[issue39301] Specification of bitshift on integers should clearly state floor division used

[Mark Dickinson]

Mark Dickinson  added the comment:

Closing; I think this is addressed.

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resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed

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[issue39301] Specification of bitshift on integers should clearly state floor division used

[Mark Dickinson]

Mark Dickinson  added the comment:

Thanks Zackery for the fix! Nick: is this good enough to close, or would you 
like to see something more here?

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[issue39301] Specification of bitshift on integers should clearly state floor division used

[Mark Dickinson]

Mark Dickinson  added the comment:


New changeset c2a177adf3575d4eb81030fba851f78d7a8e3f51 by Miss Islington (bot) 
in branch '3.8':
bpo-39301: State that floor division is used for right shift operations 
(GH-20347) (GH-20415)
https://github.com/python/cpython/commit/c2a177adf3575d4eb81030fba851f78d7a8e3f51


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[issue39301] Specification of bitshift on integers should clearly state floor division used

[Mark Dickinson]

Mark Dickinson  added the comment:


New changeset b068d892c1ba7b996e43aceb974bfadac3c577ed by Miss Islington (bot) 
in branch '3.7':
bpo-39301: State that floor division is used for right shift operations 
(GH-20347) (GH-20416)
https://github.com/python/cpython/commit/b068d892c1ba7b996e43aceb974bfadac3c577ed


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[issue39301] Specification of bitshift on integers should clearly state floor division used

[Mark Dickinson]

Mark Dickinson  added the comment:


New changeset cc0f50d62c75a1d171f5de9b56caef64e79eb013 by Miss Islington (bot) 
in branch '3.9':
bpo-39301: State that floor division is used for right shift operations 
(GH-20347) (GH-20414)
https://github.com/python/cpython/commit/cc0f50d62c75a1d171f5de9b56caef64e79eb013


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[issue39301] Specification of bitshift on integers should clearly state floor division used

[miss-islington]

Change by miss-islington :


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pull_requests: +19676
pull_request: https://github.com/python/cpython/pull/20417

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[issue39301] Specification of bitshift on integers should clearly state floor division used

[miss-islington]

Change by miss-islington :


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pull_requests: +19674
pull_request: https://github.com/python/cpython/pull/20415

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[issue39301] Specification of bitshift on integers should clearly state floor division used

[miss-islington]

Change by miss-islington :


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pull_requests: +19675
pull_request: https://github.com/python/cpython/pull/20416

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[issue39301] Specification of bitshift on integers should clearly state floor division used

[Mark Dickinson]

Mark Dickinson  added the comment:


New changeset af7553ac95a96713be847dd45bc5a8aeb0a75955 by Zackery Spytz in 
branch 'master':
bpo-39301: State that floor division is used for right shift operations 
(GH-20347)
https://github.com/python/cpython/commit/af7553ac95a96713be847dd45bc5a8aeb0a75955


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[issue39301] Specification of bitshift on integers should clearly state floor division used

[miss-islington]

Change by miss-islington :


--
nosy: +miss-islington
nosy_count: 4.0 -> 5.0
pull_requests: +19673
pull_request: https://github.com/python/cpython/pull/20414

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[issue39301] Specification of bitshift on integers should clearly state floor division used

[Mark Dickinson]

Change by Mark Dickinson :


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versions: +Python 3.10

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[issue39301] Specification of bitshift on integers should clearly state floor division used

[Mark Dickinson]

Mark Dickinson  added the comment:

> I confess that I've no idea what the "without overflow check" bit means

After some digging, it is indeed to do with the int/long unification:

- In Python 2.1, arithmetic operations on ints would raise OverflowError on 
overflow.
- Except that left shift did not raise: shifted out bits were simply lost.  
This is described in PEP 237.

In Python 2.2, arithmetic operations (including left shift) were changed to 
produce a long instead of overflowing.

The "without overflow check" part of the doc almost certainly refers to the 
Python 2.1 behaviour (at least for left shift; I'm guessing that for right 
shift, where overflow isn't possible, this was just a redundant copy-paste), 
and was never updated. It should be removed for both left shift and right shift.

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[issue39301] Specification of bitshift on integers should clearly state floor division used

[Zackery Spytz]

Change by Zackery Spytz :


--
keywords: +patch
nosy: +ZackerySpytz
nosy_count: 3.0 -> 4.0
pull_requests: +19610
stage: needs patch -> patch review
pull_request: https://github.com/python/cpython/pull/20347

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[issue39301] Specification of bitshift on integers should clearly state floor division used

[Nick Coghlan]

Nick Coghlan  added the comment:

Aye, adding "floor" to the existing footnote would be the minimal fix. I'm just 
wondering whether it's also worth stating that this means that positive 
integers saturate at zero, while negative integers saturate at -1.

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[issue39301] Specification of bitshift on integers should clearly state floor division used

[Mark Dickinson]

Mark Dickinson  added the comment:

Is the fix as simple as adding the word "floor" before "division" in the 
"equivalent to division by [...]" phrase?

> A right shift by n bits is equivalent to floor division by pow(2, n) without 
> overflow check. 

This text was probably written before // was introduced; maybe we can rewrite 
in terms of //? "x >> n" should be identical to "x // pow(2, n)" for any 
nonnegative "n".

I confess that I've no idea what the "without overflow check" bit means, but I 
suspect it's no longer relevant post int/long-unification.

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[issue39301] Specification of bitshift on integers should clearly state floor division used

[Mark Dickinson]

Change by Mark Dickinson :


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nosy: +mark.dickinson

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[issue39301] Specification of bitshift on integers should clearly state floor division used

[Nick Coghlan]

New submission from Nick Coghlan :

While reviewing ISO-IECJTC1-SC22-WG23's latest draft of their Python security 
annex, I noticed that 
https://docs.python.org/3.7/library/stdtypes.html#bitwise-operations-on-integer-types
 doesn't explicitly state that *floor* division is used for right shift 
operations, so right-shifting a negative number by more bits than it contains 
gives -1 rather than 0.

This is consistent with the way the language spec defines both binary 
right-shifts (as division by "pow(2, n)" and floor division (as rounding 
towards negative infinity), so this is just a documentation issue to note that 
we should make it clearer that this behaviour is intentional.

--
assignee: docs@python
components: Documentation
messages: 359786
nosy: docs@python, ncoghlan
priority: normal
severity: normal
stage: needs patch
status: open
title: Specification of bitshift on integers should clearly state floor 
division used
type: enhancement
versions: Python 3.7, Python 3.8, Python 3.9

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