[issue11205] Evaluation order of dictionary display is different from reference manual.

[Nick Coghlan]

Nick Coghlan  added the comment:

I'm unlikely to ever get to this (especially as 3.5 is now in security-fix only 
mode), so closing it again.

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status: open -> closed

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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Nick Coghlan]

Nick Coghlan added the comment:

Temporarily reopening this as a docs bug - I think it's worth mentioning in the 
"Porting to Python 3.5" section of the What's New docs and as a "version 
changed" note in the dis module docs, as even though it's obscure, anyone that 
was inadvertently relying on the prior deviation from the spec is going to be 
confused by the behavioural change in 3.5.

(The specific case where this came up was Russell Keith-Magee encountering the 
semantic change in BUILD_MAP's expectations for argument order on the stack for 
his VOD bytecode transpiler)

--
assignee:  -> docs@python
components: +Documentation -Interpreter Core
nosy: +docs@python
priority: high -> normal
resolution: fixed -> 
stage: resolved -> needs patch
status: closed -> open
versions: +Python 3.5 -Python 2.7, Python 3.3, Python 3.4

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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Steve Dougherty]

Steve Dougherty added the comment:

Anyone care to review issue11205-v3.patch ?

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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Terry J. Reedy]

Terry J. Reedy added the comment:

I downloaded and tried to apply to 3.5 (then default) as it was last Saturday, 
before the .b1 cutoff.  Only ceval.py and test_compile.py patches worked.  
Everything else failed.  You need to update your repository (3.5 is now a 
branch and default is 3.6) and then the patch.

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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Roundup Robot]

Roundup Robot added the comment:

New changeset 6f05f83c7010 by Benjamin Peterson in branch '3.5':
in dict displays, evaluate the key before the value (closes #11205)
https://hg.python.org/cpython/rev/6f05f83c7010

New changeset ba9e4df5368c by Benjamin Peterson in branch 'default':
merge 3.5 (#11205)
https://hg.python.org/cpython/rev/ba9e4df5368c

--
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status: open - closed

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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Steve Dougherty]

Steve Dougherty added the comment:

I've added a change to the bytecode magic number as well. I don't see a magic 
tag anymore, either in the code or for recent versions.

There are autogenerated changes to Python/importlibs.h. Is my understanding 
correct that these are not to be committed?

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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Eric Snow]

Eric Snow added the comment:

Changes to importlib.h should always be committed.  It is the frozen 
importlib._bootstrap module, which is the implementation of the import system 
used by the interpreter.  So failure to commit changes to importlib.h means 
your changes to importlib._bootstrap will not have any effect.

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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Steve Dougherty]

Steve Dougherty added the comment:

I've added the importlib.h changes and changed the name of the test to be more 
descriptive.

--
Added file: http://bugs.python.org/file39228/issue11205-v3.patch

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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Steve Dougherty]

Steve Dougherty added the comment:

I've added a patch to change the order of evaluation and of STORE_MAP's 
arguments. It includes a test incorporating the review on the previous version.

I noticed I had to remove existing .pyc files to avoid TypeErrors about values 
being unhashable. I take it the version numbers in the filenames are sufficient 
to prevent that problem in a release?

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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Nick Coghlan]

Nick Coghlan added the comment:

The pyc issue suggests the magic number embedded in pyc files to indicate
bytecode compatibility needs to be incremented. If I recall correctly, that
lives in Lib/importlib/_bootstrap.py these days.

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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Raymond Hettinger]

Changes by Raymond Hettinger raymond.hettin...@gmail.com:


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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Steve Dougherty]

Steve Dougherty added the comment:

Any word on either changing the documentation to match the behaviour or fixing 
this as a bug?

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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Guido van Rossum]

Guido van Rossum added the comment:

This needs a code patch. But it can only be fixed for 3.5.
On Apr 7, 2015 11:21 AM, Steve Dougherty rep...@bugs.python.org wrote:


 Steve Dougherty added the comment:

 Any word on either changing the documentation to match the behaviour or
 fixing this as a bug?

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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Terry J. Reedy]

Changes by Terry J. Reedy tjre...@udel.edu:


--
versions:  -Python 3.2

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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Ned Batchelder]

Ned Batchelder added the comment:

Since this is documented in the Python Language Reference, it doesn't make much 
sense to have it describe one way for 3.3 and another for 3.4, does it?  By 
definition, doesn't that make this an implementation dependency?  We should 
update the docs to say that pairs in dicts will be evaluated left-to-right, but 
that the order of key and value is implementation dependent, since it actually 
is.

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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Terry J. Reedy]

Terry J. Reedy added the comment:

I strongly agree with Guido that we should fix the code. To me, defined 
left-to-right evaluation of sub-expressions within an expression is a feature. 
C's 'implementation defined' is a nuisance.

I do not think we should temporarily change the doc. In other cases where we 
have only applied a bugfix to a future version, we have not temporarily doc'ed 
the bug as 'correct'. The change *should* be in 
What's New.

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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Guido van Rossum]

Guido van Rossum added the comment:

I am sticking with my opinion from before: the code should be fixed.  It 
doesn't look like assignment to me.

I am fine with making this a feature only fixed in 3.4.  (You can even fix 
the docs in 3.3 as long as you fix them back for 3.4.)

Minor note for Ezio: dict(k=v, ...) doesn't have this problem because k is just 
a keyword, not an expression.

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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Ezio Melotti]

Ezio Melotti added the comment:

I came across the same problem in #16777.

IMHO the current behavior is better, and the documentation should be fixed 
instead, for the following reasons:
 1) it's consistent with assignments, where the RHS is evaluated before the LHS 
(see also msg128500).  This should also be the behavior of the dict(k=v) 
syntax, and what I personally expect;
 2) changing it back is not backward-compatible with any code written during 
the last 10 years;
 3) keeping the current behavior and fixing the docs is simpler than fixing the 
code to match the docs;

In addition, I would avoid writing code with side-effects in a dict literal, 
even if the order was documented and guaranteed.  The fact that we don't see 
many reports about this seems to indicate that people don't write such code, or 
if they do they rely on the current order.

--
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type:  - behavior

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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Nick Coghlan]

Changes by Nick Coghlan ncogh...@gmail.com:


--
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priority: high - low
versions: +Python 3.4

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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Terry J. Reedy]

Terry J. Reedy added the comment:

Today in pydev thread chained assignment weirdity Guido said
http://permalink.gmane.org/gmane.comp.python.devel/135746
I agree that we should be *very* conservative in changing the
meaning of existing opcodes (adding new one is a different story).
...
Hm. I really don't think that is a good development for Python to
compromise in the area of expression evaluation order where side
effects are involved.
...

I haven't looked at the proposed fixes, but I think correctness is more 
important than saving an extra bytecode (OTOH keeping the set of opcodes the 
same trumps both). I can't imagine that this extra opcode will be significant 
in many cases.

To which Nick C. replied
Since you've indicated the implementation is in the wrong here and you
also want to preserve opcode semantics, I think Skip's patch is
correct, but also needs to be applied to dict comprehensions (now we
have them). The extra bytecode is only ROT_TWO, which is one of the
cheapest we have kicking around.

To which Guido said Ok, somebody go for it! (Also please refer to my 
pronouncement in the bug

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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Antoine Pitrou]

Changes by Antoine Pitrou pit...@free.fr:


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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Terry J. Reedy]

Changes by Terry J. Reedy tjre...@udel.edu:


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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Sandro Tosi]

Changes by Sandro Tosi sandro.t...@gmail.com:


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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Terry J. Reedy]

Terry J. Reedy tjre...@udel.edu added the comment:

If I understand, since the present patch uses present opcode semantics, adding 
a rotate, it could go into 2.7 and 3.2. Correct?

3.3 could get an improved patch that instead changed opcodes to expect key and 
value in the other order.

--
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stage:  - patch review
versions:  -Python 3.1

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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Skip Montanaro]

Changes by Skip Montanaro s...@pobox.com:


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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Raymond Hettinger]

Changes by Raymond Hettinger raymond.hettin...@gmail.com:


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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Éric Araujo]

Changes by Éric Araujo mer...@netwok.org:


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[issue11205] Evaluation order of dictionary display is different from reference manual.

[takayuki]

New submission from takayuki takayuki...@star.odn.ne.jp:

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=revrevision=30148
, but current implementation seems not to reflect these accomplishment.

In present, which behaviour is legal?

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nosy: takayuki
priority: normal
severity: normal
status: open
title: Evaluation order of dictionary display is different from reference 
manual.
versions: Python 2.7

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[issue11205] Evaluation order of dictionary display is different from reference manual.

[yeswanth]

Changes by yeswanth swamiyeswa...@yahoo.com:


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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Skip Montanaro]

Skip Montanaro s...@pobox.com added the comment:

Here's a patch.

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Added file: http://bugs.python.org/file20753/compile.diff

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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Skip Montanaro]

Skip Montanaro s...@pobox.com added the comment:

Working on a test case too.

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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Skip Montanaro]

Changes by Skip Montanaro s...@pobox.com:


Removed file: http://bugs.python.org/file20753/compile.diff

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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Skip Montanaro]

Skip Montanaro s...@pobox.com added the comment:

It's not so easy as first appeared.

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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Skip Montanaro]

Skip Montanaro s...@pobox.com added the comment:

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

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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Skip Montanaro]

Skip Montanaro s...@pobox.com added the comment:

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

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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Georg Brandl]

Georg Brandl ge...@python.org added the comment:

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

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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Skip Montanaro]

Skip Montanaro s...@pobox.com added the comment:

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.

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[issue11205] Evaluation order of dictionary display is different from reference manual.

[R. David Murray]

R. David Murray rdmur...@bitdance.com added the comment:

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

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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Georg Brandl]

Georg Brandl ge...@python.org added the comment:

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.

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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Skip Montanaro]

Skip Montanaro s...@pobox.com added the comment:

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

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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Antoine Pitrou]

Changes by Antoine Pitrou pit...@free.fr:


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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Raymond Hettinger]

Raymond Hettinger rhettin...@users.sourceforge.net added the comment:

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).

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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Raymond Hettinger]

Raymond Hettinger rhettin...@users.sourceforge.net added the comment:

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

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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Skip Montanaro]

Skip Montanaro s...@pobox.com added the comment:

 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?

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[issue11205] Evaluation order of dictionary display is different from reference manual.

[Raymond Hettinger]

Raymond Hettinger rhettin...@users.sourceforge.net added the comment:

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.

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