Evaluation order of dictionary display is different from reference manual.

[takayuki]

BPO	11205
Nosy	@gvanrossum, @birkenfeld, @rhettinger, @terryjreedy, @ncoghlan, @nedbat, @benjaminp, @ezio-melotti, @merwok, @bitdancer, @sandrotosi, @freakboy3742, @ericsnowcurrently
Files	compile.diff issue11205.patch issue11205-v2.patch issue11205-v3.patch

^{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 2017-10-06.02:29:41.330>
created_at = <Date 2011-02-13.09:39:12.878>
labels = ['type-bug', 'docs']
title = 'Evaluation order of dictionary display is different from reference manual.'
updated_at = <Date 2017-10-06.02:29:41.295>
user = 'https://bugs.python.org/takayuki'

bugs.python.org fields:

activity = <Date 2017-10-06.02:29:41.295>
actor = 'ncoghlan'
assignee = 'docs@python'
closed = True
closed_date = <Date 2017-10-06.02:29:41.330>
closer = 'ncoghlan'
components = ['Documentation']
creation = <Date 2011-02-13.09:39:12.878>
creator = 'takayuki'
dependencies = []
files = ['20754', '39218', '39225', '39228']
hgrepos = []
issue_num = 11205
keywords = ['patch']
message_count = 33.0
messages = ['128482', '128487', '128489', '128490', '128492', '128493', '128494', '128498', '128500', '128502', '128507', '128520', '128521', '128522', '128530', '138223', '175120', '178133', '179585', '185438', '185446', '240222', '240225', '242157', '242161', '242183', '242200', '242204', '244336', '244339', '244340', '263219', '303791']
nosy_count = 19.0
nosy_names = ['gvanrossum', 'georg.brandl', 'rhettinger', 'terry.reedy', 'ncoghlan', 'freakboy', 'nedbat', 'benjamin.peterson', 'ezio.melotti', 'eric.araujo', 'r.david.murray', 'sandro.tosi', 'freakboy3742', 'docs@python', 'swamiyeswanth', 'takayuki', 'python-dev', 'eric.snow', 'sdougherty']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue11205'
versions = ['Python 3.5']

[takayuki]

Running the following code shows "2 1 4 3", but in reference manual
http://docs.python.org/reference/expressions.html#expression-lists
the evaluation order described as
{expr1: expr2, expr3: expr4}

def f(i):
    print i
    return i

{f(1):f(2), f(3):f(4)}

I found some of past discussions about this problem, for example,
http://mail.python.org/pipermail/python-dev/2002-November/030461.html
http://mail.python.org/pipermail/python-dev/2002-November/030458.html
http://bugs.python.org/issue448679
http://svn.python.org/view?view=rev&revision=30148
, but current implementation seems not to reflect these accomplishment.

In present, which behaviour is legal?

[smontanaro]

Here's a patch.

[smontanaro]

Working on a test case too.

[smontanaro]

It's not so easy as first appeared.

[smontanaro]

Okay, this looks better. I was confusing myself with leftover .pyc
files I think. Test included.

[smontanaro]

Georg, I think this might need to sneak into 3.2.

[birkenfeld]

I don't think so -- it's a very minor deviation from the spec and not a critical bug.

[smontanaro]

3.2 and earlier versions are all frozen, but for 3.3 it might make sense to bump the version number of the bytecode and change STORE_MAP to take the key and value in the opposite order, thus allowing to remove the ROT_TWO I had to add to make this work.

[bitdancer]

Interesting. I would actually have expected the observed behavior. I think of the : in a dictionary literal as an assignment.

[birkenfeld]

BTW, it would be nice to know if this behavior was consistent with the docs at any time (the merge of the AST branch in 2.5 might be an obvious candidate where it was broken).

Also interesting would be what other implementations of Python do.

[smontanaro]

As Georg suggested, it is correct in 2.4, wrong in 2.5.

[rhettinger]

<sigh> If only a test had been checked-in eight years ago ...

It looks like a number of things have to be changed in order to bring behavior back to what the docs promise. Dict literals and dict comprehensions need to be fixed in both the compile.py and compile.c. To avoid introducing a ROT_TWO, the store STORE_MAP and MAP_ADD opcodes need minor modifications (just switch the u and w variable assignments).

[rhettinger]

takayuki, a special thanks to you for submitting such a well-researched bug report :-)

[smontanaro]

To avoid introducing a ROT_TWO, the store STORE_MAP and MAP_ADD
opcodes need minor modifications (just switch the u and w variable
assignments).

Either of which would not be possible in anything other that 3.3 or
later, right?

[rhettinger]

How much to change and how far to backport may make for a good python-dev discussion.

ISTM that changing generated code goes hand-in-hand with changing opcode semantics (as long as the magic number gets bumped). OTOH, none of this may be worth backporting at all since no one seems to have noticed or cared for eight years.

I don't have any feelings about it one way or the other, but it would great to see Py3.2.1 get fixed properly.