correct the doc of CALL_FUNCTION_VAR and CALL_FUNCTION_VAR_KW. #6365 bpo-33216: Clarify the documentation for CALL_FUNCTION_* #8338 [2.7] bpo-33216: Clarify the documentation for CALL_FUNCTION_* (GH-8338) #8783 bpo-33216: Improve the documentation for CALL_FUNCTION_* (GH-8338) #8784 [3.7] bpo-33216: Improve the documentation for CALL_FUNCTION_* (GH-8338) (GH-8784) #9354 [3.6] bpo-33216: Improve the documentation for CALL_FUNCTION_* (GH-8338) (GH-8784) #9355

^{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/larryhastings'
closed_at = <Date 2018-09-17.12:41:55.798>
created_at = <Date 2018-04-03.19:38:13.494>
labels = ['docs']
title = '[3.5] Wrong order of stack for CALL_FUNCTION_VAR and CALL_FUNCTION_VAR_KW'
updated_at = <Date 2018-09-17.13:09:45.656>
user = 'https://github.com/mvaled'

bugs.python.org fields:

activity = <Date 2018-09-17.13:09:45.656>
actor = 'miss-islington'
assignee = 'larry'
closed = True
closed_date = <Date 2018-09-17.12:41:55.798>
closer = 'serhiy.storchaka'
components = ['Documentation']
creation = <Date 2018-04-03.19:38:13.494>
creator = 'mvaled'
dependencies = []
files = []
hgrepos = []
issue_num = 33216
keywords = ['patch']
message_count = 17.0
messages = ['314888', '314891', '314922', '317473', '317475', '317476', '317523', '317531', '317535', '321971', '323606', '323607', '325530', '325531', '325537', '325540', '325541']
nosy_count = 7.0
nosy_names = ['vstinner', 'larry', 'docs@python', 'eric.snow', 'serhiy.storchaka', 'miss-islington', 'mvaled']
pr_nums = ['6365', '8338', '8783', '8784', '9354', '9355']
priority = 'low'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue33216'
versions = ['Python 3.5']

[mvaled]

The documentation of the dis module says that:

CALL_FUNCTION_VAR (argc)

Calls a function. *argc* is interpreted as in CALL_FUNCTION. The
top elements on the stack are the keyword arguments, followed by the
variable argument list, and then the positional arguments.

However, inspecting the how f(b=1, *args) is actually compiled shows a different order:

>>> import dis
>>> dis.dis(compile('f(b=1, *args)', '<>', 'eval'))

1 0 LOAD_NAME 0 (f)
3 LOAD_NAME 1 (args)
6 LOAD_CONST 0 ('b')
9 LOAD_CONST 1 (1)
12 CALL_FUNCTION_VAR 256 (0 positional, 1 keyword pair)
15 RETURN_VALUE

Notice that the top of the stack contains the explicit keyword arguments.

The documentation of CALL_FUNCTION_VAR_KW is also incorrect.

[mvaled]

Correction, the documentation for CALL_FUNCTION_VAR said:

Calls a function. *argc* is interpreted as in :opcode:`CALL_FUNCTION`. The top element on the stack contains the variable argument list, followed by keyword and positional arguments.

In a previous comment I pasted the version I'm putting in the PR.

[serhiy-storchaka]

I confirm that this issue is 3.5-only. Before 3.5 the order matched the documentation. After 3.5 these opcodes were completely rewritten and they use now other way of passing arguments.

[larryhastings]

Serhiy, why did you mark this as a release blocker? This is a proposed documentation fix.

[vstinner]

If the issue is specific to Python 3.5, we can no longer fix it since 3.5 doesn't accept bugfixes anymore. No?

[larryhastings]

Documentation changes are okay.

[serhiy-storchaka]

Because I don't want that this fix be missed in the next bugfix release. I think this documentation is important enough. There may be no chance to document it later.