Left to right

[tim-one]

BPO	448679
Nosy	@tim-one, @loewis
Files	ref5.diff evalorder.patch

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

Show more details

GitHub fields:

assignee = None
closed_at = <Date 2002-12-16.14:05:20.000>
created_at = <Date 2001-08-07.06:55:27.000>
labels = ['docs']
title = 'Left to right'
updated_at = <Date 2002-12-16.14:05:20.000>
user = 'https://github.com/tim-one'

bugs.python.org fields:

activity = <Date 2002-12-16.14:05:20.000>
actor = 'niemeyer'
assignee = 'niemeyer'
closed = True
closed_date = None
closer = None
components = ['Documentation']
creation = <Date 2001-08-07.06:55:27.000>
creator = 'tim.peters'
dependencies = []
files = ['108', '109']
hgrepos = []
issue_num = 448679
keywords = []
message_count = 6.0
messages = ['5810', '5811', '5812', '5813', '5814', '5815']
nosy_count = 3.0
nosy_names = ['tim.peters', 'loewis', 'niemeyer']
pr_nums = []
priority = 'normal'
resolution = 'accepted'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue448679'
versions = ['Python 2.3']

[tim-one]

The Ref Man doesn't explicitly say anything about
Python's left-to-right evaluation order. Strict left-
to-right was Guido's intent, though, and it comes up a
few times per year on c.l.py (indeed, I was just
replying to a msg asking about it).

The docs should get fleshed out.

There's also a bug:

>>> counter = 0
>>> def i():
...     global counter
...     counter += 1
...     return counter
...
>>> {i(): i()}
{2: 1}
>>> {i(): i(), i(): i()}
{4: 3, 6: 5}
>>>

That is, key:value *pairs* are evaluated left to
right, but within a single key:value pair, the value
gets evaluated first. When I asked Guido about that
some years ago, he agreed it was a bug.

[loewis]

Logged In: YES
user_id=21627

With the patch attached, this behaviour is cast into stone
(or, atleast into a SF tracker :)

[niemeyer]

Logged In: YES
user_id=7887

I'm attaching a suggested solution for the problem,
including documentation and reordering of dict evaluation.

[niemeyer]

Logged In: YES
user_id=7887

Martin, could you please review the proposed solution?

[loewis]

Logged In: YES
user_id=21627

The patch looks fine, please apply it.

It would be good to check that Jython meets all your
expectations in this respect, and that it, in particular,
executes all the examples in the documentation in the right
order.

This change should *not* be backported to 2.2 IMO.

[niemeyer]

Logged In: YES
user_id=7887

Martin, I've checked with Jython and the examples are ok as
well.

I have applied as:

Doc/ref/ref5.tex: 1.71
Python/compile.c: 2.267
Lib/compiler/pycodegen.py: 1.62
Misc/NEWS: 1.561

Thank you!